Single Asteroid Impact Shaped Deimos Smooth and Scarred

Simulating the South-Polar Crater on Deimos

For decades, astronomers viewed Mars’s two moons as odd, potato-shaped captures from the asteroid belt, though newer theories point toward a possible impact origin on the Martian surface. While the larger sibling Phobos spans roughly 26 kilometres and displays deep grooves and heavy cratering, Deimos measures only about 12 kilometres across and wears a much smoother exterior. A team of researchers set out to explain why a world so small features a gargantuan depression at its south pole spanning roughly 10 kilometres from rim to rim.

Lead author Sabina Raducan of the University of Bern and her colleagues utilized a high-performance computing cluster to run advanced hydrodynamic simulations. The Bern Smoothed Particle Hydrodynamics code allowed scientists to construct a three-dimensional model of Deimos, computationally fill in its southern crater to reconstruct its pre-impact shape, and slam virtual projectiles into it.

The team tested impactor diameters ranging from 300 to 360 metres at a standard estimated speed of 8.2 kilometres per second. The models revealed that an asteroid measuring 320 ± 25 metres across, striking at an angle of about 45 degrees, matched the physical contours of the present-day moon with an average vertical mismatch of only two metres.

How Deimos Avoided Total Destruction

An impact of that magnitude on such a small celestial body walks a fine line between severe deformation and complete obliteration. The preferred computer runs showed a subcatastrophic collision that possessed enough energy to reshape the landscape globally without shattering the moon into pieces.

Instead of launching most of the excavated material into space, the low gravity of Deimos captured roughly 10 to 20 percent of the debris. This material spread across the entire globe, settling into a regolith blanket several metres thick on average and reaching depths up to 200 metres on the Mars-facing hemisphere. The Bern SPH code incorporates a porosity treatment and a tensile fracture model, helping scientists calculate how internal pore spaces absorbed the massive shock.

This global blanket of debris explains why Deimos looks so different from typical heavily battered asteroids. The falling regolith filled in older, pre-existing scars, subduing plains and burying at least 14 larger craters under 100 to 150 metres of loose dust.

Internal Structure and Porous Rubble Pile Mechanics

The simulations required specific internal mechanical properties to replicate the shallow, wide nature of the south-polar cavity. If Deimos were a solid, monolithic boulder, the impact crater would have retained much steeper walls and a significantly deeper bowl.

Instead, the data points to a target that is highly porous, weak, and mechanically similar to rubble-pile asteroids like Bennu, Ryugu, and Dimorphos. Crater rims slumped inward and weak material crushed its internal voids to absorb the energy of the collision. Testing surface cohesion values down to approximately one pascal proved necessary; anything significantly higher resulted in unrealistic, steep-walled structures.

Observations from spacecraft add physical weight to these computer models. In March 2025, the ESA’s Hera spacecraft performed a gravity-assist maneuver at Mars and observed Deimos, testing its optical navigation systems against the moon’s surface features. Hera’s close-up checks confirmed that beneath a seemingly uniform, dusty veneer lie ancient, buried craters.

Preparing for the Japanese MMX Sample Return Mission