One asteroid strike may explain two mysteries of Mars’ moon Deimos

The research is the first scientific study to incorporate observations collected during Hera's flyby of Deimos. The findings could also help guide future missions, including the Japan Aerospace Exploration Agency (JAXA) Martian Moons eXploration (MMX) mission.

The Mystery of Deimos' Smooth Surface

Deimos is the smaller and more distant of Mars' two moons. It has a roughly oval shape and a prominent depression near its south pole. Its surface also looks strikingly different from that of Phobos, Mars' other moon.

While Phobos is heavily marked by craters, Deimos appears smoother because much of its surface is covered by loose dust and rocky debris known as regolith. Spacecraft have photographed the moon with increasing detail over several decades, but scientists have not been certain what created either the southern depression or the widespread layer of debris.

A new study led by Dr. Sabina Raducan investigated whether the two features could share the same origin. The international collaboration included researchers from the Observatoire de la Côte d'Azur, the University of Arizona, and the University of Tokyo, among other institutions.

Raducan worked in the Division of Space Research and Planetary Sciences (WP) at the Physics Institute at the University of Bern until October 2025. She is now Science Program Manager at the International Space Science Institute and a Senior Fellow at the Vrije Universiteit Brussel.

Using high-resolution simulations produced with the "Bern Smoothed Particle Hydrodynamics (SPH)" code, the team found that the distinctive depression near the south pole was most likely produced by a single asteroid impact that was powerful but not strong enough to destroy Deimos. According to the simulations, the same collision could also have generated the regolith that now blankets much of the moon.

The research, published in Nature Astronomy, is the first scientific publication to use observations gathered when ESA's Hera spacecraft flew past Deimos. Hera is currently traveling toward its main destination, the asteroid moon Dimorphos.

Simulating a Massive Asteroid Collision

To reconstruct what may have happened, the scientists used the Bern SPH code, which has been developed at the University of Bern over roughly two decades. The software is designed to model collisions involving asteroids, comets, and planets.

In the simulations, colliding objects are represented by millions of individual particles. Researchers can adjust factors such as gravity, density, and material strength to see how different types of impacts unfold.

The University of Bern has extensive expertise in numerical impact modeling. The same approach was previously used to simulate NASA's DART spacecraft crashing into Dimorphos.

"The code runs on a high-performance computing cluster here at the University of Bern and is one of the few codes capable of performing this type of simulation," explains study leader Sabina Raducan, who is also co-chair of the Hera Impact Physics Working Group for ESA's Hera mission.

The researchers tested many possible scenarios, changing the impactor's size, speed, and impact angle while also varying assumptions about the internal structure of Deimos.

"We carried out about a hundred simulations -- each one took about a week."

They then compared the results with observational data collected by ESA's space probe.

Hera's main mission is to closely examine what happened when NASA's DART spacecraft struck Dimorphos. Those observations are intended to help researchers evaluate the deflection of asteroids as a potential method to defend Earth against asteroid impacts.

In March 2025, Hera passed Mars and used the planet's gravity to alter its trajectory toward Dimorphos. The maneuver also gave the spacecraft a rare chance to observe Deimos from close range.

One Impact Could Explain Two Major Features

The simulations favored a specific collision scenario. According to the researchers, an asteroid about 320 meters across striking Deimos at an angle of 45 degrees could have produced the size and shape of the depression now visible near the moon's south pole.

The same impact could also explain why regolith is spread across the entire moon. Enormous quantities of material would have been thrown outward during the collision and redistributed across Deimos, burying many older surface features. In some locations, the deposited material may exceed 200 meters in depth.

"Our simulation thus shows that a single impact was sufficient to decisively shape the current landscape of Deimos," explains co-author Martin Jutzi from the Division of Space Research and Planetary Sciences (WP) at the University of Bern, who also serves as co-chair of the Hera Impact Physics Working Group.

"The impact was violent enough to redistribute material globally, but not so strong that it would have shattered the moon."

Comparing the simulations with spacecraft observations also revealed clues about the physical makeup of Deimos. Its uppermost layers appear to be exceptionally weak, while the moon's interior seems highly porous. That porous structure would have absorbed and weakened some of the energy from the collision, helping Deimos survive the impact.

"In terms of its physical properties, Deimos more closely resembles the so-called rubble-pile asteroids than our Earth's Moon," says Raducan. "But that doesn't necessarily mean that Deimos is actually an asteroid. It could also have formed from material ejected during impacts on Mars."

What Future Missions Could Discover

Other explanations for Deimos' southern depression and smooth surface are still possible. However, the new impact scenario provides a single mechanism capable of explaining both features and makes specific predictions that future spacecraft can test.

One important opportunity will come from the Japan Aerospace Exploration Agency (JAXA), which is preparing the Martian Moons eXploration (MMX) mission for launch in 2026. MMX is designed to study both Martian moons in detail and return samples from Phobos to Earth.

"Our study provides important, concrete predictions for this Japanese MMX mission, such as the thickness and distribution of the regolith layer and the mechanical properties of Deimos's material," explains Raducan.

"This gives MMX a clearer picture of what its instruments -- and ultimately the sample collection -- can expect."