A single asteroid impact reshaped Mars' small moon Deimos, carving out its distinctive south pole crater and coating the moon in a layer of dust, according to a new study published Tuesday in the journal Nature Astronomy.
Deimos, Mars' outermost moon, is a small, irregular, potato-shaped rock. It takes its name from the son of the Greek god Ares, meaning "dread" in ancient Greek. Ares is known as Mars in Roman mythology.
The moon orbits about 24,000 kilometers (14,913 miles) from the Martian surface and has been observed by orbiters since the 1970s, including a recent flyby by the European Space Agency's Hera mission in March 2025. Hera is on its way to study the aftermath of NASA's DART mission, which deliberately crashed a spacecraft into a small moon orbiting a larger asteroid.
Orbital images have shown that Deimos, unlike its heavily cratered sister moon Phobos, whose name means fear in ancient Greek, is much smoother, and covered in a puzzling layer of dust and debris known as regolith.
Deimos, which measures about 12 kilometers across, has a distinctive feature: a 10-kilometer-wide impact basin at its south pole that resembles a sharp depression, similar to those seen in mountain ranges. Scientists have long puzzled over what created the crater and led to the formation of the dust layer.
By comparing images from Hera's flyby with impact simulations, the study's authors suggest a single impact from a large asteroid remodeled Deimos, producing both the striking polar crater and the moon's global dust layer.
The study offers a prediction that could be tested by the Japan Aerospace Exploration Agency's (JAXA) Martian Moons Exploration mission, known as MMX, which is set to launch later this year. The mission aims to capture detailed, unprecedented observations of both moons to help determine how and when they formed, and even to bring samples collected from Phobos back to Earth.
"Our study provides important and concrete predictions for this Japanese MMX mission," said Dr. Sabina Raducan, the study's lead author, science program manager at the International Space Science Institute and a senior researcher at the Free University of Brussels. "This gives MMX a clearer view of what its instruments, and ultimately its sample collection, can expect."
A giant impact
The researchers began by creating impact scenarios for Deimos using a computer code called Bern Smoothed Particle Hydrodynamics, or SPH.
The code, developed at the University of Bern in Switzerland over two decades, is designed to simulate collisions between asteroids, comets and planets, analyzing impacts by tracking millions of individual particles involved in each scenario.
This analysis allows researchers to understand the different variables at play during an impact, such as density, gravity and the compositional strength of the rocky bodies involved.
"We ran about a hundred simulations, each one took about a week," said Raducan, who is also a coordinator of the Impact Physics Working Group for Hera's science team.
The size, speed and angle of the asteroid that could have struck Deimos varied across each simulation, as did the moon's internal structure. The team then compared their simulation data with detailed observations of Deimos made by the Hera probe.
The researchers focused on the most likely scenario, an asteroid about 320 meters across striking Deimos at a 45-degree angle, creating the south pole crater and dust layer.
The impact released a large amount of debris across Deimos' irregular surface, burying many of its features under as much as 200 meters (656 feet) of material, and making it look much smoother than Phobos.
"Our simulation shows that a single impact was enough to decisively shape Deimos' current landscape," said Dr. Martin Jutzi, a co-author of the study and senior researcher at the University of Bern's Space Research and Planetary Sciences division.
"The impact was violent enough to redistribute material globally, but not so strong as to destroy the moon."
In images from the Hera probe, the outlines of older craters are still visible beneath Deimos' dust layer. Had Deimos been more solid, shockwaves from the impact would have reverberated through the moon, destroying or even erasing surface features. Instead, according to the study's authors, Deimos' fractured interior dampened the force of the impact.
The authors did not suggest when the ancient impact on Deimos occurred, only that it must have happened long after the moon formed.
A mysterious origin
Scientists have long debated whether Phobos and Deimos are rocky fragments blasted off Mars by an impact, or space rocks captured by the red planet's gravity.
Observations and computer models suggest Deimos' internal structure is highly porous, making it more similar to "rubble pile" asteroids, loose clusters of space rocks held together by gravity, than to Earth's Moon.
"But that doesn't necessarily mean Deimos is in fact an asteroid. It could also have formed from material ejected during impacts on Mars," Raducan said.
The MMX mission is expected to gather data that could definitively resolve the mystery of Phobos' and Deimos' origins, while also offering insight into the early history of the solar system, when the gravity of the largest planets caused space rocks to collide with other planets and with each other.
The mission will reach the moons in 2027, spending two years mapping Phobos and selecting a landing site for sample collection before moving off to observe Deimos. A sample from Phobos is expected to be returned to Earth in 2031.
Interest in Deimos and Phobos is growing as more orbiters capture images of the enigmatic moons, said Dr. Terik Daly, a planetary scientist at Johns Hopkins University's Applied Physics Laboratory. Daly was not involved in the study but is a member of MMX's Science Working Team.
"The paper describes a viable model for Deimos' unusual shape, grounded in long-standing questions about its origin and evolution," Daly wrote in an email. "As with many modeling studies, there are assumptions that future observations will need to test, and it is hoped that Japan's Martian Moons Exploration mission will make observations that could offer an opportunity to evaluate the ideas presented in the paper."
