Thin layers of sedimentary rock in Mars’s Gale Crater suggest that the planet once had a moon much larger than the two that orbit it today, according to work to be presented at AGU’s Annual Meeting 2025 on 17 December. Unlike the current Martian moons Phobos and Deimos, the gravitational pull of the hypothesized moon would have been strong enough to create tides in bodies of water on or below the planet’s surface.
The team analyzed images from cameras on the Curiosity rover, which has been trundling across Gale Crater since 2012. The Mars Hand Lens Imager, for instance, captures images with resolutions up to 13.9 micrometers per pixel.
Pictures of a rocky outcrop snapped during four Martian days in late 2017 and early 2018 revealed a section of fine, repeating layers in alternating light and dark colors. The researchers interpret those layers as tidal rhythmites, or sediments deposited by the regular back-and-forth sloshing of the tides.
“Our study provides sedimentary evidence for the case of tidally deposited rhythmites, hinting at a past larger moon for Mars.”
“Our study provides sedimentary evidence for the case of tidally deposited rhythmites, hinting at a past larger moon for Mars,” Ranjan Sarkar, a planetary scientist at the Max Planck Institute for Solar System Research in Gottingen, Germany, told Eos via email. “This, in turn, aligns with the hypothesis that Mars has repeatedly had larger moons that were tidally destroyed into rings, which then reformed into successively smaller moons.” That is, the larger moon or moons would have been pulled apart by the force of Martian gravity, which would have exerted a stronger pull on the planet-facing side of the moon than the opposite side.
The layering was detected at Vera Rubin Ridge on the flank of Mount Sharp, a sedimentary peak in the middle of Gale Crater. The studied area was about 35 centimeters long and 20 centimeters thick. Individual bands in the rock ranged from submillimeters to millimeters thick, with wider, light-toned bands and darker, thinner bands.
This graphic, for presentation at AGU’s Annual Meeting 2025, traces Curiosity’s path to the Jura outcrop on Vera Rubin Ridge. Color-enhanced images from the rover show the layered rocks interpreted as evidence of tidal rhythmites, with similar layers in an Earth setting shown for comparison. Click image for larger version. Credit: Ranjan Sarkar, Priyabrata Das, Suniti Karunatillake
Comparison with other observations along the ridge suggests the layers were deposited roughly 3.8 billion years ago, when Gale Crater contained a lake.
“Back-of-the-Envelope” Profile
Not all rhythmites are tidal: Similar sedimentary layers can be deposited by winds, seasonal variations in precipitation or glacier melts, or other processes, the researchers note.
“The finely laminated rhythmites in this crater are most likely varves, or deposits that reflect seasonal changes in the climate,” said Bob Craddock, a geologist at the National Air and Space Museum who was not involved in the study. More water flows into a lake during the warmer summer months, producing thicker sediment layers with larger grains compared to those laid during winter, he said. “As this continues through time, you get rhythmites.”
“It’s very tricky. We can’t be decisive, so our argument is one of consistency.”
Sarkar, however, said the structure of these layers doesn’t match what would be expected of seasonal deposits. “Annual varves usually show simple light-dark couplets, but we observe alternating thick-thin bands showing paired dark laminae,” he said. Such patterns “are commonly used as markers of tidal sedimentary signatures on Earth.”
“It’s very tricky,” said team member Suniti Karunatillake, a geologist and geophysicist at Louisiana State University. “We can’t be decisive, so our argument is one of consistency.…We felt that the observations are generally more consistent with a tidal setting.”
The layers probably were deposited with a “monthly” cycle of about 30 days, Karunatillake said. Even if Phobos or Deimos were much closer to Mars than they are today, neither is massive enough to create such a tidal cycle. Instead, combining this new work with modeling by previous researchers, the team estimated the tides were raised by a body at least 18 times the mass of Phobos, the larger moon, orbiting at an altitude of about 3 times the radius of Mars.
Phobos, photographed by the Mars Reconnaissance Orbiter, is not massive enough to have raised tides on Mars. It could be a remnant of a larger moon that was destroyed in a giant impact. Credit:
NASA/JPL-Caltech/University of Arizona
“That’s our back-of-the-envelope calculation,” Karunatillake said. “Anything smaller and it would be difficult to induce this type of tidal activity, especially when you consider that Gale Crater is quite small as a water body on the planetary scale.”
The possibility of a smaller moon causing the observed tidal activity might be more realistic, Karunatillake added, if there were a connection between Gale Crater and the northern ocean, but no connection has yet been seen. However, even a subterranean link, such as the network of flooded caves and tunnels beneath Earth’s Yucatán Peninsula that leads to the Caribbean Sea, would suffice. “There are instances where you get tidal variations inland, as long as there’s a subsurface connection with the ocean,” he said.
Pondering the Martian Moons
Planetary scientists have pondered the origins of Phobos and Deimos extensively in recent decades. The original theory said they were captured asteroids, but it’s not easy for a planet to nab even one asteroid, much less two.
Some studies have suggested that Mars originally had a larger moon—either a captured asteroid or one that formed from an early giant impact. That body then could have been pulverized by the gravity of Mars or by its own collision, forming a ring that then gave birth to smaller moons. In fact, such a scenario could have played out multiple times. “Our study provides actual (ground) evidence, from measured laminae periodicities, for the predicted/hypothesized past larger moon,” Sarkar said.
The researchers are considering conducting a detailed celestial mechanics study to refine their estimates of the mass, distance, and orbital period of the proposed moon. They’re also examining two other sites in Gale Crater that appear to show similar tidal rhythms.
Any inconsistencies among the sites would “dispute our model, and possibly falsify it,” Karunatillake said. “But any agreement would take us toward a stronger argument for an ancient large moon.”
—Damond Benningfield, Science Writer
Citation: Benningfield, D. (2025), Sediments hint at large ancient Martian moon,
Eos, 106, https://doi.org/10.1029/2025EO250434. Published on 20 November 2025.
Text © 2025. The authors.
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