Scientists have long debated how Mars’ outermost moon, Deimos, acquired its irregular, lumpy appearance. A new study published in Nature Astronomy proposes that a single, large asteroid impact reshaped the moon, forming the 10‑kilometer‑wide crater at its south pole and depositing a global layer of dust and rubble.
Background on Deimos
Deimos, named after the Greek personification of dread, orbits roughly 24,000 kilometers (about 14,913 miles) above the Martian surface. At roughly 12 kilometers (7.5 miles) in diameter, it is the smaller of Mars’ two moons and appears smoother than its sibling Phobos, which is heavily cratered.
Since the 1970s, orbiters have photographed Deimos, most recently the European Space Agency’s Hera mission in March 2025. Hera’s close‑up images revealed a thick regolith blanket covering much of the moon’s surface, obscuring older craters beneath a layer up to 200 meters (656 feet) deep.
Impact simulations
Researchers led by Dr. Sabina Raducan used the Bern Smoothed Particle Hydrodynamics (SPH) code to run about a hundred impact scenarios. Each simulation varied the asteroid’s size, speed, angle, and the internal structure of Deimos. The most plausible model involved an asteroid about 320 meters (1,050 feet) across striking the moon at a 45‑degree angle.
The impact would have been powerful enough to fling debris across the entire surface, creating the prominent southern polar basin and laying down the observed dust layer, yet not so violent as to shatter the moon. The team notes that Deimos’ porous, “rubble‑pile” interior likely absorbed much of the shock, preventing wholesale disruption.
Implications for moon origins
The findings add a new piece to the puzzle of whether Deimos (and Phobos) are captured asteroids or fragments ejected from Mars by ancient impacts. The porous structure suggested by the simulations aligns with a rubble‑pile composition, which could be consistent with either scenario.
“Our simulation shows that a single impact was sufficient to decisively shape the current landscape of Deimos,” said co‑author Dr. Martin Jutzi of the University of Bern.
Future testing by Japan’s MMX mission
The study’s predictions can be examined by the Japan Aerospace Exploration Agency’s Martian Moons eXploration (MMX) mission, slated for launch by the end of this year. MMX plans to orbit both Phobos and Deimos in 2027, map their surfaces in detail, and return a sample from Phobos to Earth by 2031.
Dr. Raducan emphasized that MMX will provide “concrete predictions” for the mission’s instruments, helping scientists confirm whether the impact model matches the moons’ actual geology.
Looking ahead
As more high‑resolution data arrive from Hera and, later, MMX, researchers hope to refine the timeline of the impact and better understand the early history of the inner solar system, when planetary gravity frequently captured or shattered passing space rocks.
For now, the notion that a single asteroid strike could turn a small, irregular rock into the potato‑shaped moon we see today offers a compelling explanation for Deimos’ distinctive look.
Original reporting: KTVZ (Central Oregon) — read the source article.
