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 kilometres) 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 kilometres) in diametre, also has an identifying feature, a 6.2-mile-wide (10-kilometre-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. 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.” One giant impact 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. The researchers zeroed in on the most likely scenario of an asteroid measuring about 1,050 feet (320 metres) 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 metres) — 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.” Outlines of ancient craters can still be seen beneath Deimos’ dust layer in the Hera images. If Deimos were more solid, shock waves from the impact would have resonated through the moon and disrupted or even erased surface features. Instead, Deimos’ fractured interior dampened the force of the impact, according to the study authors. The authors did not include a suggestion of when this ancient impact occurred on Deimos, only that it must have happened well after the moon formed. A mysterious origin Scientists have long questioned whether Phobos and Deimos are rocky chunks that an impact blasted away from Mars, or if they are space rocks that the red planet’s gravity captured. The observations and computer models suggest that Deimos’ internal structure is highly porous, making it more like rubble-pile asteroids — groups of space rocks loosely held together by gravity — than like Earth’s moon. “But that doesn’t necessarily mean that Deimos is actually an asteroid. It could also have formed from material ejected during impacts on Mars,” Raducan said. The MMX mission is expected to collect data that could definitively solve the mystery of Phobos and Deimos’ origins, as well as provide a window into the early history of our solar system when the gravity of the largest planets caused space rocks to crash into planets as well as one another. The mission will arrive at the moons in 2027, spend two years mapping Phobos and selecting a landing site for sampling before backing away to observe Deimos. A sample from Phobos is expected to be dropped off on Earth in 2031. Interest in Deimos and Phobos is growing as more orbiters capture imagery of the enigmatic moons, said Dr. Terik Daly, planetary scientist at the Johns Hopkins University Applied Physics Laboratory. Daly was not involved in the study but is a member of the Science Working Team for MMX. “The paper outlines a viable model for Deimos’ unusual shape, building on longstanding questions about its origin and evolution,” Daly wrote in an email. “Like many modeling studies, there are assumptions that future observations will need to test and Japan’s Martian Moons eXploration mission is expected to make observations that could provide an opportunity to evaluate ideas presented in the paper.” By Ashley Strickland, CNN
