A Single Asteroid Strike May Explain Two Mysteries of Mars' Moon Deimos

An international team led by the University of Bern has found evidence that a single asteroid collision may have reshaped Deimos, the smaller of Mars' two moons, explaining both its large southern depression and its unusually smooth, dusty surface. The study, led by Dr. Sabina Raducan and published in Nature Astronomy, is the first to use observations from a flyby of Deimos by ESA's Hera spacecraft, and could help guide future missions including Japan's Martian Moons eXploration (MMX).

Deimos has a prominent depression near its south pole, and looks strikingly different from Phobos, Mars' other moon: while Phobos is heavily cratered, Deimos appears smoother because loose dust and rocky debris, called regolith, covers much of it. Scientists were unsure what created either feature, so the team, joined by researchers from the Observatoire de la Côte d'Azur, the University of Arizona and the University of Tokyo, tested whether the two shared one origin.

Using the "Bern Smoothed Particle Hydrodynamics" code, previously used to model NASA's DART spacecraft crashing into the asteroid moon Dimorphos, the team ran roughly a hundred simulations, varying the impactor's size, speed and angle. The best match: an asteroid about 320 metres across striking Deimos at 45 degrees could have carved the southern depression while throwing out enough material to blanket the moon in regolith, in places more than 200 metres deep. "Our simulation thus shows that a single impact was sufficient to decisively shape the current landscape of Deimos," said co-author Martin Jutzi. "The impact was violent enough to redistribute material globally, but not so strong that it would have shattered the moon."

Comparing the simulations with data from Hera, which passed Mars in March 2025 en route to the asteroid moon Dimorphos and used the flyby to observe Deimos up close, also showed that Deimos' uppermost layers are exceptionally weak and its interior highly porous, a structure that would have absorbed some of the impact's energy and helped the moon survive. "In terms of its physical properties, Deimos more closely resembles the so-called rubble-pile asteroids than our Earth's Moon," Raducan said. "But that doesn't necessarily mean that Deimos is actually an asteroid. It could also have formed from material ejected during impacts on Mars."