A massive crater on Mars’ moon Phobos, named Stickney Crater, could hold clues to whether the moon is a captured asteroid or formed from debris of an ancient impact on Mars. Researchers are modeling geophysical measurements of the crater to understand Phobos’ internal structure, with Japan’s upcoming MMX mission expected to provide physical evidence.
Planetary scientists have long debated the origin of Phobos. Some believe it is an asteroid captured by Mars, while others propose it formed from debris created by a large impact on the planet. Understanding Phobos’ interior is key to resolving this mystery. Benjamin Haser, a doctoral student in planetary science at Germany's Universität der Bundeswehr München, presented research at the European Geosciences Union general assembly in Vienna detailing these efforts.
The 9 km (5.6 mile) diameter Stickney Crater is central to the debate. If Phobos originated from debris from a Martian impact, the collision that created the crater may have occurred around 4.2 billion years ago. However, if Phobos was captured as an asteroid, the impact forming Stickney Crater could have happened much later, approximately 2.6 billion years ago. Current estimates suggest Phobos has a porous interior that may contain water ice.
Researchers, including Haser and co-author Thomas Andert, published a 2026 paper in *The Monthly Notices of the Royal Astronomical Society* outlining their work. They are investigating whether the Stickney impact created a concentrated region of denser material beneath the crater. Haser stated, “The Stickney event is one of the most important episodes in Phobos' history, and learning more about it could help scientists determine where the moon came from.” He also emphasized that Phobos, despite its small size (22.2 km or 13.8 miles in diameter) and rapid orbit around Mars (every 7 hours and 39 minutes), should not be considered simply a “rock in orbit.” One scenario suggests a giant impact on Mars blasted material into orbit, forming a disc that eventually created Phobos and Deimos, Mars’ two moons.
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