Mars’s outer moon Deimos owes both its massive 10-kilometre south-polar depression and its strangely smooth, dust-covered terrain to a single 320-metre asteroid impact, according to a study published on August 18, 2026, in Nature Astronomy. Researchers found the subcatastrophic collision redistributed material globally without shattering the porous body.
The Contradiction of Deimos and Its Smooth Surface
Mars has two moons, but neither resembles a typical rocky world. Phobos, the inner satellite, spans roughly 22 kilometres and is battered with heavy craters and deep grooves. Deimos, sitting further out, measures only about 12 kilometres across and presents a far more puzzling silhouette. Its terrain looks softly dusted over, marked by shallow old craters, muted plains, and bright streaks running downslope through thick loose material.
Yet that gentle appearance conceals a violent wound. A broad depression spans roughly 10 kilometres across the moon’s south pole, taking up a staggering portion of the tiny body’s diameter. For years, astronomers debated how a potato-shaped satellite could combine such a massive cavity with a surface so smooth it looked brushed by dust. Observations by the European Space Agency’s Hera spacecraft during a Mars gravity-assist maneuver in March 2025 offered a closer look, testing navigation systems on the lumpy moon and revealing ancient craters buried deep beneath layers of regolith.
Now, a newly published study in Nature Astronomy points to a single, elegant explanation for both features. An asteroid measuring roughly 320 metres across struck the south polar region at an oblique angle, excavating the enormous cavity while blanketing the entire moon in pulverized debris.
Simulating a Cataclysm on a Porous Rubble Pile
To test whether one impact could accomplish this dual feat, researchers built a detailed three-dimensional shape model of Deimos, computationally filling in the missing volume of the southern depression to recreate the pre-impact moon. Lead author Sabina Raducan of the University of Bern and her colleagues utilized the Bern Smoothed Particle Hydrodynamics (SPH) code running on a high-performance computing cluster at the University of Bern.
The SPH code models celestial bodies as millions of adhering particles whose interplay is governed by gravity levels, material strength, and cohesion. Because the code is among the few capable of handling such large surface deformations and violent disruptions, the team spent about a week on each simulation, running approximately a hundred scenarios with varied impactor masses and approach angles.
The numerical modeling required treating Deimos not as a solid boulder of rock, but as an exceptionally weak, porous body. Both the target and impactor were modeled using the Tillotson equation of state for basalt with a modified bulk modulus set two orders of magnitude lower than intact basalt, matching porous soil materials. Two independent comparisons converged on precise impact parameters: a 320-metre projectile arriving about 45 degrees from the local surface normal at an estimated average speed of 8.2 kilometres per second.
Global Regolith Blanket and Subcatastrophic Survival
The physics of an impact on such a small body differ drastically from those on Earth or Mars. When a tiny world is struck, gravity is too weak to capture everything immediately, but strong enough to prevent most material from escaping entirely. The team evolved each main simulation for eight hours, observing how the cavity opened and how ejecta moved.
In the best-fitting simulation, no more than roughly 1% of Deimos’s mass reached escape velocity. Between 10 and 20% of the displaced material was redistributed across the surface, forming a global blanket at least several metres thick and reaching up to 200 metres deep on the Mars-facing hemisphere. The gradual migration of this loosely packed dust effectively buried older impact scars, explaining why 14 craters wider than 450 metres display systematically shallow profiles beneath roughly 100 to 150 metres of regolith.
Larger projectiles tested by the researchers carried enough energy to fragment Deimos completely. The preferred impact sat just below that destructive threshold, representing a subcatastrophic collision violent enough to remodel the landscape while leaving the moon in one gravitationally connected piece.
Why the South Polar Crater Remained Shallow
The geometry of the south-polar depression presented its own puzzle because it is remarkably shallow for a cavity spanning 10 kilometres. According to the team’s analysis, the final shape depended heavily on what happened immediately after excavation. Weak crater rims collapsed inward, and internal rubble slumped into the bowl rather than maintaining the steep walls typical of craters on stronger bodies.
By simulating Deimos under current orbital conditions in a Mars-centred, co-rotating frame, the researchers assessed the structural stability of the moon’s present-day shape across various bulk cohesion values. At very low cohesion below 1 pascal, material on steep slopes began to drift under tidal and rotational stresses. Conversely, surface cohesion around one pascal stabilized the surface while allowing the rims to slump into the observed shallow depression, successfully mirroring the moon’s current morphology.
Implications for Future Exploration
By confirming that Deimos mechanically resembles rubble-pile asteroids like Bennu, Ryugu, and Dimorphos rather than a solid monolith, the research refines expectations for surface sampling. The smooth plains and muted craters are not signs of ancient geological calm, but the fallout of a single, violent asteroid strike that reshaped a fragile world.
