A single 320-meter asteroid impact at a 45-degree angle created the southern depression and global dust blanket on Mars’s moon Deimos, according to a study published in Nature Astronomy. The research relies on high-resolution computer simulations and close-range flyby data from the European Space Agency’s Hera mission.
For decades, planetary scientists puzzled over the stark contrast between Mars’s two tiny moons. While Phobos is heavily scarred by impact craters and deep grooves, its outer sibling Deimos appears remarkably smooth and dust-covered, featuring a mysterious 10-kilometer-wide basin carved out near its southern pole. A newly published international study reveals that this youthful, padded appearance is the result of a single, non-destructive collision that managed to reshape the celestial body without destroying it entirely.
High-Performance Simulations and the Bern SPH Code
To solve the mystery of how the potato-shaped moon acquired its distinctive geography, researchers turned to advanced numerical modeling. The investigation was led by Sabina Raducan, who served as a researcher in the Division of Space Research and Planetary Sciences at the Physics Institute at the University of Bern until October 2025 before becoming a science program manager at the International Space Science Institute and a senior fellow at Vrije Universiteit Brussel. The team collaborated with institutions including the Observatoire de la Côte d’Azur, the University of Arizona, and the University of Tokyo.
The team utilized a specialized computational framework developed over two decades at the Swiss university. The code runs on a high-performance computing cluster here at the University of Bern and is one of the few codes capable of performing this type of simulation,
explained Sabina Raducan, who also co-chairs the Hera Impact Physics Working Group for the European Space Agency.
“In this case we created a detailed shape model of Deimos out of SPH particles, having filled in the southern depression to leave it ready for the impactor to strike. Then we ran about a hundred simulations – each one taking about a week at a time to complete – to experiment with various impactor masses and angles of approach.”
Sabina Raducan, University of Bern, International Space Science Institute, and Vrije Universiteit Brussel
The modeling work relies on the Bern Smoothed Particle Hydrodynamics code, which breaks down colliding bodies into millions of interacting particles governed by variables like gravity, material strength, and density. The best-fit model pointed to a slanting impact from an asteroid roughly 320 meters wide striking the moon at high speed at an angle of about 45 degrees.
Global Regolith Redistribution and the Hera Flyby
Instead of shattering the 12-kilometer-wide moon—which is roughly the size of Luxembourg city and orbits about 24,000 kilometers from Mars—the impact carved out the southern depression while launching massive plumes of debris across the surface. Much of this material fell back down to form a global regolith blanket exceeding 200 meters in depth in certain areas, effectively acting as a cosmetic filler that buried ancient surface scars.
Co-author Martin Jutzi of the University of Bern noted that the collision distributed material globally without crossing the threshold of complete fragmentation. The simulation is consistent with patterns of brightness linked to the gradual migration of loose, dust-like regolith rather than cohesive rock.
Crucially, the study represents the first scientific publication to incorporate observational data from the European Space Agency’s Hera mission. While en route to its primary target, the asteroid moon Dimorphos, Hera performed a gravity-assist flyby of Mars in March 2025, capturing close-range observations of Deimos. Sir Brian May, a member of the Hera science team, produced spectroscopic depictions of the flyby images that allowed researchers to identify additional buried craters that had escaped casual detection.
Rubb-Pile Interiors and Future Martian Exploration
The survival of these pre-impact craters beneath the heavy dust layer provides a clear window into the moon’s internal composition. A solid body would have resonated like a bell during such a violent collision, erasing older topological features through shockwaves. Because ancient outlines remain intact, researchers conclude that Deimos possesses a highly porous, fractured interior akin to a rubble-pile asteroid, which efficiently dampened the impact forces.
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Michael Kueppers, the European Space Agency’s Hera project scientist, pointed out that the simulation implies the moon shares structural similarities with many known asteroids. Whether Deimos formed as a captured asteroid or coalesced from debris ejected during ancient impacts on Mars remains an open question for planetary science.
Beyond answering a long-standing topographical puzzle, these findings establish critical baselines for upcoming missions. The impact models provide necessary predictions regarding regolith thickness, mechanical properties, and particle distribution for the Japan Aerospace Exploration Agency’s scheduled Martian Moons eXploration sample-return mission.
