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Streamflow drought—when substantially less water than usual moves through rivers—can seriously disrupt the welfare of nearby communities, agriculture, and economies. Synchronous drought, in which multiple river basins experience drought simultaneously, can be even more severe and far-reaching.

In 2018, the Hayabusa2 mission successfully encountered asteroid Ryugu. The Japan Aerospace Exploration Agency (JAXA) spacecraft arrived at, touched down on, collected samples of, and lifted off from the asteroid. It returned samples to Earth in 2020.

With plenty of fuel left for its extended mission, called Hayabusa2# or “Hayabusa2 Sharp,” the spacecraft raced off to its next objective, a high-speed flyby of asteroid 98943 Torifune in 2026. If all goes well with that rendezvous, the craft will attempt its final objective: an encounter with and touchdown on asteroid 1998 KY26 in 2031.

But that final objective may prove more difficult than initially imagined. New ground-based observations of 1998 KY26 have revealed that the asteroid is 3 times smaller than previously thought and spins twice as fast.

“We found that the reality of the object is completely different from what it was previously described as,” Toni Santana-Ros, lead author of a new study on 1998 KY26 and an asteroid researcher at Universidad de Alicante and Universitat de Barcelona in Spain, said in a statement.

Small and Fast

Astronomers discovered 1998 KY26 in 1998 when it came within 2 times the Earth-Moon distance. Radar and visual observations shortly after discovery estimated that the asteroid was about 30 meters across and rotated once every 10.7 minutes, the fastest-rotating asteroid known at that time. As the asteroid moved away, it became too faint to see for more than 2 decades. When Hayabusa2’s mission scientists selected targets for its extended mission, they relied on those 1998 calculations.

The asteroid completes one spin every 5 minutes and 21 seconds, less time than it takes to listen to Queen’s “Bohemian Rhapsody.”

Finally, in 2024, 1998 KY26 came close enough to Earth—12 times the distance to the Moon—to observe again. Using four of the most powerful ground-based telescopes available, Santana-Ros and his colleagues watched the diminutive asteroid tumble and spin from multiple angles, allowing them to calculate a more accurate spin rate than was possible with the limited radar and photometry in 1998.

They calculated that the asteroid completes one spin every 5 minutes and 21 seconds, less time than it takes to listen to Queen’s “Bohemian Rhapsody.” The team then combined those new observations with the 1998 radar data to recalculate the asteroid’s size. They found that instead of being roughly 30 meters in diameter, 1998 KY26 is just 11 meters, or about the length of a telephone pole. The team published these results in Nature Communications on 18 September.

Asteroid Ryugu (left) is roughly 82 times the size of asteroid 1998 KY26 (right). Credit: ESO/M. Kornmesser; Asteroid models: T. Santana-Ros, JAXA/University of Aizu/Kobe University, CC BY 4.0

“The smaller the asteroids get, the more abundant they are—but that also means that they are harder to find,” explained Teddy Kareta, a planetary scientist at Villanova University in Pennsylvania who was not involved with the new discovery. “The fact that this new paper finds such a small size for KY26 is tremendously interesting on its own—Hayabusa2 will be able to explore an extremely understudied population—but it also means that we might not have a tremendous number of known objects to compare to as well.”

A Challenge and an Opportunity

The new size and spin measurements of 1998 KY26 will make Hayabusa2’s planned touchdown more challenging, the researchers wrote. However, this is not the first time that an asteroid rendezvous mission has had to adjust its expectations mid-flight. Both Ryugu and Bennu, the first target of NASA’s Origins, Spectral Interpretation, Resource Identification, Security–Regolith Explorer (OSIRIS-REx) mission, had rougher surfaces than expected, requiring the respective missions to adjust their sample collection methods. Too, the OSIRIS-REx team learned that Bennu was actively spitting out material only when the spacecraft got close, which led them to change their plan for orbital insertion.

Despite the new challenges with 1998 KY26, Hayabusa2#’s team has a big advantage: 6 years to rework their game plan.

“The Hayabusa2 team are incredibly smart, hardworking, and have a ton of experience under their belts, but I’m sure that this kind of result is causing a bit of hand-wringing and concern for them even if the spacecraft is fully capable,” Kareta said.

“We have never seen a ten-metre-size asteroid in situ, so we don’t really know what to expect and how it will look.”

“I’m sure even the team has their doubts about whether or not the original plan was possible, but if I had to bet money, I still think the team will try [to touch down],” they added. “You set yourself up for success by building a great spacecraft and collecting a great team of engineers and scientists to staff it, but it’s still a bet every time you try something new.”

Even if a touchdown on 1998 KY26 ultimately proves impossible and Hayabusa2# simply flies on by, asteroid scientists will still gain valuable information about an incredibly common but hard-to-spot type of small asteroid.

“We have never seen a ten-metre-size asteroid in situ, so we don’t really know what to expect and how it will look,” Santana-Ros wrote.

“In many ways, a spacecraft visit to it now is even more exciting than it was before,” Kareta said.

—Kimberly M. S. Cartier (@astrokimcartier.bsky.social), Staff Writer

Citation: Cartier, K. M. S. (2025), Hayabusa2’s final target is 3 times smaller than we thought, Eos, 106, https://doi.org/10.1029/2025EO250353. Published on 18 September 2025. Text © 2025. AGU. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

In May, a landslide above Blatten in the canton of Valais buried most of the village under a mass of ice, mud and rock, an event that has prompted in-depth research. At a recent conference in Innsbruck, UZH researcher Christian Huggel presented his findings on the link between the landslide and climate change.

Source: AGU Advances

Streamflow drought—when substantially less water than usual moves through rivers—can seriously disrupt the welfare of nearby communities, agriculture, and economies. Synchronous drought, in which multiple river basins experience drought simultaneously, can be even more severe and far-reaching.

Recent observations and modeling suggest that on the Indian subcontinent, where major rivers support more than 2 billion people, the likelihood of synchronous drought is increasing as summer monsoons weaken, the Indian Ocean warms, and anthropogenic emissions and excessive groundwater pumping continue. However, little is known about the long-term patterns of synchronous drought in India, in part because streamflow data don’t offer information about the distant past.

By combining several decades of streamflow measurements from 45 gauge stations along India’s major rivers with high-resolution temperature and precipitation data and data from a range of paleoclimate proxies, Chuphal and Mishra have now reconstructed streamflow records across more than 800 years.

To look farther back in time, the researchers turned to the Monsoon Asia Drought Atlas, which comprises tree ring data indicating summer drought conditions across Asia between 1200 and 2012. They also considered historical records of climate patterns like El Niño, the Pacific Decadal Oscillation, and the Indian Ocean Dipole to explore connections among drought frequency, reoccurrence, and synchronicity. And they used two models from the Paleoclimate Modeling Intercomparison Project Phase 4 (PMIP4) that are part of the Coupled Model Intercomparison Project Phase 6 (CMIP6) to simulate precipitation and temperature data, as well as a hydrological model to simulate streamflow from 1200 to 2012.

With all this information, the researchers created their own reconstruction model that captured historical droughts driven by monsoon failures and connected low river levels to periods of drought-induced famine. Their findings revealed an increased frequency in synchronous drought between 1850 and 2014 compared with preindustrial centuries—an increase they surmise was likely caused by the warming climate. The researchers also suggest that future synchronous droughts may threaten water security throughout India. (AGU Advances, https://doi.org/10.1029/2025AV001850, 2025)

—Rebecca Owen (@beccapox.bsky.social), Science Writer

Citation: Owen, R. (2025), Droughts sync up as the climate changes, Eos, 106, https://doi.org/10.1029/2025EO250324. Published on 18 September 2025. Text © 2025. AGU. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

In recent years, the North American beaver (Castor canadensis) has been increasingly recognized as a valuable on-site engineer to help communities meet water management goals. Beavers are famously “eager” to build dams, which slow the flow of streams and allow wetland areas to grow.

Until now, however, land managers didn’t have a way to estimate how much water beaver reintroduction could actually bring to a habitat. Not every beaver dam results in a sprawling ponded complex; sometimes they result in smaller areas with less water retention than meets the needs of the community.

In a study published last month in Communications Earth and Environment, researchers from Stanford University and the University of Minnesota were able to link the amount of surface water in beaver ponds across the western United States to the features in those landscapes that make beaver ponds bigger.

Big, Beautiful…Beaver Ponds

Oftentimes, beavers will chain together multiple dams and ponds to form beaver pond complexes. The complexes increase an area’s water retention, cool water temperatures, and provide natural firebreaks. These wetland habitats also give the semiaquatic rodents ample room to roam and allow other species (such as amphibians, fish, and aquatic insects) to flourish.

Beaver pond complexes like the one in Happy Jack Recreation Area create habitat for wetland creatures big and small, like this (very large) moose. Credit: Emily Fairfax

“Our models highlight the landscape settings where ponds grow largest, helping target nature-based solutions under climate stress.”

The advantages of beaver pond complexes aren’t going unnoticed—the reintroduction of beavers to the North American landscape is an increasingly popular strategy for land managers looking to naturally improve a waterway.

“Managers need to know where beaver activity—or beaver-like restoration—will store the most water and maximize the environmental benefits, such as providing cooling and enhancing habitat quality” said Luwen Wan, a postdoctoral scholar at Stanford and the new study’s lead author. “Our models highlight the landscape settings where ponds grow largest, helping target nature-based solutions under climate stress.”

While improving water retention is a goal of many watershed management projects, especially in the increasingly drought-prone western United States, the researchers also emphasized that creating the largest possible ponds might not be the right solution for every area.

“It’s worth thinking about what we are actually asking of these beavers, and is that reasonable?”

“Bigger ponds are not always better,” said Emily Fairfax, coauthor on the study and assistant professor at the University of Minnesota. Fairfax explained that larger ponds are great for when the goal of the project involves water retention, but smaller ponds could be a better fit for a project in which the goals are pollution removal or increasing biodiversity. “It’s worth thinking about what we are actually asking of these beavers, and is that reasonable?”

How to Design a Dream Stream

Speaking on the main findings of the study, Wan said that she and her colleagues “found a clear link between the total length of beaver dams and the size of the ponds they create.” Additionally, they observed that the biggest ponds were found “where dams are longer, stream power is lower to moderate, and woody vegetation is of moderate [6–23 feet, or 2–7 meters] height.”

Included in the study were 87 beaver pond complexes across the western United States, encompassing almost 2,000 dams. Using high-resolution aerial imagery from the National Agriculture Imagery Program (NAIP), the team was able to connect the observed ponded area to different landscape measurements like soil characteristics, stream slope, vegetation metrics, and more.

The researchers chose NAIP imagery for its high spatial resolution and ability to cover large areas (visiting every beaver pond in the field would take too much time). Wan noted that while NAIP aerial imagery was the right fit for this project, it isn’t perfectly beaver proof. The imagery is updated every 2–3 years during the growing season, which may introduce some errors, like missing ponds even when dams have already been constructed.

Using remote sensing to predict where beaver reintroduction would be a successful match to the needs of a watershed isn’t a new idea. One frequently used model mentioned in the study is the Beaver Restoration Assessment Tool (BRAT). BRAT allows researchers to identify how many dams a given stream would likely be able to host. “That’s really important information to have,” said Fairfax, “but that doesn’t tell us how big the dams are, or how much water they could be storing.”

When Beavers Aren’t Best

Findings from this study are also helpful when selecting sites for beaver dam analogs (BDAs). These human-made structures are alternatives to beaver reintroduction that mimic beaver dams to achieve the same ecosystem benefits the beavers would bring. They are often the right tool when a waterway is too degraded to host a beaver population.

BDAs raise water levels and allow the preferred foods of beavers (such as willows and alders) to take root, giving “a little push” to the process of reestablishing a beaver population, explained fluvial geomorphologist and associate professor Lina Polvi Sjöberg from Umeå University in Sweden. Polvi was not involved in the new study.

Fairfax added that BDAs are a useful tool but are not equivalent to actual beaver dams. With beaver dams, a living animal is always present, so the land managers can count on the “maintenance staff on-site” to constantly update and monitor the waterway.

The Beavers Are Back in Town

North American beaver populations are still on the rebound from a long history of trapping and habitat loss that came with European colonization of the continent. “We are at maybe 10% of the historic population, and we actually don’t know if it’s still growing,” Fairfax said. Modern threats to beaver populations include highways and man-made dams, she added, which prevent beavers from freely moving back to places they once were.

Not everyone is quick to welcome North America’s largest rodent back to their neighborhood with open arms. Though public perceptions of beavers are shifting from pest to watershed management partner, the potential for contention still remains. Beavers occasionally build their dams in less-than-ideal locations, a situation that can result in flooded private properties and damaged infrastructure. The study notes that human influence (like trapping and land use conflicts) is a factor that land managers must consider but is not captured in statistical models.

Beavers Worldwide

The researchers found what makes beaver dams bigger in the western United States, but scientists say it will be important to replicate this study in different regions of North America, especially as beaver habitat expands northward as a result of climate warming.

“North American beavers are all one species, Castor canadensis. A beaver in Arizona is the same species as a beaver in Alaska. They all have the same instincts,” said Fairfax, “but beavers also do learn and adapt to their environments pretty strongly.”

She added that beavers will use the materials available to them, such as a colony in Yukon, Canada, that has been observed using rocks as dam-building material. “Whenever we build a model that describes what beavers are doing, there is a chance that it’s going to have a strong geospatial component to it,” Fairfax said.

Polvi agreed, stating that she hadn’t seen many studies using remote sensing methods to estimate the suitability of a stream for beaver reintroduction outside of the western United States. Putting things into a wider perspective, she added that some defining features of the American West, like the semiarid climate and large expanses of undeveloped public land, aren’t applicable to other regions of the world.

In an email, Wan said the next steps from this study include further exploring beavers’ ponded complexes across larger areas and “quantifying the ecosystem services these ponds provide, such as enhancing drought resilience.”

—Mack Baysinger (@mack-baysinger.bsky.social), Science Writer

Citation: Baysinger, M. (2025), What makes beaver ponds bigger?, Eos, 106, https://doi.org/10.1029/2025EO250341. Published on 18 September 2025. Text © 2025. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

Source: Global Biogeochemical Cycles

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

多年来，海洋学家们一直困惑于为何算法会在南极偏远海域的卫星图像中检测到神秘的高浓度颗粒无机碳 (PIC)。在其他地区，高PIC是单细胞浮游植物大量繁殖的标志，这种浮游植物被称为颗石藻(coccolithophores)，这些植物闪亮的碳酸钙外壳会将光线反射回卫星。然而，长期以来人们一直认为这些极地水域温度过低，不适合颗石藻生长。

如今，得益于 Balch 等人的最新船载测量数据，这个谜团终于解开了。他们发现了一种名为硅藻(diatoms)的不同类型的浮游植物，当硅藻的反射性硅质外壳（或称硅藻壳，frustules）浓度极高时，其反射率可以模拟 PIC 的反射率。这种反射率可能导致卫星算法将这些遥远的南部海域错误地归类为高 PIC 区域。

同一研究团队此前的船上观测已证实，来自颗石藻的PIC是大方解石带的成因——大方解石带是一个巨大的、季节性的、反射性的水环，环绕南极洲北部较温暖的水域。然而，在更南端，南极大陆周围异常明亮的区域仍然无法解释，推测的成因包括松散的冰块、气泡或反射性的冰川“粉”（被侵蚀的岩石颗粒）被释放到海洋中。

研究人员乘坐R/V Roger Revelle号从夏威夷向南航行，进入较少被探索的水域，这里以冰山和波涛汹涌的大海而闻名。他们测量了PIC和二氧化硅的含量，确定了光合作用速率，进行了光学测量，并在显微镜下观察了微生物。这些数据表明，这些偏远地区的高反射率主要是由硅藻壳引起的。

然而，研究人员也惊讶地发现极地水域中有一些颗石藻，这表明这些浮游植物可以在比以前想象的更冷的海水中生存。

由于颗石藻和硅藻在海洋碳固定中都发挥着重要作用，因此这些发现可能对地球的碳循环具有重要意义。研究人员表示，这项研究还可以为改进卫星算法提供参考，以便更好地区分PIC和硅藻壳。(Global Biogeochemical Cycles, https://doi.org/10.1029/2024GB008457, 2025)

—科学撰稿人Sarah Stanley

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

Read this article on WeChat. 在微信上阅读本文。

Text © 2025. AGU. CC BY-NC-ND 3.0
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Editors’ Vox is a blog from AGU’s Publications Department.

Today, in Eos, American Geophysical Union (AGU) Publications recognizes a number of outstanding reviewers for their work in 2024, as selected by the editors of each journal.

Peer review is a crucial component of the scientific process and is vital for promoting clarity and accuracy in how science is communicated.

Peer review is a crucial component of the scientific process and is vital for promoting clarity and accuracy in how science is communicated. In an era with so many ways to share ideas and research, a healthy and thriving system of peer review ensures that we encourage clear communication and maintain the highest integrity in our scientific publications. At AGU, the peer review process is conducted by scientists, starting with the journal editors. It is then the peer reviewers who take time away from their own research to volunteer time and expertise to help other scientists improve their articles and to aid publication decisions. The work of these colleagues ensures that thousands of articles each year receive independent feedback as part of a robust process of consideration and evaluation for publication. We are thankful for their efforts to make our science stronger.

Discoveries and solutions in the Earth and space sciences rely on increasingly complex approaches and datasets reflected in the papers that share their results. Peer reviewers bring their substantial expertise to evaluate detailed and intricate science conducted by teams of researchers large and small. Reviewers must assess insights gleaned from studies utilizing more and new techniques, data, and simulations that increase in scale and scope each year. As a result, both the value and challenge of peer reviewing keeps growing. Science benefits when our community rises up to support the opportunities afforded by the work reported in AGU journals by providing thoughtful and insightful feedback through peer review.

The outstanding reviewers listed here have provided in-depth, valuable, and timely feedback and evaluations, often through multiple revisions, and multiple manuscripts, that have led to clearer and greatly improved final published papers. Their contributions helped raise the quality of submissions received from around the world, delivering valuable feedback that makes for better scientific discourse.

Many Reviewers: A Key Part of AGU Journals

While we recognize these few outstanding reviewers, we also must acknowledge the incredible service to the community by all the researchers who have conducted reviews to help ensure the quality, timeliness, and reputation of AGU journals. We also welcome new and first-time reviewers who have joined the family of community servants who act as integrity stewards and have been providing authors with valuable feedback to improve their science and communication. In 2024, AGU received over 20,000 submissions, which was a significant increase from 2023, and published 7,517 papers. Most submissions were reviewed multiple times—in all, 17,947 reviewers completed 44,656 reviews in 2024.

The past several years continued to be a rollercoaster for researchers, editors, and peer reviewers. The challenges of maintaining the peer review system remain at an all-time high. Volunteer reviewers in Europe and the United States receive more invitations than they can accept, while research output in China is now the highest of any country. AGU journals continue to make progress in balancing the efforts of colleagues serving our community via conducting peer reviews even as they often struggle to invite a proportional number of reviewers across the globe. Likewise, early career researchers observe some of their more senior colleagues being overburdened by invitations and wonder why they receive so few invitations themselves. AGU is committed to building further entrance points to peer reviewing including its co-reviewing program and peer reviewing programs in individual journals.

Reviewers play a central role in the rapid feedback and publishing of new science that is at the heart of advancing the Earth and space sciences.

Amidst these challenges, each AGU journal worked to maintain low time frames from submission to first decision and publication, and consistently maintained industry-leading standards. Reviewers play a central role in the rapid feedback and publishing of new science that is at the heart of advancing the Earth and space sciences.

Editorials in each journal express our appreciation along with reviewer recognition lists. Our thanks are a small acknowledgment of the large service that reviewers bear in improving our science and its role in society.

Additional Thanks

In addition, we are working to highlight the valuable role of reviewers through events at AGU’s Annual Meeting and other meetings.

We will continue to work with the Open Researcher and Contributor ID (ORCID) network to provide official recognition of reviewers’ efforts, so that reviewers receive formal credit there. As of 10 July 2025, we have over 116,000 ORCIDs up from 100,000 ORCIDs one year ago.

Getting Your Feedback

We value your feedback, including ideas about how we can recognize your efforts even more, improve your experience, and increase your input on the science. Feel free to send your comments to publications@agu.org. We look forward to hearing from you!

Once again: Thanks to our Outstanding Reviewers of 2024!

—Matt Giampoala (mgiampoala@agu.org, 0000-0002-0208-2738), Vice President, Publications, American Geophysical Union; and Steven A. Hauck II (0000-0001-8245-146X), Chair, AGU Publications Committee

Citation: Giampoala, M., and S. A. Hauck II (2025), In appreciation of AGU’s outstanding reviewers of 2024, Eos, 106, https://doi.org/10.1029/2025EO255029. Published on 18 September 2025. This article does not represent the opinion of AGU, Eos, or any of its affiliates. It is solely the opinion of the author(s). Text © 2025. The authors. CC BY-NC-ND 3.0
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Editors’ Highlights are summaries of recent papers by AGU’s journal editors. Source: Journal of Geophysical Research: Planets

In four years, NASA’s Psyche mission will arrive at asteroid 16 Psyche, a mysterious metallic-type (or M-type) asteroid that will be the first of its kind to be visited by a space mission. Observations of 16 Psyche’s surface suggest that it is highly metal-rich, but the bulk density of the asteroid is inconsistent with being totally made of metal. There are several hypotheses for Psyche’s origin and metallic spectra, including ferrovolcanism, which hypothesizes that metallic melts are squeezed out of the crystallizing core of the asteroid and erupt as lava flows on the surface.

The model adopts a primitive meteorite bulk composition and determines the composition and density of different internal layers of the asteroid. The metal core crystallizes from the outside moving inward (solid Fe+FeNi layer). The subsequent build-up of pressure in the liquid Fe-S layer may be high enough to allow it to erupt outward to the surface. Credit: Jorritsma and van Westrenen [2025], Figure 3

Jorritsma and van Westrenen [2025]  are the first scientists to look at whether ferrovolcanism is actually possible given what we know of the meteoritic precursors for Psyche’s composition. By calculating core sizes, compositions, and densities for different meteorite types the scientists calculate buoyancy forces and excess pressures to determine if the metallic liquid would be mobile enough to produce ferrovolcanism.

They found that some meteoritic compositions could produce ferrovolcanism while others could not. If NASA’s Psyche mission finds evidence of ferrovolcanism on the asteroid’s surface, these models can help constrain its early history and composition.

Citation: Jorritsma, J. J., & van Westrenen, W. (2025). Constraints on the feasibility of ferrovolcanism on asteroid 16 Psyche. Journal of Geophysical Research: Planets, 130, e2024JE008811. https://doi.org/10.1029/2024JE008811

—Laura Schaefer, Associate Editor, JGR: Planets

Text © 2025. The authors. CC BY-NC-ND 3.0
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Planet satellite imagery has started to reveal the events that killed 70 people at in northern India. It clearly indicates that intense rainfall triggered landslides that transitioned into channelised debris flows.

On 5 August 2025, a series of terrifying landslides struck Dharali in Uttarkashi, Uttarakhand, northern India. I blogged about this at the time – the Wikipedia page for the disaster indicates that about 70 people were killed.

In the aftermath of the disaster, there was some very strange speculation about the cause. For example, the Times of India quotes an expert from the India Meteorological Department as follows:

“Only very light to light rain was observed in the affected area over 24 hours. The highest rainfall recorded anywhere in Uttarkashi was merely 27mm at the district headquarters.”

They quote another scientist as follows:

“This volume is insufficient to trigger floods of such severity, suggesting a powerful event such as a glacier burst or a GLOF.”

They also quote a senior geologist as follows:

“Such disasters occur when water accumulates at higher elevations and discharges suddenly … Heavy rainfall alone can’t cause such a disaster.”

This is of course a nonsense. Intense rainfall is highly spatially variable, especially in high mountains. Take a look at this “Black Rainfall” event in Hong Kong (which was not in a high mountain area, where the events are even more extreme, but Hong Kong has a fantastic rain gauge network):-

Hong Kong Observatory data showing the distribution of precipitation in a “Black rainfall” event in 2020. Credit: Hong Kong Observatory

The western and southern parts of Lantau island received less than 40 mm of rainfall that day, whilst the area around Sha Tin received over 400 mm. The distance between the boundary of >400 mm and <40 mm is a few kilometres.

Cloudburst precipitation is incredibly spatially variable, so it is not the case that a failure to record heavy rainfall on a sparse rain gauge network means that such events did not occur. And, as I have shown on numerous occasions on this blog, intense rainfall most certainly can, and does, trigger catastrophic debris flows.

However, there was a stronger piece of evidence that suggests that this event was not a glacier burst or a GLOF. On 5 August 2025, the debris flows occurred in two separate, but adjacent catchments, one at Dharali but another at Harshil, about 500 m to the west. These two catchments are not connected, but they are separated by a ridge. This would suggest a common trigger – in the absence of an earthquake, a localised rainfall event is highly likely (especially in the peak of the monsoon).

Of course, to be sure we need either fieldwork or satellite imagery. In the rainy season, this is really hard to achieve but as the monsoon withdraws this becomes possible. And sure enough, on 11 September 2025, Planet captured a fabulous satellite image that starts to reveal the story.

So let’s start with Harshil – the picture here is straightforward. The slider below shows Harshil and the lower part of the catchment using Planet imagery draped onto the Google Earth DEM. Note that the change in the topography is not captured in the DEM.

A comparison of Harshil before and after the 5 August 2025 Dharali disaster. Images by Planet, using the Google Earth DEM.

The path of the debris flow is clear, and upstream there is a very obvious large landslide. Thus, the most likely cause of the Harshil debris flow is that slope failure.

The image below shows the landslide itself, but again note that the topography in the DEM has not updated, so the morphology is distorted. But this is clearly a large rock slope failure.

I will caveat to say that at this stage we cannot definitively say that this landslide occurred on 5 August 2025, but Occam’s razor implies that this is the cause.

The landslide upstream from Harshil after the 5 August 2025 Dharali disaster. Image copyright Planet, used with permission. Image dated 11 September 2025.

This slope failure has released a huge amount of sediment into the catchment. This will cause problems for Harshil in the future.

So now let’s turn to Dharali itself. This is more complex. We have to go high up into the catchment above the village to see what’s happened. The slider below shows before and after Planet images of the upper part of the catchment above Dharali:-

Planet images showing the upper part of the catchment above Dharali before and after the 5 August 2025 disaster. Images by Planet using the Google Earth DEM.

There has been substantial change on both sides of the catchment. Let’s start with the west (the right side of the image above as the image is from the south looking towards the north):-

The west side of the upper part of the catchment above Dharali after the 5 August 2025 disaster. Image copyright Planet, used with permission. Image copyright Planet, used with permission. Image dated 11 September 2025.

The juxtaposition of the sediments lower down the valley suggests to me that this was the cause of the first (most catastrophic) debris flow at Dharali. The image strongly indicates that this was a channelised debris flow initiated by a landslide in glacial sediments in the upper part of the catchment. There was a huge amount of entrainment of channel sediments downstream. Note that there is a pre-existing, smaller, landslide at this site.

Thus, the major debris flow at Dharali was probably initiated by a shallow landslide that transitioned into a channelised debris flow.

Events on the east side of the catchment are much less clear. I suspect that the processes here generated at least some of the smaller debris flows that struck Dharali after the first event. This is the Planet image for this side of the valley:-

The east side of the upper part of the catchment above Dharali after the 5 August 2025 disaster. Image copyright Planet, used with permission. Image copyright Planet, used with permission. Image dated 11 September 2025.

The changes to the fan in the centre of the image are notable, but what caused this? I am not entirely sure. A closer comparison of the glacial sediment above this fan looks like this:-

Planet images showing the upper part of the catchment above Dharali before and after the 5 August 2025 disaster. Images by Planet using the Google Earth DEM.

In the foreground is a trough that has suffered extensive erosion. But there is also major change in the slopes above that trough. I have a suspicion that the feature that I have highlighted below might be quite significant:-

The upper eastern part of the catchment above Dharali before the 5 August 2025 disaster. Image copyright Planet, used with permission. Image dated 11 September 2025.

It appears to me that a landslide has occurred on this area of glacial sediment. Did this generate one or more channelised debris flows? There are other changes too – so maybe there were repeated shallow failures that generated the smaller debris flows observed at Dharali?

In conclusion, we can say:

1. The Dharali disaster was caused by rainfall. There is no other credible trigger that explains simultaneous events in two separate, adjacent, catchments.
2. Given the rain gauge record, this must have been a highly localised, cloudburst event.
3. Harshil was destroyed by a debris flow that probably originated from a rock slope failure in the catchment above the village.
4. The first Dharali debris flow was probably triggered by a rainfall triggered shallow landslide in glacial deposits in the upper part of the catchment, which transitioned into a channelised debris flow.
5. The subsequent debris flows at Dharali were caused by other erosive events in these glacial tills. The origin is not entirely clear, but there is evidence of at least one further shallow landslide that may have been the origin for one or more events.

This is a very provisional analysis based on lower resolution (but still brilliant) imagery. These hypotheses need to be explored through fieldwork and/or high resolution imagery.

Finally, it is absolutely inevitable that the fans at Dharali and Harshil will suffer similar events in the future. Neither will be safe for habitation.

Reference and acknowledgement

Thanks to loyal reader Jack for really interesting and stimulating discussions about these events. And special thanks to Planet for the astonishing imagery.

Planet Team (2025). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://www.planet.com/

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SummaryThe Xiangshan volcanic basin locates in southeast China hosts the world’s third-largest volcanogenic uranium deposit. However, the structure of the volcanic system remains poorly resolved, limiting insights into the uranium mineralization. To address this, we conducted a joint inversion of gravity and magnetic data collected in the basin. Our inversion results reveal a southeast-dipping porphyroclastic lava conduit beneath the peak of Mount Xiangshan, characterized by low density and high magnetic susceptibility. A southwest-dipping volcanic conduit has also been identified beneath the rhyodacite crater in the Shutang area of the western basin. It connects to the porphyroclastic lava conduit in the deep. Both of these volcanic conduits are controlled by an EW-trending, low-density basement fault zone. This spatial relationship indicates that the volcanic eruptions in the western basin share a common subvolcanic plumbing system, which collectively acted as principal pathways for ore‑forming hydrothermal fluids and uranium enrichment. These results underscore the role of volcanic-intrusive architecture in controlling the mineralization processes in the Xiangshan volcanic basin.

SummaryThe South American continent (SAC), a region of pronounced geodynamic and hydrological activity, exhibits crustal deformation and gravity field anomalies driven by the interplay of tectonic forces and surface/subsurface mass redistribution. While previous studies have mainly focused on gravity changes driven by terrestrial water storage (TWS), mass variations of the solid Earth remain inadequately addressed. In this study, we resolve deep-seated mass transport Gravity Recovery and Climate Experiment (GRACE) satellite gravimetry, hydrological model outputs, GPS-derived vertical crustal motions, and glacial isostatic adjustment (GIA) correction. Our results reveal an internal mass variation of 0.21 ± 0.45 cm yr -1 in equivalent water height (EWH), independent of surface hydrological contributions. Interpreting this signal as predominantly driven by crust-mantle boundary (Moho) displacements, we estimate an average Moho depth uplift rate of 0.37 ± 0.80 cm yr -1 across SAC, based on the crust–mantle density contrast. The Moho interface depth variations exhibit significant spatial heterogeneity. Through uncertainty analysis, four distinct regions (A, B, C, and D) are identified: Region A exhibits Moho uplift and Region B exhibits subsidence, with part contributions from the isostatic adjustment. Key uncertainties in these estimates stem from sedimentation effects and the accuracy of current observations or models. Subsidence in Region C and uplift in Region D are related to the co-seismic and post-seismic effects of the 2010 Chile earthquake. These findings underscore the significance of solid Earth mass flux in active continental regions and unravel the mechanisms governing crust-Moho mass redistribution.

SummaryRecent advancements in high-resolution Digital Elevation Models (DEMs) derived from LIDAR and satellite radar technologies have added a new dimension to the determination of height systems and geoid models. However, their benefits are limited by simplified assumptions inherited from past practices. In mountainous areas, taking into consideration of topography as the Bouguer plate or employing inaccurate terrain corrections can constitute to a problematic approach. Even though the gravity reduction procedures mentioned above have been enhanced in geoid determination studies, the Helmert orthometric heights based on them are still used in some countries such as Türkiye and Taiwan. It is inevitable that this contradiction will negatively affect geoid modeling studies that are intended to be verified or combined with GNSS/leveling data. Another issue arises by ignoring density variations of topographic masses. Through a comparative analysis, this study reviews combined and individual impacts of terrain roughness and density variations on geoid models in the Konya Closed Basin (KCB) and the Auvergne regions, with a focus on their distinctive topographical characteristics. Using 1″ DEMs of the SRTM mission and 30″ UNB_TopoDensT lateral density models, we reveal that terrain corrections in gravity reductions significantly affect geoid heights, with deviations of up to 11.9 cm in KCB and 4.2 cm in Auvergne. Incorporating lateral density models has resulted in geoid height discrepancies of up to 26.8 cm in KCB and 6.7 cm in Auvergne. A validation strategy implemented through GNSS/leveling paths showed that terrain corrections markedly improved geoid model accuracy, particularly in relation to elevation. However, the contribution of the UNB_TopoDensT model to geoid accuracy is questionable in terms of accuracy. Notably, applying density values below 2.4 g·cm⁻³ in high-altitude regions can lead to disruptive effects on geoid determination. This result is underscoring of the need on a realistic modeling of topographical densities in high elevated and rugged terrains. A further conclusion that emerged from these analyses is that gravimetric geoid models should be verified by rigorous orthometric heights, which are observed to fit them better at the 1-2 cm level, instead of the Helmert orthometric heights.

AbstractThis study introduces a novel method for performing 3D inversion of magnetotelluric (MT) data. The proposed method, referred to as the radiation boundary scheme, employs a two-step simulation strategy for the computation of both forward and adjoint responses. One of the key advantages of the scheme is its ability to handle arbitrarily shaped inversion domains, thereby optimizing the number of unknown model parameters by discarding model parameters that are not constrained by the data. Consequently, it significantly improves accuracy and computational speed as compared to traditional inversion algorithms. The effectiveness of the developed algorithm is demonstrated through a comprehensive analysis of 3D inversion using synthetic and continental-scale (SAMTEX) MT data. The method’s efficiency facilitates a detailed analysis of large-scale MT data inversion. Through numerical experiments, it is observed that using a coarse mesh for inversion, the resolution is compromised in the shallower part, resulting in inferior imaging and, consequently, affecting the estimation of resistivity value in the deeper subsurface. The detailed numerical experiments indicate that performing a fine-scale inversion on a small portion of the survey data utilizing a coarsely inverted model may run into a local minimum. Hence, caution should be exercised in employing such an approach. Instead, the investigations suggest simultaneously executing a fine-scale inversion on the entire data set. The forward/adjoint problem can be solved with a two-order higher tolerance as compared to the conventional finite-difference-based inversion algorithm. Therefore, the proposed algorithm holds significant value for the MT inversion of large data sets.

Publication date: 15 September 2025

Source: Advances in Space Research, Volume 76, Issue 6

Author(s): Polina Kuzmenkova, Anna Dmitrieva, Ilya Lagoida, Polina Sukhova, Victor Shutenko, Ivan Astapov

Publication date: 15 September 2025

Source: Advances in Space Research, Volume 76, Issue 6

Author(s): Huseyin Cavus, Gani Caglar Coban, Haimin Wang, Abd-ur Raheem, Jason T.L. Wang, Mahboubeh Asgari-Targhi

Publication date: 15 September 2025

Source: Advances in Space Research, Volume 76, Issue 6

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Publication date: 15 September 2025

Source: Advances in Space Research, Volume 76, Issue 6

Author(s): Jing Li, Yongtao Bai, Sifeng Bi, Michael Beer

Publication date: 15 September 2025

Source: Advances in Space Research, Volume 76, Issue 6

Author(s): Amitabh Nag, Erin H. Lay, Brian A. Larsen, Megan D. Mark, Philip A. Fernandes, Alex R. Attanasio

Publication date: 15 September 2025

Source: Advances in Space Research, Volume 76, Issue 6

Author(s): Su Zhou, Lingmin Wang, Zongxian Wu, Zhijin Zhou, R. Selvakumaran, Sneha A. Gokani

Publication date: 15 September 2025

Source: Advances in Space Research, Volume 76, Issue 6

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