A violent impact might have changed Mars’ potato-shaped moon - CNN

Mars has two tiny moons that lack a confirmed origin story for how they came to orbit the red planet — but new research may explain some of the mysteries surrounding outermost satellite Deimos.

Named for the son of the Greek god Ares, Deimos, which means dread in ancient Greek, is a small, lumpy rock shaped like a potato. Ares is also known as Mars in Roman mythology.

The moon is about 14,913 miles (24,000 kilometers) from the Martian surface, and orbiters have observed it since the 1970s — including a recent flyby of the European Space Agency’s Hera mission in March 2025. Hera is en route to study the aftermath of NASA’s DART mission that intentionally crashed a spacecraft into a small moon orbiting a larger asteroid.

Orbital imagery has shown that unlike its heavily cratered sibling moon, Phobos, which means fear in ancient Greek, Deimos is much smoother — and covered in a perplexing layer of dust and rubble called regolith.

Deimos, which is about 7.5 miles (12 kilometers) in diameter, also has an identifying feature, a 6.2-mile-wide (10-kilometer-wide) impact basin at its southern pole, which resembles a dramatic dip similar to those seen in mountain ranges. Scientists have long questioned what created the crater and led to the dust layer.

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By comparing Hera’s flyby images with impact simulations, the authors of a new study published Tuesday in the journal Nature Astronomy suggest that one large single asteroid impact reshaped Deimos, resulting in the moon’s dramatic polar crater and global dust layer.

The study offers a prediction that could be tested by the Japan Aerospace Exploration Agency’s Martian Moons eXploration mission, known as MMX, which is expected to launch by the end of the year. The mission aims to capture unprecedented, detailed observations of both moons to help determine how and when they formed and even return samples collected from Phobos to Earth.

“Our study provides important, concrete predictions for this Japanese MMX mission,” said lead study author Dr. Sabina Raducan, science program manager at the International Space Science Institute and a senior fellow at the Free University of Brussels. “This gives MMX a clearer picture of what its instruments — and ultimately the sample collection — can expect.”

The researchers began by creating impact scenarios for Deimos using a computer code called the Bern Smoothed Particle Hydrodynamics, or SPH.

This code, developed at Switzerland’s University of Bern over two decades, is designed to simulate collisions between asteroids, comets and planets, and analyze the impact by looking at millions of individual particles involved in each scenario.

This breakdown enables researchers to understand the different variations and factors at play during an impact, such as the density, gravity and compositional strength of the rocky bodies.

“We carried out about a hundred simulations — each one took about a week,” said Raducan, who is also one of the chairs of the Impact Physics Working Group for the Hera science team.

The size, velocity and angle of impact for the asteroid that might have struck Deimos varied in each simulation, as did the moon’s internal structure. Then, the team compared the data from its simulations with Hera’s up-close observations of Deimos.

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The researchers zeroed in on the most likely scenario of an asteroid measuring about 1,050 feet (320 meters) across striking Deimos at a 45-degree angle, creating the south pole crater and the dust layer.

The impact released a large amount of debris across Deimos’ lumpy surface, effectively covering many of its surface features by as much as 656 feet (200 meters) — and making it appear much smoother than Phobos.

“Our simulation thus shows that a single impact was sufficient to decisively shape the current landscape of Deimos,” said study coauthor Dr. Martin Jutzi, senior researcher at the University of Bern’s Division of Space Research and Planetary Sciences.

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