Asteroid impact shaped the appearance of Mars’ moon Deimos

Deimos, the smaller and outermost of Mars’ two moons, is roughly oval in shape and has a deep depression at its south pole. Unlike its heavily scarred sister moon, Phobos, Deimos is covered by a loose layer of dust and rubble – a so-called regolith layer – which gives it a smoother, dustier appearance. Although numerous space probes have provided increasingly detailed images of its surface over the past decades, the origin of the debris layer and the depression at the south pole remains unclear.

This is where a new study by an international research team comes in, led by Dr. Sabina Raducan, in collaboration with, among others, the Observatoire de la Côte d’Azur, the University of Arizona, and the University of Tokyo. Raducan was a researcher in the Division of Space Research and Planetary Sciences (WP) at the Physics Institute at the University of Bern until October 2025 and is now Science Programme Manager at the International Space Science Institute and a Senior Fellow at the Vrije Universiteit Brussel. Using high-resolution computer simulations from the “Bern Smoothed Particle Hydrodynamics (SPH)” code, the researchers were able to show that the distinctive depression near Deimos’s south pole was most likely formed by a single, non-destructive asteroid impact. This impact is also believed to have created the regolith layer present on Deimos. The study is the first scientific publication to use data from the flyby of Deimos by the ESA space probe Hera, which is currently en route to the distant asteroid moon Dimorphos. The study was published in Nature Astronomy.

Bern code simulates impact

To investigate how the depression and the surface structure of Deimos formed, the researchers used the Bern SPH code, developed at the University of Bern over the course of two decades. It is designed to simulate collisions between asteroids, comets, or planets. Using the Bern computer code, colliding bodies are broken down into millions of particles, whose behavior during impact is controlled by the interaction of various reconfigurable variables such as gravity, density, and material strength. The University of Bern is a global leader in the numerical modeling of impacts, and the method was also used to simulate the collision of NASA’s DART spacecraft with the asteroid 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. In numerous simulations, they varied the size, velocity, and impact angle of the potential impactor, as well as the internal structure of Deimos. “We carried out about a hundred simulations – each one took about a week.” The researchers then compared these simulations with observational data from ESA's space probe. Hera will study in detail the consequences of the NASA DART probe’s impact on Dimorphos – and thus evaluate the deflection of asteroids as a potential method to defend Earth against asteroid impacts. In March 2025, the Hera mission used a flyby of Mars for a so-called gravity assist maneuver to set a course for its actual target, Dimorphos. This presented a unique opportunity: Hera observed Deimos at close range.

A single impact caused Deimos’s depression and surface structure

The study’s results clearly point to one scenario: an asteroid striking at a 45-degree angle with a diameter of about 320 meters caused the observed South Pole depression in terms of its extent and shape. At the same time, this scenario explains the thin layer of regolith observed across the entire moon. During the collision, large amounts of material were hurled across the surface, covering many of the existing surface features – in some places to a depth of more than two hundred meters. “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.” The comparison between the model and observations also shows that the uppermost layers of Deimos are exceptionally weak and that its internal structure is highly porous. This resulted in the impact forces being dampened. “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.”

Important predictions for the Japanese space mission

While alternative explanations for Deimos’s smooth surface and southern basin remain possible, this study offers a unified explanation for both features and provides practical predictions that can be tested by future space missions. For example, the Japan Aerospace Exploration Agency (JAXA) is currently preparing the Martian Moons eXploration (MMX) mission, which is scheduled to launch in 2026. The mission’s goal is to observe the two Martian moons in detail and bring samples from Phobos back 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.”