One Asteroid Strike May Have Reshaped an Entire Martian Moon

An international team led by the University of Bern in Switzerland has found evidence that a single asteroid impact may have dramatically reshaped Deimos, the smaller of Mars' two moons, carving a large depression near its south pole and burying much of its surface in debris. The findings, published in the journal Nature Astronomy, are the first scientific study to use data collected during the European Space Agency's Hera spacecraft's flyby of Deimos.

Deimos has a roughly oval shape and looks very different from Phobos, Mars' other moon: while Phobos is heavily marked by craters, Deimos appears smoother because it is covered by loose dust, broken rock and other debris called regolith. Scientists had not known exactly how the depression near its south pole formed or where the widespread regolith came from.

Led by Dr. Sabina Raducan, the team used high-resolution computer simulations, developed at the University of Bern over roughly two decades, that represent colliding objects as millions of individual particles and let researchers adjust factors such as gravity, density, material strength, impact speed and angle. The researchers ran about a hundred simulations, each taking about a week, and compared the results with observations Hera collected during a March 2025 flyby of Deimos, made while the spacecraft used Mars for a gravity assist on its way to the asteroid moon Dimorphos.

The simulations pointed most strongly to one scenario: an asteroid about 320 meters across striking Deimos at a 45-degree angle, powerful enough to redistribute material across the entire moon — in some areas more than 200 meters deep — but not so strong that it broke Deimos apart. "Our simulation thus shows that a single impact was sufficient to decisively shape the current landscape of Deimos," said co-author Martin Jutzi of the University of Bern.

The researchers say Deimos survived the impact because its weak, porous interior helped absorb the force of the collision. The findings could help guide the Martian Moons eXploration (MMX) mission that the Japan Aerospace Exploration Agency (JAXA) is preparing to send to Phobos and Deimos.