Mars has two small moons, Phobos and Deimos. While Phobos is larger and heavily covered with impact marks, Deimos looks much smoother and dustier. Now, a new international study suggests that a single asteroid impact may explain why Deimos looks the way it does today.
The research, led by Dr. Sabina Raducan, shows that an asteroid about 320 meters (1,050 feet) wide could have struck Deimos at an angle of around 45 degrees. The impact may have created the large depression near the moon’s south pole and spread a thick layer of dust and broken rock across much of its surface.
The findings, published in Nature Astronomy, provide a new explanation for two major mysteries about Deimos: its southern depression and its unusual layer of loose surface material.
Deimos: A Small and Mysterious Moon
Deimos is the smaller and more distant of Mars’ two moons. It is roughly oval-shaped and only about 15 kilometers across. Unlike Earth’s Moon, Deimos does not have a solid, smooth surface. Instead, its exterior appears to be covered by loose dust, rocks and broken material known as regolith.
One of its most noticeable features is a large depression near its south pole. Scientists have studied images of Deimos for decades, but the exact origin of this feature has remained uncertain.
The new study suggests that both the depression and much of Deimos’ surface regolith could have been created by the same ancient event.
Recreating an Asteroid Collision on a Computer
To investigate Deimos’ history, the research team used the Bern Smoothed Particle Hydrodynamics (SPH) computer code. Developed at the University of Bern over more than 20 years, the software is designed to simulate enormous collisions involving planets, asteroids and comets.
Instead of treating an asteroid and a moon as simple solid objects, the simulation breaks them into millions of particles. Each particle can respond to gravity, material strength, density and other physical conditions.
This allows scientists to study what happens during an impact in remarkable detail.
The same type of simulation has also been used to study the collision between NASA’s DART spacecraft and the asteroid Dimorphos.
The researchers performed around 100 different simulations. They changed factors such as the asteroid’s size, speed and impact angle, as well as the internal structure of Deimos.
Each simulation required roughly a week of computer processing.
One Impact Could Explain Two Features
After comparing the computer simulations with observations of Deimos, the researchers found one scenario that matched the evidence particularly well.
According to the model, an asteroid around 320 meters wide struck Deimos at an angle of approximately 45 degrees.
The collision was powerful enough to create the large depression near the south pole. At the same time, it launched huge amounts of rock and dust across the moon’s surface.
Some areas may have received more than 200 meters (650 feet) of impact material.
This could explain why Deimos has such a smooth and dusty appearance. Material thrown into the air during the collision would have traveled across the moon before settling back onto its surface, covering older geological features.
In other words, the impact may have acted like a giant blanket of debris, hiding much of Deimos’ earlier surface.
Deimos Was Not Destroyed
Interestingly, the impact was enormous but did not completely break Deimos apart.
Martin Jutzi, a co-author of the study from the University of Bern, explains that the collision was strong enough to significantly reshape the moon but not powerful enough to shatter it.
This tells scientists something important about Deimos itself.
The simulations indicate that the outer layers of the moon are extremely weak and that its interior is highly porous. Instead of behaving like a solid block of rock, Deimos may behave more like a loosely packed collection of rocks and debris.
This type of structure is often associated with what scientists call “rubble-pile” asteroids.
Does This Mean Deimos Is an Asteroid?
Not necessarily.
The similarity between Deimos and rubble-pile asteroids does not prove that Deimos was once an asteroid captured by Mars.
Another possibility is that Deimos formed from material thrown away from Mars during a massive impact in the distant past. Over time, this material could have come together under gravity and formed the small moon we see today.
Therefore, the study does not completely solve the mystery of Deimos’ origin. Instead, it provides new evidence about how the moon’s present-day surface and internal structure may have developed.
Hera Provides a New Look at Deimos
An important part of this research came from an unexpected opportunity involving the European Space Agency’s Hera mission.
Hera is traveling toward the asteroid system Dimorphos to study the effects of NASA’s DART impact. In March 2025, Hera flew past Mars to gain speed through a gravitational assist.
During this flyby, the spacecraft also observed Deimos from relatively close range.
These observations provided valuable new information about the moon’s surface and helped researchers compare real data with their computer simulations.
This makes the study particularly important because it is the first scientific publication to use data from Hera’s Deimos flyby.
Future Missions Could Test the Theory
The study also offers predictions that future spacecraft could test.
Japan’s Martian Moons eXploration (MMX) mission is designed to study Mars’ moons in much greater detail. The mission is expected to investigate both Phobos and Deimos and return samples from Phobos to Earth.
Researchers hope that future observations can provide more information about the thickness of Deimos’ regolith, the strength of its surface material and the structure beneath it.
These measurements could help scientists determine whether the impact scenario proposed in the new study is correct.
A New Piece of the Martian Moons Puzzle
Deimos may look like a quiet and simple object, but its surface could preserve evidence of a dramatic event from the ancient history of the Mars system.
The new research suggests that one asteroid impact may have created its southern depression while spreading debris across the moon and producing the dusty landscape we see today.
More importantly, the simulations suggest that Deimos is weak, porous and structurally different from a solid rocky body like Earth’s Moon.
Future missions will provide the opportunity to test these predictions directly.
For now, the study offers a powerful new idea: Deimos’ smooth and dusty surface may not be the result of many separate events, but the lasting signature of one enormous collision that happened billions of years ago.
Reference: Raducan, S.D., Agrusa, H.F., Asphaug, E. et al. Deimos’s shape and geology explained by a subcatastrophic impact. Nat Astron (2026). https://doi.org/10.1038/s41550-026-02956-w
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