This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 origins of the debris layer and the depression at the south pole remain 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 program 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 is published in Nature Astronomy. 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 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 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 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.
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