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Eddies encourage the ocean’s absorption of carbon dioxide from the atmosphere and help regulate the planet’s climate. Now, scientists have more details about how these ephemeral ocean features die.

Eddies are circular currents that wander around the ocean like spinning tops, ranging from tens to hundreds of kilometers in diameter. They mimic weather systems in the atmosphere and serve as a feeding grounds for sharks, turtles, and fish. Eddies often spin off major ocean currents and typically die within a matter of months.

Some fundamental questions in physical oceanography center around the life cycle of eddies: What gives rise to them, and how do they die? “It’s a big puzzle that’s been long-standing in the community,” said fluid dynamicist Hussein Aluie from the University of Rochester, N.Y.

Aluie and his colleagues found that when it comes to eddy killing, the planet’s winds are partly to blame.

Their innovative analysis of satellite data suggests that wind sucks energy out of the ocean from features smaller than 260 kilometers—features that include most eddies. Wind continually extracts about 50 gigawatts of energy from eddies around the world. The team published their research in Science Advances in July.

“Fifty gigawatts is equivalent to detonating a Hiroshima nuclear bomb every 20 minutes, year-round,” said first author Shikhar Rai, a doctoral student at the University of Rochester. “It is equivalent to operating 50 million microwave ovens continuously throughout the year.”

Although it’s long been suspected that wind zaps eddies of their spin, the latest study provides a seasonal signal and an estimate of wind power loss in major currents. Although wind may be a killer of eddies, it supercharges larger-scale ocean circulation. Wind adds about 970 gigawatts of energy to features larger than 260 kilometers, the recent research found.

Eddies boost ocean heat intake, ocean mixing at the surface, and the exchange of gases with the atmosphere, so calculating these processes relies on accurate depictions of eddies in computer models.

Blowing in the Wind

Eddies likely form from interconnected physical forces in the ocean that include density-driven motion from water of different temperatures or salinities.

Wind destroys ocean eddies by applying stress to the ocean’s surface and slowing eddies’ spin to the point of extinguishing them. Because wind stress hinges on the difference between the speed and direction of wind compared with the speed and direction of the ocean’s surface flow, wind categorically slows eddies rather than quickening them.

Eddy killing happens year-round, but the effects are particularly strong in winter, when winds grow stronger because of storms, according to the new study.

Most eddies come from western boundary currents like the Gulf Stream in the Atlantic and the Kuroshio in the Pacific, and the latest results reveal just how much energy relative to the total input wind removes from these currents’ eddies: 50% from the Gulf Stream and a whopping 90% from the Kuroshio.

“The movement of the ocean is critical in regulating the climate of the Earth,” Aluie said. Eddies can affect the trajectories of major currents: For example, eddies are widely believed to play a crucial role in causing the warm waters of the Gulf Stream to curve away from the eastern United States, keeping the climate of Canada, Greenland, and the Labrador Sea cold.

The research adds to the building evidence that wind stifles eddies. Chris Hughes, a professor of sea level science at the University of Liverpool and author of a 2008 study that found that wind sucked 60 gigawatts of energy from the ocean, said, “It’s nice to see this confirmed independently and some new diagnostics shown.”

A Blurred Photograph Coarse-graining analysis subtracts a blurred version of data (right) from a precise version (left). Credit: Paul Green/Unsplash

The research team used an emerging method in physical oceanography to conduct the new work. Typically, researchers study how the ocean changes over time. But in the latest analysis, the scientists looked at differences over space, not time.

The latest study “represents a novel application of the newly developed coarse-grain method,” said physical oceanographer Xiaoming Zhai of the University of East Anglia, who was not involved in the research.

Coarse-graining analysis can be explained with a simple example, said Aluie. Imagine a flower in a photograph. If you blur the photograph, you can’t see the texture of the flower’s petals, the grains of pollen on its anthers, or the edges of the sepals. If you now take the unblurred photo and subtract the blurry one from it, you get only the fine details of the flower.

The new study used measurements taken between 1999 and 2007 from NASA’s QuikSCAT satellite scatterometer. By “blurring” the satellite information, Rai and his colleagues used coarse-graining analysis to see the details of small-scale ocean flow, which included eddies. The method allowed them to pinpoint the 260-kilometer cutoff.

Sadly, QuikSCAT died in 2009, but an upcoming NASA mission, Surface Water and Ocean Topography (SWOT), along with wind data from other satellite missions could provide Rai and others with higher-quality data soon.

The team will continue to use spatial techniques like coarse-grain analysis in future work, which will include a look into the other side of an eddy’s life cycle: its birth.

—Jenessa Duncombe (@jrdscience), Staff Writer

Just like the planets of our solar system, most exoplanets tend to orbit their star in the same direction that the star spins. But when they don’t, exoplanet orbits overwhelmingly prefer to be perpendicular. This new understanding of planetary orbits, published in Astrophysical Journal Letters, raises questions about which planets can become misaligned from the direction that their star spins and how the orbits get that way in the first place.

From a Certain Point of View

When seeking to explain strange exoplanet phenomena, the most useful point of comparison is our own solar system. We know more about it than any other of the thousands of planetary systems discovered to date. The dynamics of the solar system are relatively neat and tidy: The orbits of the eight planets all sit very neatly in the same plane, that plane lines up almost exactly with the Sun’s equator, and the whole system rotates in the same direction.

Within the solar system, the largest angle of misalignment between a planet’s orbit and the Sun’s equator—which defines the plane of the Sun’s spin—is Earth’s at just over 7°. Exoplanet scientists have been able to make similar measurements of the spin-orbit alignment within other planetary systems. “Is 7° a small value or a large value?” asked Simon Albrecht, an astronomer at Aarhus University in Denmark and lead author on the recent study. “The jury on that is still out.”

“That alignment in our solar system is part of what led us to believe that planets form out of a disk that’s around the star,” added astrophysicist and coauthor Rebekah Dawson of Pennsylvania State University in University Park. The prevailing theory of planet formation posits that a large cloud of dust and gas collapses under its own gravity to create a star in the center. The leftover material flattens out into a disk that coalesces into one or more planets (see video at right). In that simplified model, all of the star- and planet-forming material swirls in the same direction, which should make the resulting star and planets all spin in a common direction.

However, “we have known for over a decade that there are planets that are not orbiting in the same plane as their star,” Dawson explained. Although most exoplanets orbit in the same direction as the star’s spin (prograde) and with a very small angle between spin and orbit (0°), there are plenty whose orbits don’t follow suit, including some that orbit opposite to the direction of the star’s spin (retrograde) and others that travel completely backward (180°). “The angle between the planet’s orbit and the star’s spin was some of the first three-dimensional information that we started to get about other planetary systems.…We have to imagine something that’s different or more complicated than the history that we’ve naively invoked for our solar system.”

Astronomers can calculate the angle of inclination between the exoplanet’s orbit and the star’s spin by measuring the transit of the planet in different wavelengths and comparing the different transit profiles, a method called the Rossiter-McLaughlin effect (Figure 1).

Fig. 1. If a star’s spin axis is not pointed toward Earth, some of the light from the star will appear to be moving toward observers (blueshifted), and some of the light will appear to move away from observers (redshifted). Here this apparent movement is represented by the stars (large circles) colored blue and red as they spin from left to right (dashed arrow). Exoplanets (black circle with white halo) will block varying amounts of blueshifted and redshifted light as they transit the star (solid arrow). The pattern of how much of the bluer or redder light is blocked over time, known as the Rossiter-McLaughlin effect, can reveal the direction of the planet’s orbit relative to the star’s spin. Credit: Kimberly M. S. Cartier

Usually, however, astronomers can measure only one dimension of a star’s 3D spin—the component of the spin that’s pointed at Earth. “That can tell you that something is misaligned but not by how much,” Dawson said. How much of the star’s total spin we can see and measure depends on the geometry of our vantage point: If a star’s spin axis points directly at Earth, we would measure no spin at all and see no planetary misalignment. To understand the physical reasons why planetary systems are misaligned, it’s not the perceived angle of misalignment that matters, but the true one.

A recent mathematical advancement helped Albrecht and his team calculate our viewing angle for 57 stars that host misaligned planets. With that additional information, the researchers determined that the planets’ misalignments weren’t as random as previously thought. In fact, they found that a significant number of the true misalignment angles were close to 90°, meaning that the planets orbit their stars from pole to pole rather than across the star’s equator.

More Questions Than Answers

For now, the data on perpendicular planets are outpacing the theories that explain them. There’s no obvious commonality that groups these stars and planets together that might explain why misaligned planets end up on polar orbits: The stars range from hot to cold, the planets range from Neptune mass to more massive than Jupiter, and the planetary orbits range from very close in to quite far away.

“The biggest thing these planets have in common is that we can measure this [viewing angle] for them,” Albrecht said. There are no models of planetary dynamics that predict a preference for perpendicular planets, he explained, because, quite simply, no one knew that their models needed to explain it.

No one theory can yet explain all of the perpendicular planetary systems.Regardless, Albrecht and his team offered a few potential ideas to start with, although they acknowledged that no one theory can yet explain all of the perpendicular planetary systems they analyzed. Three of the proposed explanations rely on the gravity of another object—the star, an unseen planet, or the planet-forming disk—tugging a planet’s orbit into a 90° misalignment; the fourth theory invokes a magnetic interaction during planet formation.

J. J. Zanazzi, a postdoctoral researcher at the Canadian Institute for Theoretical Astrophysics in Toronto, said that the team “did a great job summarizing the primary theories which can lead to their very exciting result that spin-orbit misalignments come in two flavors,” well aligned or perpendicular. “All the mechanisms have different strengths and weaknesses, and each mechanism fails to explain some part of [the] observation.” Zanazzi was not involved with this research.

The good news, Zanazzi said, is that “all of the astrophysical mechanisms which have been proposed make specific predictions when the mechanism does not work.…For me, a big thing observers can do in the near future is look for companion planets or stars which can cause the required tilts.” If they fail to find any that fit the bill, such a pattern would narrow down the potential explanations.

Moreover, Albrecht said, as theorists begin to refine their models to explain a cluster of polar orbits, those models can help guide the observers toward the right planetary systems to take a closer look at. Will polar orbits be more prevalent around cool stars or hot stars? Will perpendicular planets be found mostly in multiplanet systems or as loners? More observations, new theories, and time will tell.

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

During the late Pliocene, Arctic sea ice began to expand rapidly. The new ice created changes to sea level, albedo, the thermohaline circulation, and a host of other factors that still drive the planet’s climate today. But piecing together what caused the ice to expand rapidly has remained an elusive goal for scientists.

Now, a new study by Ma et al. shows that the sea expansion coincided with the formation of Siberia’s Irtysh River 2.77 million years ago. Previous work has shown that the Irtysh River was once a series of inland rivers that drained into a large paleolake in the Junggar Basin, located in northwestern China. But at some point, the basin burst, and the Irtysh began to flow northward toward the sea.

By analyzing neon-21 isotopes along with aluminum-26/beryllium-10, the researchers determined the timing of this critical event. Isotopes like these can be used to date rock and sediment samples because they are cosmogenic in nature and decay at different rates, meaning that if a sample is exposed to cosmic rays at the surface, the isotopes will be created. Then, if the sample is buried, the different nuclides will decay at different rates, providing insight into how long the sample has been sequestered from cosmic rays. With this technique, the scientists reconstructed much of the Junggar Basin’s geologic history and imply when the Siberian-Arctic river system began supplying fresh water to the Arctic Ocean.

The new water provided by the Irtysh created a layer of fresh water roughly 9 meters thick in the Kara Sea, which lies off of western Siberia. The scientists say this sudden influx of fresh water would’ve disrupted the vertical stability of the water and reinforced the stratification of vertical circulation. In combination, these changes created more sea ice in the Arctic, which then drove a series of albedo-based feedbacks, creating colder temperatures and yet more ice. The results show what an incredible impact even a single freshwater input can have in driving sea ice formation and the planet’s climate at large. (Geophysical Research Letters, https://doi.org/10.1029/2021GL093217, 2021)

—David Shultz, Science Writer

As the novel coronavirus has raced around the world, experts have wondered whether it would behave like influenza and other respiratory viruses, spiking in the winter and abating in the summer. Now, more than a year into the pandemic, researchers have enough data to confirm the seasonality of COVID-19 and determine which environmental factors may be driving it.

Of course, environmental factors alone cannot fully explain the spread of COVID-19; social and biological factors, such as population density and social distancing policies, also play a role. To isolate the impact of the environment, Choi et al. examined data on COVID-19 prevalence and environmental variables between 1 March 2020 and 13 March 2021 across five countries—Canada, Germany, India, Ethiopia, and Chile—that had relatively consistent social controls throughout the study period.

Previous studies have linked seasonal spikes of viruses like influenza, which spread by virus-laden droplets, to low humidity. But Choi and colleagues note that this pattern holds only in temperate regions; in the tropics, influenza peaks during the wet season. To account for this disparity, the team also looked at air drying capacity (ADC), defined as the rate of decrease with time of droplet surface area, given ambient temperature and humidity. Essentially, it predicts the fate of droplets under specific temperature and humidity conditions.

The team compared COVID-19 rates with the daily mean temperature, specific humidity, ultraviolet radiation, and ADC across a wide range of climate zones. Much like influenza, COVID-19 peaked in the winter months in the countries with temperate climates—Canada, Germany, and Chile—when temperature and humidity were at a low. But in the tropical countries, cases peaked during the summer monsoons, when humidity was at a high, suggesting that temperature and humidity considered separately can’t explain the seasonality of respiratory viruses like influenza and COVID-19. The seasonal values of ultraviolet (UV) radiation and ADC, however, were consistent with fluctuations in COVID-19 prevalence across all five countries, with high ADC and UV linked to low prevalence and vice versa.

Understanding the seasonality of the virus will be critical for future efforts to combat its spread, as experts have cautioned that people may need annual booster shots to protect against the virus and its emerging variants. (GeoHealth, https://doi.org/10.1029/2021GH000413, 2021)

—Kate Wheeling, Science Writer

This is an authorized translation of an Eos article. 本文是Eos文章的授权翻译。

俯冲带是板块构造的基本组成部分，在这里一个板块滑入另一个板块之下向地幔靠近。但这个过程的最初阶段——俯冲起始——对于科学家来说仍然有些神秘，因为大多数俯冲作用的地质记录都被正在发挥作用的极端力量掩盖和覆盖了。要想了解俯冲带如何开始形成，唯一的办法就是看看现今地球上的年轻例子。

这张示意图显示了大约1600万年前Puysegur边缘的构造背景。走滑运动使得来自澳大利亚板块的海洋地壳与来自太平洋板块的变薄的大陆地壳并置。在新西兰南岛附近的板块碰撞迫使澳大利亚大洋板块处于太平洋大陆板块之下，在Puysegur海沟产生了俯冲作用。图片来源： Brandon Shuck

在一项新的研究中，Shuck等人使用地震成像技术的组合构建了新西兰西南海岸Puysegur海沟的详细图像。在这个地方，东边的太平洋板块覆盖了西边的澳大利亚板块。Puysegur边缘的构造活动非常活跃，在过去的4500万年里已经发生了几次更替，从裂谷作用到走滑作用再到早期俯冲作用。该地区边缘保存完好的地质历史使其成为研究俯冲作用如何开始的理想地点。该研究团队进行的地震结构分析表明，俯冲带的形成始于地壳中现有的薄弱部分，并依赖于岩石圈密度的差异。

该俯冲带形成的必要条件大约始于4500万年前，当时澳大利亚板块和太平洋板块开始相互分离。在这一时期，伸展力导致海底扩张，并在南部形成新的高密度海洋岩石圈。而在北部，西兰大陆厚厚的、浮力强的大陆地壳只是被拉伸和略微变薄。在接下来的几百万年里，板块旋转，走滑变形使得高密度的海洋岩石圈自南向北移动，在那里撞上低密度的大陆岩石圈，促使了俯冲作用的开始。

研究人员认为，岩石圈密度的差异，加上以往构造期的走滑边界上存在的薄弱区域，促进了俯冲作用的发生。研究团队得出结论称，走滑作用可能是俯冲带形成的关键驱动因素，因为走滑作用能够有效地将非均匀岩石圈的不同部分聚集到板块边界上。(Tectonics, https://doi.org/10.1029/2020TC006436, 2021)

—科学作家David Shultz

This translation was made by Wiley. 本文翻译由Wiley提供。

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The shape and position of Earth are constantly changing. Geodesy is the branch of geophysics that studies these properties—our planet’s size, orientation, and gravity—which are crucial for answering important Earth and space sciences questions: How will sea levels rise in the coming decade? What are the precise orbits of satellites? What are the patterns in a volcano’s magma migration? How is elevation determined? Pursuing these questions requires maintenance and improvement of the geodetic infrastructure—the instruments, software, and expertise that provide precise measurements.

“It’s like a freeway system…it’s really fundamental.”“It’s like a freeway system or something—it’s really fundamental,” said David Sandwell, a marine geophysicist at Scripps Institution of Oceanography in San Diego. Sandwell chaired the committee behind a report addressing geodetic infrastructure, released in 2020 by the National Academies of Sciences, Engineering, and Medicine (NASEM). The report provides recommendations to ensure that researchers will be able to continue using geodetic approaches to tackle diverse Earth science questions, from sea level change to weather models to geological hazards. These research areas were highlighted in an earlier decadal survey funded by NASA, NOAA, and the U.S. Geological Survey (USGS).

Terrestrial Reference Frame

The primary need for the geodetic infrastructure is to define a terrestrial reference frame, a set of 3D coordinates organized around Earth’s center of mass. The more correct this reference frame is, the more accurate and stable satellite orbits are. This accuracy provides scientists with better data sets.

The terrestrial reference frame relies on four techniques. Very long baseline interferometry (VLBI) measures radio signals from distant quasars to measure Earth’s orientation in space and scale. Satellite laser ranging (SLR) relies on short pulses sent to satellites; the return times can be used to trace satellite orbits and calculate Earth’s center of mass and scale. The Global Navigation Satellite System (GNSS), which includes GPS, and the Doppler Orbitography by Radiopositioning Integrated on Satellite (DORIS) system provide additional global measurements. Raw data from these systems are combined and analyzed to produce the International Terrestrial Reference Frame (ITRF). The ITRF is a group effort, the report describes: “All parties involved work in an open international collaborative environment to provide the most accurate reference frame for science and applications.”

The report recommends the deployment of more VLBI and SLR stations and the establishment of more receivers that can interface multiple GNSS systems. Such improvements would provide more coverage and improve the accuracy and stability of the terrestrial reference frame.

Geodetic infrastructure like this is critical for determining the precise orbits of satellites described in the decadal survey, said Shin-Chan Han, a geodesist at the University of Newcastle in Australia. “Such precise orbit is mandatory to achieve all the identified important science problems,” he said. Han reviewed the NASEM report but wasn’t directly involved in its preparation.

Monitoring Land Subsidence

Geodetic infrastructure is also crucial in hydrology. GPS and interferometric synthetic aperture radar (InSAR), a satellite-based technique for measuring land deformation, have transformed how scientists study land elevation changes due to groundwater removal and recharge.

“I just can’t imagine waking up one morning and saying, oh, the GPS constellation isn’t working anymore,” said Michelle Sneed, a USGS hydrologist who was part of the committee that authored the NASEM report. One area that Sneed monitors is the San Joaquin Valley, an agriculture region in central California that has changed dramatically because of groundwater pumping for irrigation. From 1925 to 1977, the land surface in this area subsided about 9 meters because of compaction. Sneed and colleagues used continuous GPS and InSAR to assess land subsidence in the west central San Joaquin Valley and explored potential risks to the California Aqueduct.

The techniques also indicated that Coachella Valley has stabilized, likely because of projects that increased recharge and reduced reliance on groundwater. “The integration of these different geodetic techniques…adds different pieces to the stories,” Sneed said. “InSAR is this great spatial tool. But if you want a daily value of the land surface at any one point, then you need continuous GPS.”

Additional Infrastructure

Maintaining geodetic infrastructure faces significant challenges, the NASEM report notes. Making the software for processing raw geodetic data widely available is one such challenge, and compensating for an aging workforce is another. “I’m concerned about a shortage [in the] geodesy workforce,” Han said.

Maintenance and enhancement of geodetic infrastructure will be crucial for addressing the Earth science questions outlined in the decadal survey. As described in the report, “the international geodetic infrastructure is the largely invisible foundation of Earth system science and applications.”

—Jack Lee (@jackjlee), Science Writer

Imagine young students bundling up in winter clothing, strapping on snowshoes, and trekking to a site with thick snowpack where a volunteer instructor cuts out a refrigerator-sized block of snow. If the block stays coherent, the instructor asks the kids to jump on it until it fails, making them tumble into a flurry of snow. Together, the teacher and students measure the density and dimensions of the snow block to calculate its weight, which can be nearly as heavy as a car. By experiencing this mini avalanche, the students might begin to fathom what a real one might feel like.

This snow stability test is among the many experiments that SnowSchool, a nationwide program run by the nonprofit Winter Wildlands Alliance, uses to seed K–12 students’ interest in science and outdoor education. The curriculum integrates the local ecology for each of 81 active sites across the United States and uses snow as the medium to engage students, said Kerry McClay, SnowSchool’s national director.

“This fusion of fun, science, and snow ignites that sense of wonder and lets kids explore, with their curiosity in the driver’s seat.”More than half of SnowSchool’s students come from underserved populations, including numerous Title I schools—schools with at least 40% enrollment from low-income families. “Every community where a SnowSchool site is located is different,” McClay said. Some sites serve tribal communities on reservations. Another site ferries students from Oakland, Calif., to the Sierra Nevada, a rather lengthy trip that takes at least 3 hours.

Because SnowSchool is heavily subsidized by grants and fueled by donations, participation is often free, said McClay. The program provides gear, including snowshoes and winter clothing for students; resources and training for volunteers; and curricula for teachers. Interested schools simply need to apply and provide buses to transport students to their SnowSchool sites.

The more than 35,000 students in the program might explore how snowpack forms and melts, build igloos, or track wildlife, said McClay. A favorite activity of the students—many of whom have never seen the deep snowpack of the mountains—is sliding on their bellies through drifts. This fusion of fun science and snow “ignite[s] that sense of wonder and lets kids explore…with their curiosity in the driver’s seat,” said McClay.

Snow Stratigraphy

An especially illuminating experiment, he said, begins with the classic kid activity of digging a hole in snow. The instructors and students begin by digging a trench through the snowpack, down to where the snow meets the ground, sometimes 6 feet deep (nearly 2 meters), said HP Marshall, a snow scientist and professor at Boise State University who helps design materials and train volunteers for SnowSchool. “It’s like looking at tree rings,” he explained, except instead of years, each layer in the snow pit signifies a discrete weather event. The students learn to identify the previous night’s soft snow, last week’s snowstorm, and last month’s ice crust left by a rainy day.

Researcher Kelsey Dean examines snow crystals with a macroscope while working in a snow pit in Fraser Experimental Forest, Colo. Credit: Kelly Elder

Then the students get macroscopes—like microscopes, but with a large viewing area—and they look at the changing snow crystals. “It’s like a whole other universe,” said Marshall.

Digging the trench serves as a window into ice core climate research, said Marshall, and lets instructors start discussions about how scientists study climate change. In the world of climate research, scientists drill many kilometers down, extracting deep ice cores that help researchers see what the climate was like and how it changed many hundreds or even thousands of years back.

Another SnowSchool project is a crowdsourced science initiative conducted in collaboration with NASA’s SnowEx program. In this project, students from SnowSchool collect snow data on the ground that will ultimately help calibrate satellite data.

No Snow, No SnowSchool?

Near Boise, Idaho, the flagship SnowSchool site at the nonprofit Bogus Basin recreation area and ski resort beckons. At this location, students often come from predominantly Latinx agricultural communities and typically have not spent much time in a snowy environment, said Marshall. By focusing the curriculum on water availability, he viscerally links water to everyday life for students steeped in cultivating crops. Students learn the role snow plays in the water cycle, which gives them tools to talk about snow and water with their families. “Snow water resources,” Marshall said, “are so impacted by climate change.”

With the uptick in extreme events, the snowpack atop mountains is more variable and melts faster, said McClay. “Eighty percent of our water is coming from melted snow,” he said. Students see trends with snow-sourced data and begin to consider the repercussions for water supply, irrigation, agriculture, or fires. “The list goes on.”

Unfortunately, Marshall admitted, “people that live too far from the mountains can’t really engage with this program.” For these communities, “the SnowSchool organization put a lot of effort into videos and online material,” in part as a response to travel restrictions imposed by the COVID-19 pandemic.

In his visits to classrooms, Marshall has found that even a cooler filled with snow excites kids. “They want to have snowball fights, [or] see how long they can stick their hands in [it],” he said. McClay is hopeful that as SnowSchool expands, students everywhere can engage in the program—as long as there’s access to snow.

“SnowSchool,” said McClay, “is not as effective without snow.”

—Alka Tripathy-Lang (@DrAlkaTrip), Science Writer

For the past 1.2 million years (during the Late Pleistocene period), ice ages have occurred in cycles lasting roughly 120,000 years. Before this period (during the Early Pleistocene period), these cycles lasted only about 41,000 years. The cause of the change in ice age duration, known as the Mid-Pleistocene Transition, is unknown. A recent article published in Reviews of Geophysics examines possible explanations for the Mid-Pleistocene Transition. We asked the authors about the Mid-Pleistocene Transition and possible explanations for this period.

 What makes the Mid-Pleistocene Transition (MPT) particularly fascinating to study?

A change in any one of these physical systems will affect all others, which is a profound realization when thinking about current climate change.Studying glacial cycles, and particularly the MPT, requires you to think about all the possible ways the different components of the Earth’s climate can interact. A growing ice sheet will reflect more sunlight back into space, cooling down the climate. A colder climate means colder oceans, which absorb more CO2 from the atmosphere. Ice sheets cause erosion, which creates airborne dust and influences the flow of the ice. Some of this dust can rain down on top of the ice sheet, creating dirty snow that absorbs more sunlight, or it can be blown into the oceans, where it can fertilize algae growth and increase CO2 drawdown.

These interactions between the ice sheets, the global climate, the oceans, the carbon cycle, and even the solid Earth, are fascinating. A change in any one of these physical systems will affect all others, which is a profound realization when thinking about current climate change.

How did the cycle of ice ages differ between the Early Pleistocene and Late Pleistocene?

The Pleistocene (the last 2.8 million years of Earth’s geological history) is distinguished by the presence of glacial cycles (“ice ages”): periodic, dramatic climate changes that caused vast ice sheets to appear and disappear over large parts of North America and Europe.

During the Early Pleistocene, these glacial cycles occurred roughly every 41,000 years. This makes sense, because these cycles are caused by small changes in Earth’s orbit, which also occur every 41,000 years. The MPT marks the transition to the Late Pleistocene, where the glacial cycles took much longer (about 100,000 on average). Understanding how a 41,000-year change in Earth’s orbit can lead to a 100,000-year climate response is one question we tried to answer; the other is why it only did so during the Late, and not during the Early Pleistocene.

What are the various explanations for the MPT?

There are two groups of theories. The first is the “global cooling plus non-linear feedbacks” group, which says that ice sheets respond non-linearly to changes in climate. The larger ice sheets of the cold Late Pleistocene created their own cold climate environment, making them more resistant to climate warming. This allowed them to survive some of the warm interglacial periods, growing even larger during the next cold phase. The reason why this didn’t happen during the Early Pleistocene is because the world was warmer then, so that the ice sheets never reached the required size to survive a warm period.

The second group are the “erosion” theories. Ice sheets slowly grind away the land underneath them, scraping away the soil until nothing remains but bare rock. Ice slides more easily over soil than over rock, so that soil-based ice sheets tend to “flatten out” when compared to rock-based ice sheets. Also, as mentioned before, soil-based ice sheets create airborne dust which can lead to dirty snow (which absorbs more sunlight), and oceanic algae fertilization (which draws CO2 out of the atmosphere). In this theory, the MPT marks the moment when the last soil was eroded away in northern North America and Europe, and these different processed ceased.

Does the available evidence appear to more strongly support one explanation over the others?

It’s likely that all of these mechanisms at least played a role, but determining how much of a role is tricky due to the lack of detailed and conclusive data.It’s likely that all of these mechanisms at least played a role, but determining how much of a role is tricky due to the lack of detailed and conclusive data. Since the erosive action of ice sheets tends to remove evidence of earlier ice sheets, it’s difficult to figure out what the older ones looked like. Data on the state of the Earth’s climate or the composition of the atmosphere is also limited. Our most valuable source of information is the ice core record, but that only extends to 800,000 years ago – not long enough to cover the MPT.

What different research approaches could be used to resolve the question of the MPT’s cause?

Currently, a team of scientists is drilling into the Antarctic ice sheet to produce a core that, if all goes well, should go back well over a million years. We’re very excited to see what comes out of that project, particularly in terms of CO2 concentration.

At the same time, the ice-sheet modelling community is working on improving the physics of sliding and oceanic melting. Although most of the focus is on near-future ice-sheet retreat, the outcomes are also important for the MPT, since these processes were important in those times as well. And the more accurate our models can reproduce ice-sheet evolution in the past, the better they are at predicting the future.

—Constantijn J. Berends (c.j.berends@uu.nl; 0000-0002-2961-0350), Roderik S. W. van de Wal ( 0000-0003-2543-3892), and Lucas J. Lourens ( 0000-0002-3815-7770), Utrecht University, The Netherlands

Picture a young weather enthusiast walking across the stage to receive their meteorology degree. They feel pride in this culmination of their years of hard work. They also recall how that hard work always seemed to appear to others. Friends and family called them “proper” during visits home from school, creating a distance that lingered. Their colleagues and peers frequently offered their own unsolicited impressions:

“You are so articulate!”

“You need to be more professional…”

“You cannot show up like that.”

“You are not like those other Black people.”

Or in another common story, an early-career scientist reflects on the cost of their profession. They earned a degree, but they had to permanently relocate for school and the only career opportunities available to them. Visiting home and family is emotionally exhausting because it is a constant reminder of what was given up to focus on those limited opportunities. They raise a new family away from their abuelitos, missing out on making tamales with their tías or dancing to cumbia at their cousin’s quinciañera. As they slowly lose their grasp of their native language, they fear their children will also lose that deep connection with their Latino heritage. Sí se puede, but is it worth it?

Pursuing careers in this extremely white dominated field requires us, more often than not, to assimilate either internally or externally to the culture, to code-switch.On the surface these stories may sound and feel similar to most of us who pursued higher education or careers in academia. Who hasn’t felt inadequate, had trouble finding their place in a new environment, or ultimately felt as though they did not belong? The difference we authors want to express is that although the situations and experiences may sound similar, the consequences of these experiences for Black, Indigenous, and People of Color (BIPOC) professionals in geosciences are very different. Additional stress, emotional labor, and baggage cause long-lasting trauma for BIPOC professionals. We feel this trauma. It is visceral. And it bubbles to the surface even as we write this article. Pursuing careers in this extremely white dominated field requires us, more often than not, to assimilate either internally or externally to the culture, to code-switch. In the process, we lose our authenticity.

This assimilation, however, is counterproductive to the creation of a richly diverse and inclusive scientific community that is prepared to address the questions of our modern world, and more importantly, it is deeply disrespectful and harmful to the BIPOC scientists whom the community boasts about recruiting. We are asking our colleagues to form a better awareness of code-switching, why BIPOC scientists perform it, and how we can address the deficiencies in our community that require it.

Code-Switching and Identity Shifting

The term code-switching originates from linguistics, meaning “the practice of alternating between two or more languages or varieties of language in conversation.” The concept of code-switching has evolved to describe the changes in speech, appearance, and behaviors by an individual to adjust to the norms of the dominant culture in a given space. We have all code-switched at some point, but for BIPOC it can be a mandatory coping strategy to protect ourselves from judgement, discrimination, hypervisibility, and tokenism [Dickens et al., 2019].

Initiatives to increase the number of BIPOC in science, technology, engineering, and mathematics (STEM) have been working, if slowly, and now these folks are attempting to exist and thrive within the white-centric environments of academic institutions, scientific laboratories, and private industries. Wanting to fit in and be comfortable, BIPOC learn to assimilate cultural norms by “deemphasizing a negatively-valued identity and replacing it with a positively-regarded identity,” also known in psychology literature as identity shifting [Dickens and Chavez, 2018, p. 761].

These shifts can be intentional or unintentional as we evaluate the level of risk associated with the possibility of making white people uncomfortable. For example, we’re asked to participate in a diversity panel, again. Should we express to our white colleagues that we feel used as a prop or just stay quiet and humbly accept the invitation? Will we be seen as problematic or ungrateful? Even through editing of this writing, we authors felt conflicted over appeasing the editors and staying true to our story, appreciating the critique yet not wanting to lose our voice. The risk can range from feeling embarrassed or worried you made a bad impression to being harassed and fearing for your life and safety.

Some BIPOC grow up in segregated communities, only learning to identity shift after they leave home and experience predominantly white spaces such as a university, a scientific conference, or an internship at a national lab. Even within historically Black colleges and universities, academic spaces where Black culture is championed, a Black STEM student may still feel like an outcast if they feel the need to hide their perceived nerdy self to belong, as nerdiness is stereotypically associated with whiteness.

The Cost and Consequences of Code-Switching

Code-switching is exhausting, taking up mental capital that should be devoted to our research.The inner turmoil created by shifting identities can often manifest as physical and mental ailments [Dickens and Chavez, 2018]. Code-switching is exhausting, taking up mental capital that should be devoted to our research. We just want to be scientists, without having to separate our culture from our profession, and to be able to present ourselves authentically without needing to constantly account for potentially negative reactions from others.

Instead, we live with that constant nagging in the back of our minds, reminding us that we have to say the right things, react the right way, and behave in a manner that draws attention away from the obvious difference we present. When we are not able to blend in, we falsely believe that we don’t belong and fear being called out as incompetent. This phenomenon is often called imposter syndrome. This explanation, however, identifies the person feeling it as the responsible party—the one who needs to change. Imposter syndrome is, instead, a scapegoat that takes focus away from addressing the culture of bias and systemic racism that exists for women and BIPOC scientists [Tulshyan and Burey, 2021].

For BIPOC in geoscience, those feelings are compounded because of the more extreme cultural isolation that exists in the field compared with other STEM fields. In the geosciences, we are often not just one in a historically excluded group but the only one in our field or lab.

What does a reliance on code-switching force us to give up? We become accustomed to adjusting to norms within our professional workplace (e.g., at the office, at conferences, during a field campaign or expedition) or, often, in the neighborhoods we’re required to move to. Those adjusting behaviors start to become unconscious, even dominant. We start to lose our native and colloquial language and cultural norms. Returning home can make us feel like outsiders looking in. We lose the thread that connects us with the people we grew up with and the people who raised us. Ultimately, we are left to wonder where we fit in.

We are never white enough in our professional environments but become too white in our home communities. Some of us self-exclude and choose not to be seen altogether, not wanting to lose ourselves or be the representative of an entire race of people. But by choosing to stay in the shadows, we also lose the opportunities and recognition that make any profession worth pursuing.

Professionalism Versus Assimilation

We are taught to be professional, but let’s consider the origins of present-day professional standards. In the broadest sense, the concept of professionalism encompasses the conduct by which one is expected to present oneself in formal settings, often customized to one’s discipline.

We strongly believe we should elevate and celebrate the people within our scientific communities, not ask them to assimilate.For geoscientists, these settings include job interviews, research seminars, conferences, classes, labs, and field campaigns. The standards are taught by mentors and in professional development seminars that focus on how to modify people’s behavior rather than how to evaluate, modernize, or fix the many problems in the culture. We persist in perpetuating professional standards that were established by white men many decades ago when women and BIPOC were not represented. Ethnically and culturally traditional attire, hairstyles, and vernacular were inconceivable when present-day professionalism was defined. Some scholars contend that this bias in professional standards is a form of white supremacy.

BIPOC resort to code-switching to boost their perceived professionalism—we assimilate [McCluney et al., 2019]. Code-switching, then, becomes a barrier to true inclusivity [Goldstein Hode, 2017], which should be the ultimate goal of modern professional behavior based in mutual respect and ethical integrity. Inspired by the perspectives of Halsey et al. [2020], we strongly believe we should elevate and celebrate the people within our scientific communities, not ask them to assimilate.

A Path Forward Isn’t Easy

The need for code-switching will persist until we can eradicate the systemic, institutional, and personal racism against which we need a shield. The onus should be on the larger community, not on BIPOC alone, to develop strategies that lead us to modern-day professionalism that is inclusive and respectful of everyone.

How can we collectively create an inclusive community and environment where we can each be our authentic selves? It’s not easy, and we don’t have all the answers. It requires us all to challenge professional standards.

Professionalism should require mutual respect, not assimilation to a single specific set of behaviors. Everyone, but especially those in leadership or supervisory positions, should seek out and recommend professional development opportunities on cultural competencies. Look around your workplace and take steps to evaluate and assess the culture and climate, then use these data to modernize your policies and practices to focus on equitable inclusion. Understand and listen to the variety of experiences of the people around you, in particular those of your BIPOC colleagues. Accept BIPOC colleagues for who they are. By doing so, you’ll show everyone around you how to change the culture rather than changing the people. By working together, we will become better together.

The more we assimilate, the less diverse our science and our ideas become.Ultimately, the need for code-switching negatively affects the individual BIPOC professional as well as the entire science community. As challenging as it can be, we are passionate about the science we pursue and desire to contribute to it. But the more we assimilate, the less diverse our science and our ideas become. This lack of diversity makes code-switching and the persistence of the institutions that require it a detriment to the advancement of our knowledge of our rapidly changing world.

To our BIPOC friends, peers, and colleagues: We carry hope in each other, knowing that we can look across the conference table, the poster session, or the Zoom room and be able to lock eyes and feel comfort and community. We want future generations to be empowered to show up as their authentic selves and focus their time and effort on great science, without interference and the additional labor of code-switching.

Acknowledgements

The authors would like to thank Deanna Hence, Rebecca Haacker, Rosimar Ríos-Berríos, Talea Mayo, and Valerie Sloan for their encouragement, support, and helpful contributions to this article.

This is an authorized translation of an Eos article. Esta es una traducción al español autorizada de un artículo de Eos.

Dune de Frank Herbert cuenta la historia de Paul Atreides, un hijo de una familia noble enviada al hostil planeta desértico Arrakis para supervisar el comercio de una misteriosa droga llamada melange (apodada “especia”), que otorga a quien la consume habilidades sobrenaturales y longevidad. Sobreviene la traición, el caos y las luchas políticas internas.

Imagina que estás en Arrakis, rodeado por un océano de arena. El aire es irrespirable, el cielo brumoso, el paisaje misterioso. Arena por millas, hasta donde alcanza la vista. Sabes que a varios cientos de kilómetros de distancia hay una vasta red de cañones que, desde arriba, parecen haber sido tallados por enormes gusanos.

Antes de emocionarse demasiado, es importante saber que este no es el famoso planeta desértico que aparece en las novelas de Dune.

No, este Arrakis está más cerca de nuestro propio mundo.

Este Arrakis está a tan solo mil millones de kilómetros de la Tierra, en un mundo que orbita a Saturno.

Incluso hemos aterrizado una nave espacial cerca de allí.

Si aún no lo has adivinado, este Arrakis, oficialmente llamado Arrakis Planitia, pertenece a la segunda luna más grande de nuestro sistema solar, Titán. Arrakis es una vasta llanura de arena indiferenciada, pero no arena como la conocemos. La arena de Titán está hecha de grandes moléculas orgánicas, lo que la haría más suave y pegajosa, dijo Mike Malaska, científico planetario del Laboratorio de Propulsión a Chorro (JPL, por sus siglas en inglés) de la NASA en Pasadena, Calif.

Todas los rasgos de Titán (aquí fotografiadas en ultravioleta e infrarrojo por el orbitador Cassini) llevan el nombre de lugares en las novelas de Dune de Frank Herbert. Créditos: NASA/JPL/SSI

A Malaska le gusta imaginar que la arena de hidrocarburos de Titán, que en realidad se conoce como tholin, o suciedad orgánica compleja, podría duplicarse como la especia infame en el centro del extenso arco narrativo de Dune.

En los libros de Dune, la especia huele a canela, mientras que el tholin en Titán “probablemente huele a almendras amargas…y a muerte”, dijo Malaska.

Arrakis no es el único nombre de las novelas de Dune que adorna las características geológicas de Titán. Todas las llanuras y laberintos (rasgos en forma de cañón tallados en la superficie) indiferenciados de Titán que tienen nombre llevan el nombre de planetas de la serie Dune. Está Buzzell Planitia, que lleva el nombre del “planeta del castigo” utilizado por una antigua orden de mujeres con habilidades sobrenaturales. Está Caladan Planitia, que lleva el nombre del planeta natal del héroe principal de Dune, Paul Atreides. Está Salusa Labyrinthus, que lleva el nombre de un planeta prisión. Y más.

“Estoy asombrado [de] cuánto se parece Titán a la descripción de Arrakis”, dijo Malaska. Además de las vastas llanuras de arenas de hidrocarburos que se extienden a lo largo de la superficie de Titán, el complejo clima de tormentas y lluvia de metano de la luna se siente como de Dune. “Titán es Dune”.

Y, por supuesto, están las dunas. Los campos de dunas de Titán rodean el ecuador de la luna de 16.000 kilómetros de largo. La luna tiene más dunas que la Tierra tiene desiertos.

Rosaly Lopes, otra científica planetaria del JPL, fue una de las primeras personas en ver las dunas de Titán. Ella y otros miembros del equipo Cassini estaban analizando imágenes de uno de los primeros sobrevuelos de Titán de la nave espacial, allá por 2005, y vieron extraños rasgos curvados en la superficie.

“Cuando vimos las dunas por primera vez, no sabíamos que eran dunas”, dijo Lopes. No fue hasta un sobrevuelo posterior de Cassini que confirmaron que Titán tenía dunas en todo alrededor de su ecuador.

Aunque Herbert se inspiró originalmente en las dunas de arena de la costa de Oregón, también podría haber estado imaginando Marte.De hecho, Lopes fue la primera en sugerir nombrar las llanuras y laberintos de Titán en honor a los planetas del universo Dune en 2009, aunque no recuerda exactamente cómo surgió la idea. Ella dijo que tenía sentido, considerando las dunas de Titán.

Los científicos planetarios no nombran los rasgos hasta que existe una necesidad científica para ellos, dijo Lopes. Primero se debe elegir un tema, ya sean aves míticas para áreas interesantes en el asteroide Bennu, o dioses del fuego para volcanes en la luna de Júpiter Io (Lopes nombró a dos de ellos, Tupan y Monan, en honor a deidades de culturas indígenas en su país de origen de Brasil). Hay otros rasgos literarios en el sistema solar, como los cráteres de Mercurio que llevan el nombre de artistas y escritores famosos.

Aunque Herbert se inspiró originalmente en las dunas de arena de la costa de Oregón, Malaska imagina que Herbert, y sus muchos lectores, también podrían haber estado imaginando Marte, el único planeta desértico que conocíamos en la época en que se publicó Dune, en 1965. De hecho, ese mismo año, la NASA hizo su primer sobrevuelo exitoso de Marte con su nave espacial Mariner 4 y la humanidad pudo ver de cerca el Planeta Rojo.

Pero los campos de dunas de Titán son únicos en el sistema solar, y es lógico que esta misteriosa luna lleve el nombre de un revolucionario universo de ciencia ficción.

—JoAnna Wendel (@JoAnnaScience), Escritora de ciencia

This translation by Daniela Navarro-Pérez (@DanJoNavarro) of @GeoLatinas and @Anthnyy was made possible by a partnership with Planeteando. Esta traducción fue posible gracias a una asociación con Planeteando.

Seasonal snowpack covers 46 million square kilometers annually—31% of Earth’s land area—but that number is shrinking. Snowpack is accumulating later, melting earlier, and retreating at an even faster rate than Arctic sea ice. This reduction in snowpack has implications for water locally and climate globally.

“The Earth gets rid of enormous amounts of heat by painting itself white in the winter, and that’s going away.” “Snow is an enormous regulator of heat on Earth because of its high reflectivity,” said Matthew Sturm, group leader of the Snow, Ice and Permafrost Group at the University of Alaska Fairbanks Geophysical Institute. “The Earth gets rid of enormous amounts of heat by painting itself white in the winter, and that’s going away.”

Just how substantial will changes brought by shrinking snowpack be? SnowEx, a multiyear NASA research program, hopes to find out. SnowEx has tested sensors in Western states since 2017; this winter the research continues in Alaska, a state with applicable infrastructure, experience, and plenty of snow.

Matthew Sturm, who specializes in tundra research, probes for snow depth using a GPS-enabled automatic depth probe. Credit: Matthew Sturm A Satellite for Snow

Every 10 years, an independent panel assesses NASA’s satellite fleet and recommends research areas that are currently unmet. The 2017 decadal survey suggested snow (and, specifically, snow water equivalent) as a possible mission focus for NASA’s Explorer program.

“[Snow water equivalent] is a critical component of hydrologic cycling and the Earth’s energy balance, but it’s really difficult to measure,” said Carrie Vuyovich, project scientist for SnowEx Alaska and a research physical scientist at NASA’s Goddard Space Flight Center. Field observations provide valuable data, but only in limited areas. “It means a huge amount of landscape is missing information,” she said. “Satellites are really the ideal observers to cover that amount of area.”

To prepare for a potential satellite mission, SnowEx scientists are developing and refining aircraft-mounted sensors adaptable across a range of conditions. There’s no guarantee of a satellite launch—“It’s a competitive process,” said Vuyovich—but the snow science community can prepare for the potential opportunity by testing sensors calibrated to the temporal and spatial intricacies of snow.

Tundra Crust and Taiga Woods

A snow-focused satellite should work in all regions, from deep mountain powder to dense tundra crust. Sensors must also react to complicated conditions: wet snow, deep snow, snow covered by trees. SnowEx’s mobility allows it to refine algorithms and accuracy in various conditions, and Alaska is essential for testing those abilities.

SnowEx tundra research will take place in the Toolik/Imnavait area of northern Alaska. The area is home to unique snow formations like sastrugi, snow ridges shaped by wind erosion. Credit: Matthew Sturm

“The large fraction of global snowpack is here at higher latitudes,” said Svetlana Stuefer, an associate professor of civil and environmental engineering at the University of Alaska Fairbanks. As deputy project scientist for SnowEx Alaska, Stuefer is helping coordinate the campaign and identifying locations that represent the world’s two largest snow biomes: Arctic tundra and boreal forest, also called taiga.

“Tundra and taiga take up a lot of room, but they pose two different problems,” said Sturm, a senior adviser to SnowEx Alaska.

Tundra snow is shallow, stratified, and often located on permafrost. Remote sensors must recognize and respond to those conditions. Taiga is more complex. “Sorting out what’s on the ground and what’s on the trees is very difficult,” said Sturm. Snow suspended from tree branches reflects light (which is good for climate control) but may sublimate into the atmosphere without contributing to groundwater. A snow-focused satellite would need sensors attuned to both climate and water issues.

“I’m excited to see where [SnowEx] goes. They have a lot of challenges ahead of them, but I think it can be an important tool,” said Daniel Fisher, a senior hydrologist with the U.S. Department of Agriculture’s Alaska Snow Survey not involved in the project. “I don’t think [remote sensing] will ever be a silver bullet, but I do think it will play an important role in understanding and measuring the snowpack across the state,” he said.

Fixing the Data Drought

SnowEx scientists plan to fly lidar and stereophotogrammetry sensors in Alaska this winter. Another aircraft will carry the Snow Water Equivalent Synthetic Aperture Radar and Radiometer (SWESARR), a specialty SnowEx instrument developed at Goddard to calculate snow water equivalent (SWE) using active and passive microwaves. Field staff will measure snow conditions on the ground to compare observations.

Charlie Parr, a research technician at the University of Alaska Fairbanks, measures the density of layers of snow. Credit: Matthew Sturm

Better snow data could benefit a range of interests, from road crews to flood forecasters to subsistence trappers. Increased SWE data would particularly help water managers; one in six people relies on seasonal snowpack for drinking water.

Then there are the recreationalists, like backcountry skiers who scan avalanche reports while brewing their morning coffee.

“Right now, operationally, we are extremely reliant on point-based observations,” said Andrew Schauer, a lead forecaster for the Chugach National Forest Avalanche Information Center not involved in SnowEx. Avalanche centers are challenged by a lack of data, he said, but aerial observations could fill that gap if updated quickly. “I’m excited to see what becomes of the SnowEx program,” he said.

By preparing sensors for all winter conditions, SnowEx scientists hope to be ready should NASA ask for a mission proposal. Research in Alaska is an important step to reaching that goal.

“[SnowEx Alaska] positions us to be competitive,” said Sturm. “I don’t think there’s any question that a satellite for snow would help humanity.”

—J. Besl (@J_Besl), Science Writer

It sounds like the beginning of a joke: What do you get when you put a team of geologists and nurses in the same room? But the answer is no laughing matter for Kentuckians.

Although smoking itself is a well-known health risk—and Kentucky dances around the top spot for cigarette smokers in the United States—smokers who live in a home with radon gas exposure can be 10 times more likely to be diagnosed with lung cancer.

But how prevalent is radon in Kentucky homes? The first step to answering the question is understanding the underlying geology—the source of radon gas. In an innovative collaboration, the Kentucky Geological Survey (KGS) and the Bridging Research Efforts and Advocacy Toward Healthy Environments (BREATHE) research group at the University of Kentucky have created an interactive radon hazard map available to the public.

The map is web based and searchable and merges detailed geologic mapping, radon home test kits, and color-coded hazard values to help residents understand their risk to the clear, odorless, tasteless gas. The researchers hope the map will be a useful tool for both residents and public health experts.

Partnership Forged in Bedrock

The collaboration began about 5 years ago. The University of Kentucky published a news article on researchers in the School of Nursing working on radon exposure. A geologist at KGS (which is also affiliated with the University of Kentucky) saw the story, and his interest was immediately piqued.

“Well, you know, radon comes from rocks, and rocks don’t start and stop at county boundaries.”In the original article, the radon map they were using showed risk at the county level. “Our geologist said, ‘Well, you know, radon comes from rocks, and rocks don’t start and stop at county boundaries,’” said Bill Haneberg, state geologist and director of KGS and lead author of a paper in GeoHealth that detailed the interactive radon mapping project.

Haneberg explained that Kentucky already has exceptionally detailed geologic maps. (“For so many years, the motivating factor for these great maps we have in Kentucky was coal,” he added.) “There was this phenomenal collaborative program between the U.S. Geological Survey (USGS) and the KGS,” he said. From 1960 to 1978, geoscientists mapped the entire bedrock geology of the state at a 1:24,000 scale, and the maps have all been digitized.

Geologists at KGS shared the maps with the BREATHE team. Ellen Hahn, director of BREATHE and a coauthor of the paper, was impressed with their detail and scalability.

“Our initial work was based on 1,000 indoor radon values in northern Kentucky,” Hahn said. The initial collaboration between KGS and BREATHE used this smaller data set and geologic mapping to look for any patterns. “We were looking for statistical correlations between the types of rock formation and indoor radon values that we had from that data set,” she noted. “We did indeed find interesting results from that.” For instance, homes built on limestone, dolostone, and some shales have higher indoor radon concentrations than homes on siltstone, sandstone, and surficial deposits.

Scaling Up the Project

From the northern Kentucky data set, the team then scaled up the project to the entire state. Hahn explained that two companies had been distributing and analyzing home radon kits in Kentucky for more than 20 years. She contacted those companies and asked for historical data, making sure that personal information was kept private—all addresses were changed to coordinates, for instance. In the end, the team ended up with more than 71,000 radon tests across the state.

“It turns out in Kentucky, the highest rate of potential is associated with Mississippian limestones.”The team grouped the indoor radon tests located in a specific geologic formation to look at the values of measured radon. Using the 75th percentile of radon measurements, rock units were color coded for hazard. When a person clicks on the map, a box pops up containing information about the rock unit, measured radon levels, and number of radon kits tested for that formation.

“I think that map is quite cool. It’s really nice to look at, and I think it’s pretty accessible,” said Douglas Brugge, an environmental health professor and chair of the Department of Public Health Sciences at the University of Connecticut. He was not involved in the study. “I think what most people are going to want to do is look for where they live, they’re going to want to zoom in on the map for their locality and get a sense of risk in their area.”

“The highest potential may surprise a lot of people,” said Haneberg, explaining that Kentucky has a lot of uranium-rich, Devonian age black shales. “But it turns out in Kentucky, the highest rate of potential is associated with Mississippian limestones. In fact, if you look at that map, there’s a big red belt going around the edge of the Illinois basin—those are Mississippian limestones, including the same limestones that host Mammoth Cave.” Haneberg noted that Devonian shales, with all of their uranium, came in third.

Credit: BREATHE

The EPA suggests an action level of 4.0 picocuries per liter, meaning that radon remediation should be done on homes that register those radon levels. On the map, the worst rock units for potential radon release were in excess of 16 picocuries per liter. Haneberg pointed out that the map is not a definitive measurement of the radon at an individual home, as that number is influenced by variations in geology and home construction.

Researchers said the map demonstrates the value of cross-disciplinary science. “I think environmental health hazards usually require people who are more on the physical sciences side and people who are on the public health, epidemiology side,” Brugge noted. Although geologists can focus in on the exposure to hazards, public health experts can link the exposure to health concerns and communicate risk to the public.

If the public can understand that the interactive map is a first step in understanding their exposure, then the efforts were successful, said Brugge. “If it motivates [residents] to do testing when they wouldn’t have done it, then that’s a good thing.”

Clear Communication and Collaboration

When communicating with the public, “I think it’s really important to distinguish between hazard and risk—it’s a subtle thing,” said Haneberg. Geologists look for hazards—what is the likelihood of this rock unit releasing radon? Risk, on the other hand, “involves the consequences” of those hazards, he explained.

That’s where the public health experts come in. “We bring the knowledge of disease and how it affects the body…how environmental exposures affect people’s risk of developing the disease,” Hahn noted. “We’re very familiar with prevention of disease through exposures.” Part of that prevention is behavior change—whether that is quitting smoking or adding radon remediation to a house.

The radon mapping and communication strategy saved approximately one premature lung cancer death and between $3.4 million and $8.5 million every year.Hahn and her team have been working on how best to get radon risk information to the public. She said they targeted high-risk counties for their initial outreach efforts. “We invited the Cooperative Extension agents, the health department, and other professionals…to a lunch-and-learn—bring your lunch, and we’ll teach you something about radon.” Each participant got infographics, free radon test kits, and some presentation materials to go out into their communities.

In a related study, Hahn and her colleagues ran an economic analysis to assess the value of using geologic data to help communicate radon risk potentials in Kentucky. They wanted to understand how geologic maps may have reduced lung cancer by fostering increased testing and increased mitigation. “We were able to find that we actually save lives and money,” Hahn said.

They found that the radon mapping and communication strategy saved approximately one premature lung cancer death and between $3.4 million and $8.5 million per year.

—Sarah Derouin (@Sarah_Derouin), Science Writer

Tropical cyclones’ strong winds are associated with ocean mixing and cold wakes. The cold water that is brought to the surface is rich in nutrients and can trigger photosynthesis that can be observed by satellites. Using satellite, Da et al. [2021] show that there are statistically significant trends in sea surface temperature cooling and primary production associated with tropical cyclones in the past 35 years. Furthermore, this tropical cyclone-induced increase in ocean primary production has partially mitigated the overall decline in primary production due to anthropogenic climate change.

Citation: Da, N. D., Foltz, G. R., & Balaguru, K. [2021]. Observed global increases in tropical cyclone-induced ocean cooling and primary production. Geophysical Research Letters, 48, e2021GL092574. https://doi.org/10.1029/2021GL092574

—Suzana Camargo, Editor, Geophysical Research Letters

Environmental conditions are intimately connected with human health outcomes. The COVID-19 pandemic has brought this into sharp focus. Some countries have plentiful data about their populations to inform resource distribution and policy making. Across the continent of Africa, however, comprehensive data on human health and environmental indicators is generally lacking for a variety of reasons, and results in poorer health outcomes and disjointed health interventions. A new special collection on the connections between COVID-19, environmental and human health across continental Africa seeks new data, research, and analysis to improve our understanding.

COVID-19 in Africa

As of mid-June 2021, there were approximately 5.1 million cases and 136,000 deaths associated with COVID-19 in Africa.COVID-19 cases first appeared on the continent of Africa during the spring of 2020, with most of the cases occurring north of the Sahara and in South Africa, plus in Ethiopia.

As of mid-June 2021, there were approximately 5.1 million cases and 136,000 deaths associated with COVID-19, with an estimated case-fatality ratio of 2.64 percent, which is only exceeded by the continent of South America.

South Africa has the highest number of cases and fatalities within the continent, with 1.7 million cases and more than 58,000 fatalities. The top five countries in terms of both the number of COVID-19 cases reported and fatalities reported are South Africa, Morocco, Tunisia, Egypt, Ethiopia. Other countries experiencing high cases and fatalities include Kenya, Nigeria, Algeria, Zambia, Sudan, Nigeria, Zimbabwe, Ghana, and Sudan.

A new wave of COVID-19 first observed in Southern Africa is now spreading across the continent has been linked to the Delta variant.

While the total numbers of COVID-19 cases and fatalities within Africa are considerably lower than in Europe, South America, North America, and Asia, there are reasons to be concerned, which is the motivation behind the call for new research papers for a special collection. These include:

Low and middle-income countries with growing populations that lack critical health facilities and equipment such as intensive care units, ventilators, and health care staff Limited vaccinations have been distributed across the continent and new variants continue to emerge The indirect causal connections between COVID-19 cases, spread, and seriousness have been found on other continents but not necessarily in Africa Limited in situ environmental data requiring the use of remotely sensed measurements. COVID-19 and air pollution

When the first cases began to show up in Africa during March 2020, many governments immediately began to partially or fully shut down activities tied to their economies. Consequently, while atmospheric pollution may have been reduced, many individuals with limited income may have become unemployed during the pandemic.

Air pollution is linked to COVID-19 with greater uncertainty in relation to precipitation and temperature.Our current level of knowledge suggests that air pollution is linked to COVID-19 with greater uncertainty in relation to precipitation and temperature.

In high particulate matter (PM) regions, COVID-19 seriousness and mortality are greater and there is the same dimension of environmental injustice that occurs in communities of color in the United States.

Results also show that COVID-19 transmission (R0) is higher in polluted areas where high long-term exposure to PM2.5 concentrations (composed of sulfate, ammonium, and black carbon) aerosols exceeds healthy standards.

In sub-Saharan Africa, poor air quality occurs at the household level from indoor cooking and heating, at local level from mega-city pollution, and on regional scales from large-scale desert dust and biomass burning during the dry seasons. Aerosols from desert dust are surmised to cause the drive of more premature death relative to biomass burning. During the dry season in West Africa, hazardous dust concentrations are associated with infant mortality, meningitis, and respiratory disease.

A shortage of data and a lack of research

Access to data remains the main obstacle to linking the environment and health in Africa.Access to health and environmental data remain the main obstacles to linking the environment and health in Africa.

During the COVID-19 pandemic, daily COVID-19 cases have been reported in many countries; however, spatially varying data at higher administrative levels within African countries may not be available.

In addition, some countries show a low number of tests per 100,000 persons leading to potential under-sampling, which could be related to available tests or the expense of testing to many low-income citizens.

The lack of research and operational grade in situ measurements of particulate matter and trace gas pollutants is abysmally low throughout the continent. Hence, the use of satellite observations and low-cost PM sensors has introduced novel ways to examine potential relationships between COVID-19 and the environment.

Special collection on the COVID-19 pandemic and environmental conditions in Africa

A new special collection in the journal GeoHealth entitled The COVID-19 pandemic and environmental conditions in Africa is a call to the community to help explore the various aspects of COVID-19 across continental Africa. It also provides a platform for input from scientists in Africa to contribute from their vantage point and across widely varying cultural and environmental settings.

Papers investigating the links between the environment, COVID-19, and health, in general, in Africa are welcomed. Submissions can include recent developments in modeling and forecasting, monitoring, data analysis, weather, climate and air quality variability, epidemiology and COVID-19 related impacts. Manuscripts should be submitted via the GEMS website for GeoHealth.

—Gregory S. Jenkins (gsj1@psu.edu,  0000-0002-0753-3964), The Pennsylvania State University, USA

New Telescope, New Worlds • Exoplanets in the Shadows   • Overture to Exoplanets   • The Forecast for Exoplanets Is Cloudy but Bright   • “Earth Cousins” Are New Targets for Planetary Materials Research   • Oddballs of the Exoplanet Realm   • Thousands of Stars View Earth as a Transiting Exoplanet   • Taking Stock of Cosmic Rays in the Solar System   • Gap in Exoplanet Size Shifts with Age   • Unveiling the Next Exoplanet Act  

If exoplanets were comic book characters, the first few ever confirmed would have been greeted with cries of “Zounds!” or “Zowie!” or even “Gadzooks!” Not only were these worlds unlike anything in our own solar system, but they were unlike anything scientists had even pondered. The first two were chunks of rock orbiting a pulsar, the remnant of an exploded star. The next one was a gas giant orbiting at just a fraction of the distance between the Sun and Mercury—so close that the planet’s outer atmosphere was heated to more than 1,500°C.

Astronomers have since added more than 4,000 confirmed exoplanets to the list (although the exact number depends on which list you check). Thousands more await verification.

Most of those worlds fit into a few major categories, some of which are alien to our own neighborhood. According to NASA’s exoplanet catalog, for example, there are more than 1,300 super-Earths, which are pretty much what they sound like—rocky planets a few times the size of Earth. Hundreds more are mini-Neptunes, which are bigger than super-Earths but smaller than Neptune, the Sun’s most distant major planet.

Some exoplanets don’t fit into the major categories, though. They are the oddballs. And like many oddballs, they can be more interesting than the conformists.

Second-Chance Planets

The first two confirmed exoplanets, discovered 3 decades ago, remain among the oddest and rarest of all: “zombie” planets that probably were born after their star died. Both orbit the pulsar PSR B1257+12. A pulsar is a rapidly spinning neutron star, the corpse of a massive star that exploded as a supernova. As the neutron star spins, it emits pulses of energy that form an extremely accurate clock—and provide clues for exoplanet hunters. The tug of an orbiting object alters the timing of the pulses a tiny bit, revealing a planet’s presence.

A brilliant aurora encircles the pole of one of the planets orbiting the pulsar PSR B1257+12 in this artist’s concept. The pulsar is at upper left, and its other two known planets are also in view. Credit: NASA/JPL-Caltech

Astronomers have discovered a handful of other pulsar planets (including a third for PSR B1257+12). Pulsar timing is so precise that it can reveal orbiting objects as small as asteroids, so the dearth of discoveries suggests that pulsar planets are rare.

It’s unlikely that planets could survive a supernova, so astronomers say these must be “second-chance” planets. They may have formed from debris from a pulsar’s destroyed companion star, such as a white dwarf. “If the star is in a binary with a low-mass star or a compact companion, the pulsar irradiates the companion and the companion evaporates,” said Rebecca Martin of the University of Nevada, Las Vegas. “This can lead to a runaway effect where the companion is dynamically disrupted and forms a disk around the neutron star. Planets may form from this disk.”

Hot Jupiters

The first exoplanet found orbiting a star in the prime of life, similar to the Sun, was just as shocking as the pulsar planets (and earned its discoverers a share of the 2019 Nobel Prize in Physics). Exoplanet 51 Pegasi b is roughly half the mass of Jupiter, the giant of our solar system, yet is close enough to its star that it orbits in just 4 days (compared to 12 years for Jupiter). That makes the planet extremely hot.

And 51 Pegasi b is not even the most extreme “hot Jupiter.” Of the few hundred known examples, some are many times Jupiter’s mass, one orbits its planet in just 18 hours, and some are being blasted by so much stellar radiation that their atmospheres are eroding into space. And although 51 Pegasi b was a true oddball when it was discovered, the roster of hot Jupiters has grown so large that these worlds form a category all their own. (A swelter of hot Jupiters, perhaps?)

The exoplanet WASP-79 b (left) is so close to its parent star that its upper atmosphere is as hot as molten glass. Credit: NASA/ESA/L. Hustak (STScI)

Such worlds are hard to explain. Close to a star, temperatures should be too high, and stellar winds should be too strong to allow a planetary core to sweep up enough hydrogen and helium to grow that big.

Most astronomers have hypothesized that hot Jupiters formed farther out in their solar systems and migrated inward. As often happens in comics, though, one character can disrupt the entire narrative. HIP 67522 b, which orbits once every 7 days, belongs to a star that’s only about 17 million years old—hundreds of millions of years younger than most hot-Jupiter hosts. It seems unlikely that the planet could have formed far from the star and then migrated so close in such a short period of time. So scientists may have to go back to the drawing board to explain at least some hot Jupiters.Kepler-51 hosts three planets, all of which are oddballs. They are a few times the mass of Earth but roughly as big as Jupiter. That makes them not much denser than cotton candy.

Cotton Candy Planets

The star Kepler-51 hosts three planets, all of which are oddballs. They are a few times the mass of Earth but roughly as big as Jupiter. That makes them not much denser than cotton candy. The Kepler-51 worlds are among a dozen or so confirmed “super-puff” planets.

Although some hot Jupiters have been puffed up by the heat from their nearby stars, super-puffs are much cooler, noted Jessica Libby-Roberts, a graduate student completing her Ph.D. at the University of Colorado Boulder. That temperature difference means the super-puffs must be inflated by some other mechanism.

Despite their great size, the planets of Kepler-51 are lightweight, so they are roughly as dense as cotton candy. Credit: NASA/ESA/L. Hustak/J. Olmsted (STScI)

Kepler-51 is a relatively young star, so its planets could be puffed up by the internal heat left over from their formation, Roberts said. Other super-puffs could have formed in “a really weird” region of the disk around the star where they could grab a lot of gas in a hurry. However, super-puffs might not be especially puffy at all. Instead, high haze layers or wide bands of rings might make them appear much larger than they really are.

Except for the planets of Kepler-51, most known super-puffs are the most distant members of multiplanet systems, Roberts said. If they really are puffy, then “either super-puffs need to form really far from their stars before migrating inwards, or they need to end up at a distance far enough from their stars to hold on to all that hydrogen-helium atmosphere, or a combination of both,” Roberts said. “There is still a lot to be done in this area.”

Wrong-Way Planets An artist’s concept depicts the retrograde orbit of planet WASP-8 b. Credit: ESO/L. Calçada

Some exoplanets fit into more than one “oddball” category. WASP-17 b, for example, is a super-puff. It’s half as massive as Jupiter but twice as wide, making it one of the largest and cotton-candiest planets yet discovered. It’s also a “wrong-way” exoplanet, orbiting in the opposite direction from its star’s rotation on its axis—one of only a handful of such planets yet seen.

Scientists suggest that WASP-17 b (and other retrograde planets) could have performed an about-face as the result of the gravitational influence of another planet­, through either a single especially close encounter or a more gradual long-range nudge.

Seeing Double

If planet hunters could visit any fictional world of their choosing, there might be a mad dash for Tatooine, the home world of Luke Skywalker. The first Star Wars movie featured an iconic view of Luke watching twin suns set over the desert. Today, any planet found to orbit both members of a binary star is instantly compared to that famous world.

Twin suns set on a Tatooine-like world, which orbits both members of a binary star, in this artist’s concept. Credit: NRAO/AUI/NSF, S. Dagnello

Although quite a few planets are known to orbit one member of a binary, circumbinary planets are about as common as stormtroopers who can shoot straight—astronomers have cataloged roughly a score of them. (One of them, Kepler-64 b, orbits one binary in a two-binary system, giving it four stars).

The known circumbinaries should remain in stable orbits for “at least 100 million years,” according to Jerome Orosz of San Diego State University. Some of the planets even lie within their host stars’ habitable zone, where conditions are most comfortable for life. “It’s obviously more complicated than the habitable zone for a single star,” Orosz said. “In particular, the habitable zone around a binary star moves as the two stars orbit….Keep in mind that the known circumbinary planets are gaseous, with diameters in the range of Neptune’s to Jupiter’s. Those planets probably won’t be habitable. There are no Earth-like planets known to be in circumbinary systems.”

The search for Tatooines in other systems continues, however—perhaps leading to more zowies or zounds in the years ahead.

—Damond Benningfield (damond5916@att.net), Science Writer

New Telescope, New Worlds • Exoplanets in the Shadows   • Overture to Exoplanets   • The Forecast for Exoplanets Is Cloudy but Bright   • “Earth Cousins” Are New Targets for Planetary Materials Research   • Oddballs of the Exoplanet Realm   • Thousands of Stars View Earth as a Transiting Exoplanet   • Taking Stock of Cosmic Rays in the Solar System   • Gap in Exoplanet Size Shifts with Age   • Unveiling the Next Exoplanet Act  

The whole field of exoplanet study is frustratingly tantalizing. We now know for sure there are alien worlds. We can see them! Kinda. We see their shadows; we can see their fuzzy outlines. We are so close to the tipping point of having enough knowledge to truly shake our understanding—in the best way, says this space geek—of Earth’s place in the universe.

The first light of the James Webb Space Telescope (JWST) may be what sends us over that exciting edge. In just a few months, the much-delayed launch will, knock on wood, proceed from French Guiana and take around a month to travel to its destination at the second Lagrange point (L2). “This is certainly an exciting time for exoplanet science, with current missions like Hubble and TESS [Transiting Exoplanet Survey Satellite] providing us with new discoveries and future missions like JWST, which promises to provide incredible new data that will answer some of our current questions and also create many new ones,” said Sarah Hörst of Johns Hopkins University, Eos‘s Science Adviser representing AGU’s Planetary Sciences section who consulted on this issue. “The field is moving very quickly right now.”

That’s why our August issue is all about exoplanets—what we know and what awaits us over the launch horizon. Who gets the first peek through JWST? In March, the proposals selected for the first observing cycle were announced. Meet the slate of scientists who will be pointing the telescope at other worlds, and read what they hope to learn in “Overture to Exoplanets.”

As with all new instruments, the data collected from JWST will be pieced together with observations from ongoing missions and other facilities around the world. “Over the last decade, we’ve gotten gorgeous images from the ALMA interferometer in Chile and have seen loads of fine-scale structure, tracing pebbles in planet-forming disks,” says astronomer Ilse Cleeves in our feature article. Hörst found this synergy with ALMA (Atacama Large Millimeter/submillimeter Array) especially intriguing: “Although I’ve thought a lot about what we’ll learn about individual planets, I hadn’t really thought much about what we’ll be able to learn about planet formation process by studying the disks themselves.”

“I’m excited for all the ‘well, that’s weird’ moments. Those are my favorite things in science because that’s when you know that new discoveries are going to be made.”In “The Forecast for Exoplanets Is Cloudy but Bright,” we learn the immense challenge posed by exoplanet atmospheres, when researchers are still struggling to understand the complex dynamics of clouds on our own planet. And in “Exoplanets in the Shadows,” we look at the rogues, the extremes, and a new field being coined as necroplanetology.

What awaits us when the first science results start coming in from JWST and all the coordinated missions next year? “I’m really excited for the unexpected,” says Hörst. “I’m excited for all the ‘well, that’s weird’ moments. Those are my favorite things in science because that’s when you know that new discoveries are going to be made. I’m also really happy for all of my colleagues who have worked so tirelessly for so many years to make JWST happen.”

We’re pretty happy, too, for the scientists long awaiting this day and for the rest of us who eagerly await a wide new window on our mysterious universe.

—Heather Goss (@heathermg), Editor in Chief, Eos

New Telescope, New Worlds • Exoplanets in the Shadows   • Overture to Exoplanets   • The Forecast for Exoplanets Is Cloudy but Bright   • “Earth Cousins” Are New Targets for Planetary Materials Research   • Oddballs of the Exoplanet Realm   • Thousands of Stars View Earth as a Transiting Exoplanet   • Taking Stock of Cosmic Rays in the Solar System   • Gap in Exoplanet Size Shifts with Age   • Unveiling the Next Exoplanet Act  

When astronomers gathered to reveal “new planets” at a press conference in January 1996, the world paid attention. Hundreds of journalists and fellow astronomers packed the meeting room, where presenters confirmed the identity of one exoplanet and reported the discovery of two others—the first planets known to orbit other Sun-like stars. The story made the front pages of major newspapers (“Life in Space? 2 New Planets Raise Thoughts,” wrote the New York Times), appeared in magazines (including a Time cover story), and aired on television news (including CNN) soon after.

A quarter of a century later, exoplanets still generate headlines—sometimes. With the number of confirmed planets well beyond 4,000 and more being added to the list almost weekly, however, a sort of exoplanet fatigue has set in. Only the most spectacular discoveries show up in our daily newsfeeds: potentially habitable planets, for example, or “extreme” worlds—those that are especially hot or young or blue or close to our solar system.

Yet some of the topics in the penumbra of exoplanet discussions are just as fascinating as those in the spotlight. They remain in the shadows in part because they involve objects that are rare or that are difficult to find and study with current technology. The recently named field of necroplanetology, for example, studies planets orbiting dead or dying stars, providing the only direct look at the innards of exoplanets. Gravitational microlensing allows astronomers to detect planets at greater distances than once thought possible. Several groups of researchers are developing instruments or small spacecraft to look at Earth as an exoplanet analogue, showing us what our planet would look like to an astronomer many light-years away. And the International Astronomical Union (IAU) has begun the long process of bestowing proper names on exoplanets—a process that simply may not have had enough time to filter into the consciousness of either professional astronomers or the public.

“We’ve discovered a lot of weird things,” said Laura Mayorga, an exoplanet researcher and postdoctoral fellow at the Johns Hopkins University Applied Physics Laboratory (APL). “When we first started studying exoplanets, we found that they got stranger and stranger. They put all of our understanding to the test.… Finding something new throws everything up in the air, and it has to resettle. That makes this a really exciting time.”

Death of a Planet

Although it sounds like something from a Syfy channel original movie, necroplanetology is the newest branch of exoplanet studies—a novelty that involves intrinsically rare targets. The term was coined by Girish Duvvuri, then a student working with Seth Redfield at Wesleyan University in Connecticut, in a 2020 paper. “We’re proud of the name,” said Redfield. “It’s a great way to describe the systems we’re studying. It has a small number of practitioners, but the larger community is just starting to look into this topic.”

The name was originally applied to the study of dead or dying planets around white dwarfs, which are the hot but dead cores of once normal stars. A typical white dwarf is at least 60% as massive as the Sun but only about as big as Earth. The size of white dwarfs makes it easier to detect the remains of pulverized planets as they transit, passing across the face of the star and causing its brightness to dip a tiny bit.

Starlight filtering through an exoplanet’s atmosphere during a transit would reveal its composition. (Astronomers have used the same technique to measure the atmospheres of planets transiting much larger main sequence stars, which are in the prime of life.) “What we started finding first was not whole planets but planetary debris,” Redfield said.

“All those clues made it clear that planets can exist around white dwarfs. They can be destroyed by white dwarfs as well.”In particular, using early observations from the K2 mission of the planet-hunting Kepler space telescope, they found WD 1145+017, a white dwarf about 570 light-years from Earth. The star’s light dipped several times in a pattern that repeated itself every few hours. The researchers concluded that they were seeing the debris of a planet that had been shredded by its star’s gravity—probably chunks or piles of rock surrounded by clouds of dust.

Observations with large ground-based telescopes revealed calcium, magnesium, iron, and other heavy elements in the white dwarf’s spectrum. Such heavy elements should quickly sink toward the core of a white dwarf, where they wouldn’t be detected. Their discovery suggested that the elements had been deposited quite recently, as rubble from a disrupted planet (or planets) spiraled onto the white dwarf’s surface.

“All those clues made it clear that planets can exist around white dwarfs,” said Redfield. “They can be destroyed by white dwarfs as well. The tidal forces are quite extreme, so they can break apart and grind up a planet.… As that material accretes onto the white dwarf, we’re actually learning about the innards of the planets.”

Such a planet may have been born far from its host star and migrated close enough to be destroyed. Astronomers know that such migrations are possible because they have discovered a few hundred “hot Jupiters”: worlds as massive as the largest planet in the solar system but so close to their stars that their upper atmospheres are heated to hundreds or thousands of degrees. Some of these planets are being eroded by stellar radiation and winds, perhaps marking the beginning of the end for worlds that could be subjects for future necroplanetologists.

Stars That Take a Dip

Despite expectations of a bounty of such white dwarf systems, Redfield said, they seem to be rare. (A recent study found evidence of one intact giant planet around one white dwarf.) Astronomers have found evidence of similar processes at work around main sequence stars, though.

The best-known example is KIC 8462852 (also known as Boyajian’s Star), about 1,470 light-years from Earth. Large, but irregular, dips were discovered in the brightness of the star, which is bigger, hotter, and brighter than the Sun. Possible explanations for the decrease included the panels of a “megastructure” built by an advanced civilization orbiting the star—an idea (since abandoned) that generated plenty of headlines.

Astronomers have discovered other examples of “dipper” stars as well. Edward Schmidt, a professor emeritus at the University of Nebraska–Lincoln, reported 15 slow dippers, whose light varies over long timescales, in study released in 2019. He said he plans to publish details on 17 more in an upcoming paper.

One or more moons could be snatched away as a planet falls into its star. The planet essentially hands its moons to the star—they’re orphaned exomoons.The stars all have similar masses and temperatures, which suggests that their dipping patterns share a common explanation, Schmidt said. “It could be caused by disintegrating planets—that looks promising so far.” He’s looking through published spectra of the stars to see whether their surfaces are polluted by the residue of planets, which could solidify the idea.

A couple of systems discovered by Kepler seem to add credence to the hypothesis. Kepler-1520b, for example, shows dips in luminosity of up to 1.3%. A ground-based study found that the dimming is caused in part by clouds of dust grains, providing “direct evidence in favor of this object being a low-mass disrupting planet,” according to 2015 paper. And K2-22, discovered in Kepler’s K2 mission, appears to be a disintegrating planet more massive than Jupiter but only 2.5 times the diameter of Earth.

Another study suggested a slightly altered explanation for Boyajian’s Star and other dippers: disintegrating exomoons. Researchers suggested that one or more moons could be snatched away as a planet falls into its star. “The planet essentially hands its moons to the star—they’re orphaned exomoons,” said Brian Metzger, one of the study’s authors and a physicist at Columbia University and senior research scientist at the Flatiron Institute.

Stellar radiation could be eroding the surviving moons, releasing solid grains of material that then form a clumpy disk around the star. So the young field of necroplanetology may need a new subfield: necrolunarology.

A Second Chance at Life

For some planets, though, the death of a star isn’t necessarily the end—it may be the beginning. The first confirmed exoplanets, discovered 3 decades ago, orbit a pulsar, a dead star whose composition is more exotic than a white dwarf. A pulsar is a rapidly spinning neutron star, the collapsed core of a massive star that exploded as a supernova. As the neutron star spins, it emits pulses of energy that form an extremely accurate clock. The gravitational tug of a companion alters the timing of the pulses a tiny bit, revealing the presence of an orbiting planet.

The first identified pulsar planets orbit PSR B1257+12. Astronomers have since discovered a handful of others, but most searches have come up empty. An examination of more than a decade of observations made by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), a project that is using pulsar timing to hunt for gravitational waves, for example, found no evidence of planets around a set of 45 fast rotating pulsars. The search could have revealed planets as light as the Moon in orbital periods of 1 week to almost 5 years, said Erica Behrens, a graduate student at the University of Virginia who conducted the study during an internship at the National Radio Astronomy Observatory.

This artist’s view shows a brilliant aurora on one of the planets of the pulsar PSR 1257+12, energized by the pulsar itself (top left). The system’s other two confirmed planets also are in view. Credit: NASA/JPL-Caltech

“Since we’ve seen so few, it seems like they’re pretty rare,” Behrens said, which may explain why they’ve received so little attention since the early discoveries. “They must have formed after the star has blown up. No planet that existed while the star was still living would be able to survive the supernova.”

Theoretical work hints that instead of supernova survivors, pulsar planets may be “zombies,” born from the debris of companion stars.

Metzger and Ben Margalit, also of Columbia, have suggested, for example, that the companion could be a white dwarf. The extreme gravity of the neutron star tears the white dwarf apart—perhaps in a matter of seconds—and the debris forms a disk around the pulsar. Some of the material in the disk falls onto the neutron star while the outer edge of the disk expands and cools. Solid material in those precincts may condense to form solid bodies, which then merge to make planets.

The scenario would explain the frequency of pulsar planets, which is roughly equal to the frequency of neutron star–white dwarf binaries, Metzger said. It would not, however, explain the birth of a pulsar planet that’s been discovered in a globular cluster, where the density of stars is extremely high. “You’d have to invoke more exotic interactions,” which scientists are still trying to model, he said.

A Rogues’ Gallery of Exoplanets

Although most exoplanets have been discovered through transits or radial velocity measurements, which detect a back-and-forth shift in the wavelengths of starlight caused by the pull of orbiting planets, a few stragglers have been found through other methods. Such methods are difficult to apply, or they’re looking for objects or phenomena that are rare, so they’ve yielded far fewer discoveries than the most favored methods.

Astrometry, for example, precisely measures a star’s position to detect tiny wobbles caused by the gravitational tug of orbiting planets. Such measurements are hard to make and have yielded only one or two discoveries. However, astronomers expect observations by the Gaia spacecraft, which is plotting the positions and motions of more than 1 billion stars, to yield thousands of new Jupiter-sized exoplanets in relatively wide orbits, which would create a whole new population for study.

The most successful of the lesser known techniques, however, has been gravitational microlensing which has revealed more than 100 planets. “It’s very complementary to other techniques,” said Matthew Penny, an astronomer at Louisiana State University. “You get an instant detection of some very distant planets that would take decades to find with other techniques.”

This diagram shows how microlensing reveals a planet orbiting a star. Credit: NASA, ESA, and K. Sahu (STScI)

Gravitational microlensing relies on general relativity, which posits that if a star or planet passes in front of a more distant star, the intervening object’s gravity bends and magnifies the background star’s light, creating a double image. If the alignment is perfect, it creates a bright circle of light known as an Einstein ring. (The same technique is used on a larger scale to study galaxies and quasars billions of light-years away.)

The length and magnification of a lensing event allow astronomers to calculate the intervening object’s mass and, in the case of a planet, its distance from its star. Astronomers have measured planet-star separations of up to more than 10 astronomical units (AU), which is far wider than with other techniques.

Microlensing can reveal planets that are thousands of light-years away (the current record holder, according to the NASA Exoplanet Archive, is at 36,500 light-years, many times farther than planets discovered with other techniques). Microlensing allows astronomers to study planets in regions of the Milky Way well beyond our own stellar neighborhood, including the central galactic bulge.

Perhaps most important, microlensing is the only technique that can reveal rogue planets, which travel through the galaxy alone, unmoored to any star.

A rogue planet glides through the galaxy alone in this artist’s impression. Credit: NASA/JPL-Caltech

Rogues might form as stars do, from the gravitational collapse of a cloud of gas and dust. That process would form only massive planets—a minimum of 5 times the mass of Jupiter, Penny said. “So far,” however, he explained, “the main results are that there are not a lot of free-floating giant planets out there,” with only a handful of confirmed discoveries to date.

Most rogues probably form from the disk of material around a star, then escape. “It could be an interaction between planets,” Penny said. “If you form a lot of planets in a disk, the disk keeps order until it dissipates. But once the damping effect of the disk is gone, all hell breaks loose,” and gravitational battles can sling planets into interstellar space. There may be billions of these smaller castaway worlds.

Although three searches are dedicated to finding planetary microlensing events, they’re restricted by daylight, clouds, and the other disadvantages of looking at stars from the ground.

As with astrometry discoveries and the Gaia mission, though, a space telescope may greatly expand the numbers of confirmed exoplanets. The Nancy Grace Roman Space Telescope, which is scheduled for launch later in the decade, could find 1,400 bound exoplanets and 300 rogues during its lifetime, Penny said. The telescope’s mirror will be the same size as that of Hubble Space Telescope, but with a field of view 100 times wider. That field of view will allow Roman to see a large area toward the galactic bulge—the preferred target for microlensing planet searches. Current plans call for it to scan the region six times for 72 days per session.

“It’s the ideal platform for doing microlensing because you can never predict when a lensing event will occur, and planetary events are very short,” Penny said.

One Telescope, Many Exoplanet Studies

Roman is expected to help with other exoplanet studies as well. As a technology demonstration, it will carry a coronagraph, which blocks the light of a star, allowing astronomers to see the light of planets directly. “It’ll try to get down to Jupiter-like exoplanets that are closer than Jupiter is now,” said Mayorga. “It might get as close as 1 AU for a Sun-like star.”

Current images of exoplanets, whether from telescopes in space or on terra firma, generally cover a single pixel. To better understand those pictures, scientists use the planets of our solar system as exoplanet analogues. In essence, they take the beautiful pictures of Earth and the other worlds that fill Instagram pages and squish them down to a pixel. “That sets a ground truth for the weird things we find in the universe,” Mayorga said. “It allows us to connect that disk-integrated light to the underlying cloud bands or continents or oceans. It’s the only place we can make that connection.”

Mayorga and colleagues used Cassini images snapped during a flyby of Jupiter as one analogue. They saw how the planet’s brightness and color changed as viewed under different Sun angles or as the Great Red Spot rotated in and out of view.

Several teams are developing missions or instruments that would use Earth as an exoplanet analogue. Mayorga, for example, is involved with a concept known as Earth transit observer, a proposed CubeSat mission that would watch Earth from L2, a gravitationally stable point in space roughly 1.5 million kilometers beyond Earth. Transits of the Sun would reveal the composition of our planet’s atmosphere, including its many “biomarkers,” such as oxygen, ozone, and methane.

Another mission, LOUPE (Lunar Observatory for Unresolved Polarimetry of Earth), would monitor Earth in both optical and polarized light from an instrument that hitches a ride on a lunar orbiter or lander.

“Measuring the linear polarization of a planet over a range of time yields a wealth of information about atmospheric constituents and clouds, as well as surface features like vegetation, water, ice, snow, or deserts,” said Dora Klindžić, a member of the mission team and a graduate student at Delft University of Technology and Leiden Observatory. “By observing Earth from a distance where we can reasonably pretend we are an outsider looking at the Earth, such as from the Moon, we can learn how a planet richly inhabited with life and vegetation appears when observed from another faraway planet. In a way, we are looking at ourselves to know others.”

Interstellar Probe could provide that type of understanding from an even more distant perspective. The proposed spacecraft could travel up to 1,000 AU from the Sun to study interstellar space and would look back toward the planets of the solar system. “Ten, 20, 30 years into the mission, we would have observations of the solar system from outside looking in, as if we were flipping the telescope and taking a look at a planetary system we do know,” said Michael Paul, project manager for the mission study at APL. “Tying that with in situ data we have for Mercury, Venus, Mars, Earth, Jupiter, Saturn will better inform the models we have of other planetary systems.”

No Tatooines Here

Give an object a good name, and people are likely to pay attention. “The fact that [Boyajian’s Star] has this special name means that there aren’t many other objects like it,” said Redfield. Perhaps with catchier names, the “unsung” planets and techniques, which can produce some of the most thought-provoking discoveries, will gain their share of the spotlight.

The three exoplanets discussed at the January 1996 press conference, for example, were designated 51 Pegasi b, 47 Ursae Majoris b, and 70 Gamma Virginis b—the names of the parent stars followed by the letter b. Astronomers have used that naming scheme ever since, with extra planets in a system assigned the letters c, d, e, and so on, on the basis of the order of discovery.

The system works well, although the names get a little confusing when the star has only a long catalog designation; no one’s going to be enchanted by 2MASS J21402931+1625183 A b, for example. And such “telephone book” designations are hardly going to appeal to the public, which regularly sees planets with names like Tatooine and Vulcan and Gallifrey in movies and TV shows.

So the IAU has conducted two international competitions that have produced proper names for more than 140 exoplanets. In the most recent project, 112 countries held individual contests, with each country proposing the name for one planet and its star.

“It was great to tap into the public imagination,” said Eric Mamajek, cochair of the naming campaign steering committee and deputy chief scientist for NASA’s Exoplanet Exploration Program. “I was blown away by the ones that made it through the campaign. The names all have stories.”

Astronomers have been slow to adopt the names, though. The names don’t show up in most of the major online catalogs, for example. “Those phone book names take on the intimacy of a proper name for most astronomers,” said Redfield. “I know that HD 189733 b [an exoplanet he’s studied] is just a bunch of numbers, but for me it has the power of a proper name. I call it ‘189.’ We’re on a nickname basis.”

“I think it will be a long process,” said Mamajek. “It may take a new generation—people who grew up reading these names in textbooks.”

Perhaps that new generation will recognize the first exoplanet confirmed around a Sun-like star not as 51 Pegasi b but as Dimidium or the first pulsar planets not as PSR B1257+12 b and c but as Draugr and Poltergeist.

Author Information

Damond Benningfield (damond5916@att.net), Science Writer

New Telescope, New Worlds • Exoplanets in the Shadows   • Overture to Exoplanets   • The Forecast for Exoplanets Is Cloudy but Bright   • “Earth Cousins” Are New Targets for Planetary Materials Research   • Oddballs of the Exoplanet Realm   • Thousands of Stars View Earth as a Transiting Exoplanet   • Taking Stock of Cosmic Rays in the Solar System   • Gap in Exoplanet Size Shifts with Age   • Unveiling the Next Exoplanet Act  

The long-awaited launch of the James Webb Space Telescope ( JWST) is finally in sight. Astronomers around the world are anticipating the wealth of information the flagship will gather on everything from the oldest galaxies in the universe to the birthplaces of stars and planets.

“It really is a Swiss army knife telescope with a huge range of applications,” said Elisabeth Matthews, an astronomer at Observatoire de Genève in Switzerland.

JWST, built by a team of more than 1,200 people from 14 countries, will collect infrared (IR) light across a broad range of wavelengths. That makes it ideally suited to studying exoplanets, which bury most of their secrets deep in the infrared spectrum. In this way, among many others, JWST will build on the legacies of the Hubble and Spitzer space telescopes, both of which astronomers have used to make revolutionary leaps in our understanding of distant worlds, although neither telescope was designed to do so. JWST’s instruments, on the other hand, were designed with exoplanets in mind.

The observatory is scheduled to launch by the end of this year, and exoplanet scientists have long been planning what they want to look at first. In 2020 they submitted their proposals to the telescope’s science team, and the selections for JWST’s first observing cycle were announced in March. (An observing cycle is 1 year, or 8,760 hours, of observing time.) More than 20% of JWST’s time during its first observing cycle will be dedicated to understanding exoplanets.

The unifying theme across the exoplanet observing programs? “One word: diversity.”The unifying theme across the exoplanet observing programs? “One word: diversity,” said Stefan Pelletier, an astronomy doctoral student at Université de Montréal. “All bases are being covered in terms of science cases as well as instrument and observing configurations.”

The list of principal investigators (PIs) and coinvestigators on the accepted programs is also more diverse across many axes of identity than space telescope programs have been in the past. Compared with a recent round of Hubble proposals, a higher percentage of PIs who are women and also PIs who are graduate students will make the first JWST observations.

It’s a testament to the hard work by the team at the Space Telescope Science Institute, Matthews said, “both in making sure [members of] the exoplanet community are able to understand the telescope and design good science experiments for it and also in ensuring that the proposals for these science experiments have been carefully and equitably judged.”

Andrew Vanderburg, an astronomer at the University of Wisconsin–Madison, added, “It’s awesome that the [dual anonymous] peer review—where the reviewers don’t know who wrote the proposals, and vice versa—makes it possible for young scientists with good ideas to be awarded time on the world’s most powerful observatory from day one.”

Prologue: A Shakedown Cruise

JWST promises to be a game changer for understanding how and what types of planets form and what makes them habitable, but for this first cycle it’s unknown how the telescope’s performance will measure up to expectations. “The reviewers very much wanted a robust ‘shakedown cruise,’” said Peter Gao, an exoplanet scientist at the University of California, Santa Cruz. “Several proposals focused on new and interesting observing methods and science cases that are sure to be the testing grounds for similar, larger, and more elaborate proposals in the next cycles.”

The selected exoplanet observations tend to stay well within the telescope’s expected limitations. “JWST time is very precious, so for the first cycle it is understandable that emphasis was put on programs that are ‘safe’ in that they are almost guaranteed to generate good results,” said Alexis Brandeker, an astronomer at Stockholm University. Some observations might be “risky” in that the scientists aren’t sure what they’ll find, but if they do find something, they’ll get a good look at it.

On the science side, there’s variety both in the types of planets targeted for observations and in the types of observations being made. “These include the measurement of mineral cloud spectral features as a way to probe the composition of exoplanet clouds, exploring asymmetries in the dawn and dusk limbs of exoplanets during transits, eclipse mapping, and getting a sense of which rocky exoplanets host atmospheres,” Gao said.

And on the target side, “there is a nice balance between some of the first exoplanets to be characterized, like HD 189733 b, and weird exoplanets whose observations were difficult to interpret, like 55 Cancri e,” said Lisa Đặng, a physics graduate student at McGill University in Montreal. Instead of making limited observations of a wide range of planets, most of the selected exoplanet programs seek to observe one or a few planets in great detail.

Lights Up on a Familiar Scene

In this first observing cycle, “we are going after a lot of known exoplanets that we have observed in the past, so there aren’t many unexplored targets,” Đặng said. “This makes absolute sense since it will be the first time we are going to use these instruments in space and we don’t really know what challenges we will have to deal with yet.”

“There are some really interesting planets…that we already have tantalizing glimpses of from Hubble and Spitzer data,” said Hannah Wakeford, an astrophysicist at the University of Bristol in the United Kingdom. Wakeford, for example, will be targeting a well-studied, but still mysterious, hot Jupiter, HD 209458 b. “The data we currently have from Hubble tell us there is something in this atmosphere, and my program aims to show that it is clouds made from magnesium silicates (glass),” she said.

In 2007, astronomers used the Spitzer Space Telescope to create the first global temperature of an exoplanet, the hot Jupiter HD 189733 b. With JWST, astronomers plan to make a map of this planet’s hot spots (yellow) and cold spots (blue) not just in 2D, but also in 3D. Credit: NASA/­JPL­Caltech/H. Knutson (Harvard-Smithsonian cfa)

Tiffany Kataria, a planetary scientist at NASA Jet Propulsion Laboratory in Pasadena, Calif., is part of one of the five programs studying HD 189733b, a hot Jupiter so normal that it’s called canonical. “This planet was one of the first exoplanets whose atmosphere was observed with the Spitzer and Hubble space telescopes, yet there is still much we don’t know about the properties of its atmosphere,” she said. Kataria will make a 3D map of the planet’s glowing dayside to study its wind and temperature patterns, “which tells us a great deal about the physical processes taking place in the atmosphere.”

Néstor Espinoza’s target is hot Jupiter WASP-63 b and, more specifically, its sunrise and sunset. The program “aims to try to detect, for the first time, the infrared atmospheric signatures of the morning and evening limbs of a hot gas giant exoplanet…. It goes in the direction of exploring atmospheric structure of these distant exoplanets in 3D.” Espinoza is an astronomer at the Space Telescope Science Institute in Baltimore, Md.

Plenty of smaller planets reside among the old favorites that JWST will study, including the Earth-sized lava world 55 Cancri e. Brandeker’s program will examine changes in light when the glowing, molten planet passes behind its star. “We hope to see if consecutive eclipses show the same or different faces of the planet,” he said. Planets that orbit close to their stars are assumed to be tidally locked, having the same hemisphere always facing the star. If 55 Cancri e rotates faster or slower than it orbits, “this assumption, often taken for granted, can be questioned also for other planets. This in turn has major implications for how planets are heated, i.e., one side versus all sides.”

Another old favorite is the sub-Neptune GJ 1214 b, the target of one of Eliza Kempton’s observing programs. “Through a combination of mid-IR transmission spectroscopy, plus thermal emission and secondary eclipse observations, we aim to get a clearer picture of the atmospheric composition and aerosol properties of this enigmatic world,” she said.

“The overlap with existing observations is not a main motivator because we expect JWST to perform so much better than existing facilities,” said Kempton, an exoplanet astronomer at the University of Maryland in College Park. “But it will certainly be reassuring to see that the JWST data do agree with prior observations, and the level of agreement will help us to contextualize all data taken previously with facilities like Hubble and Spitzer.”

The surface of 55 Cancri e is probably molten, as seen in this artist’s illustration. Astronomers will use JWST to better understand how the surface heats up. Credit: ESA/Hubble, M. Kornmesser, CC BY 4.0

Newest among the old favorites soon to be studied by JWST is the TRAPPIST-1 system, which excited astronomers and the public alike when it was discovered to have seven possibly rocky Earth-sized planets.

“JWST has a small chance of finding biosignatures on TRAPPIST-1 planets …but a very good chance of telling us which molecules dominate the atmosphere and whether there are clouds.”A grand total of eight different programs will look at these planets’ atmospheric properties. “With this program,” said Olivia Lim, an astronomy doctoral student at Université de Montreal and PI for the program, “we are hoping to determine whether the planets have an atmosphere or not, at the very least, and if they do host atmospheres, we wish to detect the presence of molecules like [carbon dioxide, water, and ozone] in those atmospheres. This would be an important step in the search for traces of life outside the solar system.”

“JWST has a small chance of finding biosignatures on TRAPPIST-1 planets,” said Michael Zhang, “but a very good chance of telling us which molecules dominate the atmosphere and whether there are clouds.” Zhang is an astronomy graduate student at the California Institute of Technology in Pasadena.

Planetary Plot Twists

Some exoplanets just don’t fit inside the box as neatly as other exoplanets do, and astronomers are really hoping that JWST will help them understand why that is. Kataria leads the program to study one of these oddballs, HD 80606 b.

“HD 80606 b is an extreme hot Jupiter, and that’s saying something, given that hot Jupiters are pretty extreme to begin with!” Kataria said. “This Jupiter-sized exoplanet is on a highly eccentric, or elliptical, orbit and experiences a factor of greater than 800 variation in flux, or heating, throughout its 111-day orbit.”

“Most of the time it spends at relatively temperate distances,” Brandeker added, “but once every 111 days it swooshes very closely by the star in a few days [and] gets ‘flash heated.’”

The flash heating that HD 80606 b experiences once every 111 days likely creates intense storms and unexpected weather patterns, which JWST will monitor. These computer models of those weather patterns are based on Spitzer data taken during a 2007 flash heating event. (Blue represents colder bulk atmosphere, and red represents warmer winds.) Credit: NASA/­JPL­Caltech/G. Laughlin (UCO/Lick Observatory)

Studying HD 80606 b’s atmosphere as it heats and cools “will really help us examine the pure physics behind atmospheric energy transport, which is important for all worlds,” Wakeford said.

Kataria is also a coinvestigator on a program to make a 3D atmospheric map of a different oddity, WASP-121 b, a gas giant so hot that it bleeds heavy metals into space and orbits so close to its star that it’s shaped like a football. WASP-121 b is one example of a “super-puff” planet: These planets are roughly the size of Jupiter but far less massive, which makes their density closer to that of cotton candy. Pelletier will be looking at another super-puff, WASP-127 b. “Our hope is to gain a better understanding of the carbon budget on a planet vastly different from anything we have in our solar system,” he said.

What’s the most important thing to learn about super-puff planets? “Basically anything!” according to Gao, whose program will target super-puff Kepler-51 b. “All previous attempts at characterizing super-puff atmospheres have yielded featureless spectra and therefore very little information. If our observation is anything but a flat line, then we will have learned so much more than what we now know about these mysterious objects. It really is a fact-finding mission.”

M Dwarfs’ Breathtaking Aria

M dwarf stars are the smallest and most common stars in the universe, and astronomers have found that they host plenty of planets. Rocky habitable planets around these stars are easier to find using the two most prevalent methods—transits and radial velocity—but whether those planets can host atmospheres is still debated.

“I think the Cycle 1 observations will teach us a ton about whether rocky planets around M dwarfs can keep their atmospheres,” said Laura Kreidberg, director of research into the atmospheric physics of exoplanets at the Max Planck Institute for Astronomy in Heidelberg, Germany. “This is one of the most fundamental questions about where life is most likely to arise in the universe. There are tons of these small planets around small stars”— more than 1,500 are known so far—“but they experience more high-energy radiation over their lifetimes, so it’s not known whether they can keep their atmospheres. No atmosphere [is] bad news for life!”

The small rocky planet LHS 3844 b depicted in this illustration has been confirmed to have no atmosphere, so JWST will be able to study its surface composition. Credit: NASA/­JPL­Caltech/R. Hurt, IPAC

Both of Kreidberg’s observing programs will target rocky planets around M dwarfs. “One of the planets [LHS 3844 b] is already known to not have an atmosphere, so the goal of this program is to study the planet’s surface composition—what type of rock it’s made of—and search for any hints of volcanic activity, which could produce trace amounts of sulfur dioxide.”

Kreidberg is also looking at TRAPPIST-1 c, “which is very close to Venus in temperature. For that planet, I’m searching for absorption from carbon dioxide, to test whether the planet has a thick, Venus-like atmosphere or whether the atmosphere has been lost.”

“While we have made many models of atmospheric loss for small planets,” Gao said, “this will be our first real test of these theories. Will we find out that most characterizable rocky planets don’t actually have atmospheres and that our modeling efforts for their climates and habitability are futile? Or will we see a much more diverse set of atmospheric states? The results of these studies will be interesting and informative for future cycles regardless of what we find.”

Small-Planet Showstoppers

About half of JWST’s exoplanet-specific observing time will be dedicated to studying worlds smaller than Neptune. “This tells me without a doubt that the community is overwhelmingly interested in the little guys,” Gao said. These planets might be rocky (if they’re small enough) or could have a rock-ice core and a thick atmosphere.

“The large program on sub-Neptune and super-Earth atmospheres led by Natasha Batalha and Johanna Teske is especially exciting to me because it will provide us with a systematic survey of a class of planets that is not present in our solar system and was not readily observable with previous facilities,” Kempton said. “The potential for this program to unlock greater insight into the atmospheres of small planets is quite high.”

Small planets all start from a collection of dust and gas. How much material a future planet starts with determines how much additional gas it can attract and how much of that gas it will keep when it heats up. Planets that end with a large gaseous envelope are termed mini-Neptunes, and those with only a small amount of gas are deemed super-Earths. Credit: NASA/Ames Re-search Center/­JPL­Caltech/R. Hurt

“These planets are so small that they’re beyond the reach of current technology, so anything JWST discovers will be a big improvement on what we know,” Zhang said. “For small planets like GJ 367 b, my target, and 55 Cancri e, we basically don’t know anything, so we’ll learn the first thing about them. Do they have atmospheres? If so, are they carbon dioxide, oxygen, or exotic metal atmospheres made of sodium and silicon oxide?”

One of Espinoza’s programs will focus on super-Earth K2-141 b, a planet only slightly larger and more massive than Earth but much, much hotter. “Depending on the properties of this exoplanet like the presence or not of an atmosphere, the flux change during its orbit around the star should give rise to very different signals, which will enable us to infer what this exoplanet’s exterior is made of,” said Espinoza.

If K2-141 b does have an atmosphere, it might not be the one it started with. Lisa Đặng aims to find out. Rocky planets as hot as that one “are thought to have lost any primordial atmosphere but, instead, could sustain a thin rock vapor atmosphere [that] outgasses from the mantle,” she said. Does the atmosphere stick around or rain back down? “With our observations we are hoping to detect molecular signatures of the atmospheric constituents and also obtain a map of the planet’s atmosphere and surface.”

Ballad of Planets and Disks

JWST should build upon discoveries made not only by space telescopes like Hubble and Spitzer but also by ground-based observatories like the Atacama Large Millimeter/submillimeter Array (ALMA). These observations will probe the birthplaces of planets: the disks of dust and gas around young stars. “Over the last decade, we’ve gotten gorgeous images from the ALMA interferometer in Chile and have seen loads of fine-scale structure, tracing pebbles in planet-forming disks,” said Ilse Cleeves, an astronomer at the University of Virginia in Charlottesville. “Some of the structures likely trace planets in formation, and so it’ll be very exciting to see what JWST uncovers, both in terms of patterns in the disk and perhaps even the drivers—protoplanets—themselves!”

Matthews added that “if JWST is able to successfully detect planets in these disks, it will be an important confirmation of our understanding of how planets interact with disks.” If no planets appear in the disks, astronomers will have to rethink how, and whether, planets shape disks.

A few of the first observations with JWST will seek to map out how planet-forming disks distribute their water and other materials that are essential for life. This artist’s illustration shows where water ice may exist in a ­planet-­forming disk. Credit: A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO), CC BY 4.0

Cleeves will be studying planet-forming disks to understand how they give rise to habitable planets. “How common are habitable planets? Availability of water is a natural place to start, but we don’t have great observational constraints on how much water is present or the distribution of water in disks. We are looking forward to mapping out water ice in a nearby disk that happens to be posing in front of a host of background stars.” If a star’s light passes through a part of the disk that has ice, the ice will imprint a spectroscopic signal on the light. With so many background stars, Cleeves said, they’ll be able to say not just whether ice is present, but also where.

The makings of a world well suited for life go beyond the presence of water, however, and Melissa McClure’s three observing programs will look for them. We’ll “trace how the elemental building blocks of life—like carbon, hydrogen, oxygen, nitrogen, and sulfur—evolve between molecular clouds, where they freeze out on dust grains as ices, and protoplanetary disks, where these ices are incorporated into forming planetesimals and, ultimately, planets,” she said. “I think that within a few years we will have an understanding of how much water terrestrial planets typically have and whether they inherited that water from their birth locations in their disks or if cometary delivery was necessary.” McClure is an assistant professor and a Veni Laureate at Leiden Observatory in the Netherlands.

“This planet orbits close enough to the white dwarf that it could not have originally orbited there before the star’s death. So how did it get there?”A perhaps underrecognized component of JWST’s observing capabilities is the coronagraph that will allow direct imaging of exoplanet systems, meaning that the telescope will see light emitted by the planet itself. Coupled with JWST’s infrared capabilities, the telescope will be able to observe planets much older and colder than is currently possible. That’s Matthews’s aim. “Eps Indi Ab is similar in age to the solar system and is similarly far from its star as Jupiter is from the Sun. Because JWST is able to image much further into the infrared than Earth-based telescopes and because old planets are brighter at these very long wavelengths, our project provides a unique opportunity to study a truly Jupiter-like planet outside the solar system,” she said.

Sometimes planets survive their star’s demise, as is the case of WD 1856+534 b, a gas giant planet that orbits the slowly cooling corpse of a star, also known as a white dwarf. In this case, the planet’s survival presents a puzzle. “This planet orbits close enough to the white dwarf that it could not have originally orbited there before the star’s death. So how did it get there?” asked Andrew Vanderburg, whose program will target this system.

Bridge to Act II

Once the “shakedown cruise” is complete, Hannah Wakeford would like to see JWST used to study more worlds the size of Jupiter, Saturn, and Neptune. “There is so much we can learn that we can’t even get from our own solar system giant planets,” she said, “so it is, in my opinion, a low-risk, high-reward scenario.”

“The very first exoplanetary observations to be made by JWST are going to be a big jump into the known unknown…. As the title of an album of one of my favorite rock bands would say, ‘Expect the unexpected.’”On Vanderburg’s wish list: “Disintegrating planets. These will be great probes of the interior compositions of planets, so I hope we will get observations of them in the future.”

Cleeves called the first cycle “a great place to start. I have a feeling, though, that the most interesting next projects are those that we haven’t anticipated yet, so I’m really looking forward to the first couple of years with JWST, grappling with the data and finding those unexpected puzzles.”

Espinoza agreed. “I’m almost convinced features will show up in the data that we will perhaps not be able to explain right away,” he said. “As such, the very first exoplanetary observations to be made by JWST are going to be a big jump into the known unknown…. As the title of an album of one of my favorite rock bands would say, ‘Expect the unexpected.’”

Author Information

Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

New Telescope, New Worlds • Exoplanets in the Shadows   • Overture to Exoplanets   • The Forecast for Exoplanets Is Cloudy but Bright   • “Earth Cousins” Are New Targets for Planetary Materials Research   • Oddballs of the Exoplanet Realm   • Thousands of Stars View Earth as a Transiting Exoplanet   • Taking Stock of Cosmic Rays in the Solar System   • Gap in Exoplanet Size Shifts with Age   • Unveiling the Next Exoplanet Act  

The first time scientists measured the atmosphere of an exoplanet—a planet outside our solar system—they found something unexpected in the signal. It was 2001, and the Hubble Space Telescope was trained on HD 209458 b, a recently discovered gas giant roughly the size of Jupiter.

When astronomers looked for the presence of sodium in light waves shining through the planet’s atmosphere as it crossed in front of its star, there was a lot less of it than they thought there would be, said Hannah Wakeford, a lecturer in astrophysics at the University of Bristol in the United Kingdom. “From the very first measurement of an exoplanet atmosphere, there was evidence that something else was happening, something else was there blocking the light.”

The most compelling theory for what that something could be? Massive banks of dark, hot clouds. “Clouds are essentially liquid or solid droplets or particles that are suspended in a gaseous atmosphere,” said Wakeford. But because the planet is so hot—5 times hotter than Earth—those droplets couldn’t be made of water, as they are on Earth.

In the 2 decades since analyzing the atmosphere of HD 209458 b, astronomers have discovered more than 4,000 exoplanets. Using spectroscopy, they have measured the atmospheres of more than 100 of those objects, and it looks like many of them are cloudy. The way those extraterrestrial clouds behave and the exotic things they could be made of—liquid sand, iron, even rubies—are stretching scientists’ ideas of what terms like clouds, rain, and snow even mean in the context of the universe.

“Clouds are everywhere,” said Laura Kreidberg, an astronomer at the Max Planck Institute for Astronomy in Germany. “And to have any hope of understanding what’s going on in [exoplanet] atmospheres, we have to understand the clouds.”

Mushballs and Methane Lakes

The trouble is, clouds are complicated. Even on Earth, clouds are difficult to model (one reason weather forecasts can still lack accuracy.) Their complexity arises partly because they are simultaneously very small and very large: made up of microscopic water droplets yet so vast they can cover more than two thirds of Earth’s surface. Another reason is that there are so many kinds of clouds, and they behave in complex ways, explained atmospheric physicist David Crisp at the Jet Propulsion Laboratory, California Institute of Technology.

Clouds are “ubiquitous; they can form in many different kinds of environments, and there are many processes associated with their formation,” Crisp said.

On Earth, clouds take many forms, like these cirrus and stratocumulus clouds. Their variety and ubiquity make them difficult to account for in climate models. Credit: NASA

And they’re not made only of water, either. Most cloud particles start growing on condensation nuclei—a speck of dust or a grain of salt. And although most earthly cloud droplets are spherical and liquid, those that make up cirrus clouds are hexagonal ice crystals.

Clouds can frustrate scientists’ ability to see clearly, whether they are gazing at the heavens from the ground or peering back at Earth from space. In the 1980s, Crisp helped build the camera in the Hubble Space Telescope and now leads a NASA team that uses orbiting satellites to measure the dangerous levels of carbon dioxide accumulating in Earth’s atmosphere. “I’ve learned to hate clouds from both sides now,” he joked.

Clouds mess with models predicting future climate change, he said, because they simultaneously warm and cool the planet, depending in part on whether their droplets are mainly liquid or mainly ice. In general, low-lying, mostly liquid clouds provide shade and reflect solar energy back into space, whereas high-altitude, frozen cirrus clouds trap infrared radiation emitted by Earth’s continents and oceans and intensify surface heating. This duality has long frustrated exoplanet cloud watchers, too—scientists scrutinize cloud signals to better understand how or whether clouds are heating the atmosphere below them.

Scientists are still trying to understand whether, at a global level, those cooling and warming effects cancel each other out and how that balance could change in the future. (One recent study even suggested that at carbon dioxide levels of around 1,200 parts per million, global cloud cover could become unstable and dissipate, dramatically accelerating warming.)

“We’ve dropped a few dozen probes into the atmosphere of Venus. But you know, if you measured Earth’s atmosphere with only a dozen instruments, how much would you know about the Earth?”Despite the uncertainties, we know a lot more about Earth’s clouds than we do about those on other planets and moons of our solar system. It was only in the 1970s, for instance, that scientists figured out that Venus is enveloped in clouds of sulfuric acid. “This stuff will strip paint—and just about anything else,” said Crisp. Space missions to Venus have dropped mass spectrometers into the planet’s atmosphere that, “even though sulfuric acid is not very nice to our mass spectrometers,” have managed to send back data about the chemical makeup and concentrations of several cloud layers.

Jupiter’s atmosphere has been sampled too, and has been found to contain swirling ammonia clouds. Recent flybys of the tops of these clouds by NASA’s Juno mission identified mushballs—Jovian hailstones formed out of water-ammonia slush enrobed in an ice crust—that fall through the planet’s atmosphere. On the way down, these mushballs collide with upward moving ice crystals and electrify the clouds, causing shallow, high-altitude lightning visible from space.

The thick atmosphere of Saturn’s moon Titan glows in this composite image made with both vis-ible and ultraviolet light. On Titan, the rain, rivers, and lakes are made of liquid methane and ethane. Credit: NASA

Thanks to the Cassini spacecraft, we know that the atmosphere on Titan, the largest of Saturn’s moons, is largely made up of nitrogen, like Earth’s. There are seasons, monsoons, and wild windstorms. But Titan’s mountains are made of solid ice, and instead of a water cycle, it has a hydrocarbon cycle: On Titan, the rain, rivers, and lakes are made of methane and ethane.

But many questions remain when it comes to solar system weather. For example, we don’t know how deep into Jupiter the mushballs fall before they evaporate and rise again, said Wakeford. There are mysterious long-chain hydrocarbons floating high in the atmosphere of Titan too. “We have absolutely no idea how they got there; it’s baffling.”

What knowledge we do have is drawn from the briefest of snapshots, added Crisp. “We’ve dropped a few dozen probes into the atmosphere of Venus. But you know, if you measured Earth’s atmosphere with only a dozen instruments, how much would you know about the Earth? These planets are big places, and they have complicated climates—quite as complicated as ours.”

A Lead Blanket or Gems and Jewels

The challenges of analyzing extraterrestrial clouds are magnified when it comes to exoplanets. We can’t send a probe laden with instruments to any of them or record detailed images of their surfaces.

All we have is light, said Heather Knutson—the light coming from a far-off star. “We know there’s a planet in orbit around it, and we can indirectly infer some basic things about that planet, but it’s really a sort of poor man’s camera,” said Knutson, an astronomer at the California Institute of Technology

“If we’re going with the X-ray analogy, clouds are sort of like a lead blanket over the planet.”When an exoplanet passes in front of its star—an event called a transit—astronomers can measure the way light passes through the planet’s atmosphere on its way to us. Measuring how opaque the atmosphere is at different wavelengths of light (transit spectroscopy) offers clues to its composition. Kreidberg used an X-ray analogy to explain how it works: “Our bodies are opaque in optical light. If you shine a flashlight at a person, you can’t see through them. But if you look in the X-rays, you can see through the skin, but not through the bones.”

In the same way that our skin differs from our bones, molecules in planetary atmospheres are opaque or transparent at different wavelengths. “Whether it’s water or methane or oxygen or carbon dioxide, they have distinct opacity at different wavelengths of light,” said Kreidberg. “So if the planet looks a little bit bigger at a particular wavelength, then we can work backward from that to try to infer what’s in its atmosphere.”

https://eos.org/wp-content/uploads/2021/07/Exoplanet_Animation_Transit_Depth_vs_Wavelength_ipod_lg.m4v

But clouds get in the way of that process, said Knutson. “If we’re going with the X-ray analogy, clouds are sort of like a lead blanket over the planet. You see something that looks very featureless.”

Still, on the basis of the planet’s average atmospheric temperature—something astronomers can estimate from the brightness of the star and the planet’s distance from it—it’s possible to infer what those clouds are likely to be made of because of the varying temperatures at which different molecules condense from gas into liquid.

And the vast range of possible temperatures is something that distinguishes exoplanets from those in our solar system, said Nikku Madhusudhan, an astrophysicist and exoplanet scientist at the University of Cambridge. “Because of that vast range, you allow for a much wider range of chemical compositions [than in the solar system]. A lot more chemistry can happen.”

An artist’s impression of the gas giant HAT-P-7 b. Astronomers detected strong winds and cata-strophic storms and suspect that the clouds could be made of corundum, the mineral that forms rubies and sapphires. Credit: Mark Garlick/ University of Warwick

Here on Earth, with an average temperature of 290 K, clouds are made mostly of water. The atmospheres of some exoplanets, between 400 K and 900 K, are warm enough to condense salts and sulfides into clouds. At around 1,400–2,000 K (a third as hot as the Sun), we would expect to see clouds of molten silicates—the material that makes up the volcanic sand on some of Earth’s beaches and is used in the production of glass. On an even hotter planet like WASP-76b, which is estimated to reach 2,400 K, clouds are likely made of liquid iron. And the atmospheres of the hottest known exoplanets—giant, 2,500+ K ultrahot Jupiters orbiting very close to their stars—are the right temperature for clouds made of corundum, a crystalline form of aluminum oxide that forms rubies and sapphires on Earth.

“These are quite literally the gems and jewels that we have here on Earth forming clouds and lofted high into the atmospheres of Jupiter-sized worlds that are lit glowing from their star,” said Wakeford. She remembered walking through the Hall of Gems in London’s Natural History Museum after learning this, trying to imagine the crystals molten and forming clouds. “It just blew my mind.”

Metallic Monsoons

WASP-76 b made headlines in 2020 when a team of European researchers published a paper suggesting it had not only clouds of iron but iron rain as well.

Artwork imagining the nightside of WASP-76 b, a hot Jupiter exoplanet with clouds of iron—and possibly even iron rain. Credit: ESO/M. Kornmesser

“We see the iron, and then we don’t see the iron. So it has to go somewhere, and the physical process that we expect is rain.”Like our own Moon and many planets that orbit very close to a star, WASP-76 b is tidally locked, meaning one side of the planet always faces the star (dayside) and the other always faces away (nightside). Researchers found evidence of iron atoms in the atmosphere of WASP-76 b’s hotter dayside but not on the cooler nightside, which they argued meant that the iron must be condensing into liquid droplets as wind carries the atoms around the planet. “We see the iron, and then we don’t see the iron. So it has to go somewhere, and the physical process that we expect is rain,” said Kreidberg, who was not involved in the study. “This is some of the most convincing evidence I have ever seen for exoplanet weather.”

But Caroline Morley, an astrophysicist at the University of Texas at Austin, cautioned that the phenomenon could be more complex. Recent studies, including one co-authored by Kreidberg, have examined the microphysics of how iron droplets form, finding that the substance’s high surface tension means that it doesn’t easily condense from a gas to a liquid. There might be some other processes involved in WASP-76 b’s iron phenomenon, Morley said—perhaps the iron interacts with some other chemicals in the planet’s atmosphere, which helps it form a cloud.

“Statistically, I believe that there are exoplanets where it is raining right now,” she said. “But I think that we have not seen smoking gun evidence for rain on other planets yet.”

Crisp agreed. “Clouds we’ve detected. Rain and snow have not yet been detected—but I’d be surprised if they weren’t there. Those are logical outcomes of the systems we see.”

Metaphorical Meteorology

So when astronomers talk about possible rain on exoplanets, is it really what we would think of as rain? What do the concepts of rain and clouds even mean in the context of distant space? To some extent, it’s all a metaphor, said Wakeford.

“We have to be open to the fact that the complexity in nature may greatly surpass our imagination at the present time.”On Earth, the terms rain, clouds, and snow all apply almost exclusively to one substance: water. “Water is one of the most amazing materials in the universe,” Wakeford said, but not all substances behave like water when experiencing differences in pressure or temperature. “So when we frame these very alien clouds and rain and snow in that [water-based] context, it puts things in our minds that aren’t exactly what the physics is.”

For instance, words like snow and hail can be a bit misleading when you talk about solid particles in an atmosphere that’s hotter than a lava flow. “I tend to use rain instead of snow,” Wakeford said, “because snow to us evokes a temperature, a coldness. Rain is something that can define many different types of conditions, whereas snow for us is very much a cold thing. And this is not what’s happening here on some of these planets that are so incredibly hot.”

An imagined movie poster for the exoplanet HD 189733 b, a cobalt-blue hot Jupiter with winds approaching 8,700 kilometers per hour and rain of molten glass. Credit: NASA-JPL/Caltech

Still, Wakeford thinks a smattering of poetic license is justified to bring the public along on the journey and capture people’s imaginations. “If you start by saying, ‘It’s raining drops of glass on these planets’—that’s a starting point. I can use that; I’ve got [your attention] now. Then we can build on that and get a deeper understanding.”

When it comes to actually doing the research, though, scientists should be both circumspect and open-minded, said Madhusudhan. Although it can sometimes help to extrapolate from what we’re discovering about Earth’s clouds to these faraway planets, for instance, it’s important to remember that these worlds are so exotic that it’s possible there are processes going on in their atmospheres that we haven’t even considered. “The biggest mistake we could make is to try to simplify the complexity of exoplanetary systems just to fit a narrative.”

We may go on to discover kinds of weather we don’t even have words for, said Madhusudhan. “We have to be open to the fact that the complexity in nature may greatly surpass our imagination at the present time.”

Peering into the Infrared

So far, everything we know about clouds on exoplanets has been based on what Madhusudhan calls indirect inference: “It’s a bit more real than philosophical but a bit less real than an actual observation.” But the launch of the international James Webb Space Telescope ( JWST) near the end of this year promises to give astronomers the chance to make direct observations of exoplanet clouds for the first time.

JWST will keep Earth between it and the Sun and is designed to look at the longer wavelengths of infrared light. “Planets are easier to study in the infrared,” said Knutson. The telescope will make faraway objects look brighter than they do in visible light and will be better able to detect molecules in exoplanet atmospheres. It should also advance our understanding of alien weather.

“When you go to midinfrared, the composition of a cloud droplet starts to matter—the way that it scatters light is different for different cloud species,” said Knutson. “So we might, for the first time, directly measure what the clouds are made of.”

Morley is leading a team that will use JWST to examine a cold exoplanet called WISE J085510.83−071442.5 to test for the presence of water ice clouds and see whether they are changing as the planet rotates, implying that there are storm systems and weather. “That would give us real evidence, for the first time, that there’s water ice forming in a planet outside of the solar system,” Morley said.

Wakeford, meanwhile, will have a chance to train the telescope on HD 209458 b, the very first planet that 20 years ago was assumed to have clouds of magnesium silicate. JWST will give her a chance to prove (or disprove) that assumption with direct measurements.

Overall, “I think we think about clouds more broadly than anybody has thought about clouds in human history,” said Morley. “And we’re just on the cusp of being able to get a huge amount of really detailed information about those clouds. It’s a really exciting time to be in this field.”

Author Information

Kate Evans (@kate_g_evans), Science Writer

The properties of suspended sediments are difficult to measure in the field, yet characterizing these materials is essential to successful monitoring and management coastal and estuarine environments. To overcome the challenges of estimating the relative proportions of sand and mud in mixed sediment environments, Pearson et al. [2021] developed a new methodology. This is achieved by comparing the response of simultaneous optical and acoustic measurements, in both laboratory experiments and in application to field measurements on the ebb-tidal delta of a major inlet.

The important contribution in this paper is the development of a new indicator, the “sediment composition index”, that can be used to directly predict the relative fraction of sand in suspension. This approach may prove to be widely useful in gaining deeper understanding of material transport in the coastal ocean by improving estimates of sediment flux and increasing confidence in the interpretation of observations.

Citation: Pearson, S. G., Verney, R., van Prooijen, B. C., Tran, D., Hendriks, E. C. M., Jacquet, M., & Wang, Z. B. [2021]. Characterizing the composition of sand and mud suspensions in coastal and estuarine environments using combined optical and acoustic measurements. Journal of Geophysical Research: Oceans, 126, e2021JC017354. https://doi.org/10.1029/2021JC017354

—Ryan P. Mulligan, Editor, JGR: Oceans