The calm, smooth surface of Deimos may not be what it seems. A new study suggests that a single asteroid impact long ago may have reshaped much of the small moon, carving out a huge depression near its south pole and then scattering debris from the impact over a wide area of its surface.
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The significance of the result lies in the fact that it provides a single explanation for two features that have puzzled scientists, and it reveals information about the internal structure of Mars’ moon that may help in understanding its origin and guiding future missions to it.
The study was led by researchers from the University of Bern in Switzerland. They used a high-resolution computer simulation to reconstruct the possible collision and then compared it with new data captured by the European Space Agency's Hera spacecraft when it approached Dimos in March 2025, during a maneuver to take advantage of Mars' gravity to change its trajectory toward the asteroid Dimorphos.
The puzzle of the smooth surface
Deimos is the smaller and more distant of Mars' two moons. It is oval-shaped and has a distinct depression near its south pole. Unlike Phobos (Mars' other moon), Deimos appears smoother and less impacted because its surface is covered with large amounts of dust, fragmented rock, and debris—materials known as regolith.
For years, scientists were unsure how the southern depression formed, or where the vast amount of material covering the moon's surface came from. But the research team, led by Sabina Radukain, reconstructed dozens of scenarios using a simulation program developed by the University of Bern over nearly two decades to study asteroid, comet, and planetary collisions.
The team conducted approximately 100 simulations, each lasting about a week, varying the size of the impacting object, its speed, the angle of impact, and the internal structure of Deimos. The results showed that the scenario most consistent with the moon's shape assumes an asteroid with a diameter of about 320 meters impacting Deimos at an angle of approximately 45 degrees.
A violent collision, but it did not destroy the moon.
According to the model, the impact was powerful enough to carve out the southern depression and eject enormous amounts of material, which was then spread over large parts of Deimos. In some areas, the debris may have accumulated to a depth of more than 200 meters, obliterating much of the older surface features and giving the moon its current smooth appearance.
Martin Jutsi, a researcher in the Department of Space and Planetary Sciences at the University of Bern and co-chair of the collision physics group for the Hera mission, says the simulation shows that a single collision was enough to decisively shape the current landscape of Deimos. The impact was violent enough to redistribute material globally, but not powerful enough to shatter the moon.
But the researcher warns that this does not necessarily mean that Deimos is a former asteroid; the moon may also have formed from material ejected by massive impacts on the surface of Mars.
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Other hypotheses are still possible to explain the surface of Deimos and its southern low, but the strength of the new study lies in the fact that it presents a single scen
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