An international team led by the University of Bern has found evidence that a single asteroid impact may have dramatically reshaped Deimos, the smaller and more distant of Mars’ two moons. By combining advanced computer simulations with observations from ESA’s Hera spacecraft, researchers concluded that one powerful impact could explain both Deimos’ large southern depression and its unusually smooth, dust-covered surface.
This is the first scientific study to incorporate observations gathered during Hera’s close flyby of Deimos. The findings could also help guide future Mars-moon missions, including the Japan Aerospace Exploration Agency’s (JAXA) Mars Moon Exploration (MMX) mission.
The mystery of Deimos’ smooth surface
Deimos is the smaller and more distant of Mars’ two moons. The roughly oval-shaped satellite features a prominent depression near its south pole and looks significantly different from Phobos, Mars’ other moon.
While Phobos is heavily covered with impact craters, Deimos appears smoother because much of its surface is blanketed by loose dust and rock fragments known as regolith. Although spacecraft have captured increasingly detailed images of Deimos, scientists have not determined what caused the southern depression or the moon’s extensive debris layer.
A new study led by Dr. Sabina Raducan examined whether both features could have formed during the same event. Researchers from the Côte d’Azur Observatory, the University of Arizona, the University of Tokyo, and other institutions contributed to the international collaboration.
Raducan worked in the Department of Space Research and Planetary Sciences (WP) at the Institute of Physics at the University of Bern until October 2025. She is now a scientific program manager at the International Institute for Space Sciences and a senior research fellow at the Free University of Brussels.
Using high-resolution simulations generated with the Bern Smoothed Particle Hydrodynamics (SPH) code, the team found that Deimos’ distinctive southern depression was likely created by a single asteroid impact. The collision was powerful enough to reshape the moon but not strong enough to destroy it. The simulations also suggest that the same event produced much of the regolith now covering Deimos.
This study, published in Nature Astronomy, is the first scientific publication to use observations collected when ESA’s Hera spacecraft flew past Deimos. Hera is currently traveling toward its primary destination, the asteroid moon Dimorphos.
Simulating a major asteroid impact
To reconstruct Deimos’ history, the researchers used the Bern SPH code, a modeling tool developed at the University of Bern over approximately 20 years. The software is designed to simulate collisions involving asteroids, comets, moons, and planets.
In the simulations, colliding bodies are represented by millions of individual particles. Scientists can adjust variables such as gravity, density, material strength, impact speed, and collision angle to study how different impacts unfold.
The University of Bern has extensive expertise in numerical impact modeling. The same approach was previously used to simulate the collision between NASA’s DART spacecraft and Dimorphos.
“This code runs on a high-performance computing cluster at the University of Bern and is one of the few tools capable of performing simulations of this kind,” explains study leader Sabina Raducan, who also co-chairs the Hera Impact Physics Working Group for ESA’s Hera mission.
The researchers tested numerous impact scenarios by varying the asteroid’s size, velocity, and angle, along with different assumptions about Deimos’ internal structure.
“We ran about 100 simulations, with each one taking approximately a week to complete.”
The team then compared the simulation results with observations collected by ESA spacecraft.
Hera’s primary mission is to study the effects of NASA’s DART impact on Dimorphos. The data will help scientists assess asteroid deflection as a potential planetary-defense strategy against hazardous space rocks.
In March 2025, Hera flew past Mars and used the planet’s gravity to adjust its trajectory toward Dimorphos. The maneuver also gave the spacecraft a rare opportunity to observe Deimos from close range.
One asteroid impact may explain two major features
Several impact scenarios produced results that matched Deimos’ observed features. The researchers found that an asteroid approximately 320 meters in diameter, striking at an angle of about 45 degrees, could have created the size and shape of the depression near the moon’s south pole.
The same collision may also explain Deimos’ widespread regolith. The impact would have ejected enormous quantities of material, redistributing debris across the moon’s surface and burying many older geological features. In some areas, the deposited material may be more than 200 meters deep.
“Our simulations show that a single impact was sufficient to decisively shape Deimos’ present-day topography,” explains co-author Martin Jutzi of the Department of Space Research and Planetary Sciences (WP) at the University of Bern. Jutzi also co-chairs the Hera Impact Physics Working Group.
“The impact was energetic enough to redistribute material across the entire moon, but not powerful enough to break Deimos apart.”
Comparisons between the simulations and spacecraft observations also revealed clues about Deimos’ internal structure. Its uppermost layer appears to be extremely weak, while the interior is likely porous. This porous structure may have helped Deimos survive the impact by absorbing and reducing some of the collision’s energy.
“In terms of its physical properties, Deimos is more similar to a so-called rubble-pile asteroid than to Earth’s moon,” Raducan said. “However, that does not necessarily mean Deimos is an asteroid. It may also have formed from material ejected during an impact on Mars.”
What future Mars-moon missions could reveal
Other explanations for Deimos’ southern depression and smooth surface remain possible. However, the asteroid-impact theory offers a single mechanism that accounts for both features and makes specific predictions that future spacecraft can test.
One important opportunity will come from JAXA’s Mars Moon Exploration (MMX) mission, which is being prepared for launch in 2026. MMX will study both Martian moons in detail and collect samples from Phobos for return to Earth.
“Our study provides important, testable predictions for the Japanese MMX mission, including the thickness and distribution of Deimos’ regolith and the mechanical properties of its surface materials,” Raducan explains.
“These findings will give MMX scientists a clearer idea of what to expect from the spacecraft’s instruments and, ultimately, from its sample-collection operations.”
Source: www.sciencedaily.com