A giant crater on Mars’ moon Phobos may hold the answer to one of the solar system’s oldest mysteries: where the moon came from. The enormous Stickney Crater, which covers nearly half of Phobos’ surface, could hide a dense region of compressed material that would reveal whether Phobos is a captured asteroid or debris from an ancient impact on Mars.
New models of Phobos’ gravitational pull and its motion have been presented at the recent European Geosciences Union general assembly in Vienna. They focus on Stickney Crater, a striking feature on the moon. The subtle evidence they offer may finally settle a long-standing argument about where Phobos came from.
Stickney Crater as a Time Capsule
The debate over Phobos’ history hinges on whether the moon formed from debris thrown into orbit by a giant impact on Mars, or whether it was instead captured as an asteroid. In the first scenario, the collision that produced the 9 km (5.6 mile) diameter crater may have happened roughly 4.2 billion years ago. In the second, the impact that created the crater could have occurred much later, around 2.6 billion years ago.
Scientists use the age of the crater to determine when the material beneath it was compressed. A large collision would have produced intense heat at the point of impact, melting and compressing the rock below. This compression created a denser area beneath the crater, and that denser area would produce a slight gravitational signal that researchers might be able to detect.
What the researchers hope to determine is whether the Stickney impact left behind a concentrated region of denser material beneath the crater. Finding such a region would strengthen the case that Phobos is made from asteroid fragments. Without it, the alternative explanation — that Phobos is a captured asteroid — gains more support.
Two Explanations for Phobos’ Origin
Two main explanations have been developed by scientists to account for Phobos’ origin. Under one scenario, a giant object struck Mars and blasted material into orbit. That debris eventually formed a disc around the planet and later produced Mars’ two moons, Phobos and Deimos.
An alternate theory suggests the moons were not formed alongside Mars, but instead were asteroids captured by its gravity. Evidence from the moons’ spectral properties and research into how asteroids might be drawn in supports this possibility.
Scientists are using the gravitational field of Phobos to put these ideas to the test. Pinning down how mass sits inside the moon is a key step toward figuring out how it came into being. Present estimates suggest an interior that is full of empty space, possibly mixed with frozen water, along with a denser grouping of material near Phobos’ equatorial region.
The Stickney Survival Question
A collision of this magnitude should have destroyed a moon as small as Phobos outright. But survival was possible, Haser says, if the moon had an unusually low and relatively uniform density, which would allow it to absorb the impact somewhat like a sponge.
Haser is the one who says “You would assume that such an impact would have shattered Phobos, unless it has a very low homogeneous density, like a sponge that can absorb that kind of impact,”.
The collision also would have generated intense heat around the impact site. “And at that impact region, there must be very high temperature that melted and compressed the stone beneath it,” he says.
A denser region might have formed beneath Stickney Crater, a remnant of the compression that created it, leaving a gravitational mark that scientists may be able to track. The researchers are examining how that compressed mass alters Phobos’s gravitational signal, its moments of inertia, and its libration amplitude — the wobbling and oscillation of the tiny moon.
Phobos as a Rubble Pile
The captured asteroid explanation matches several characteristics of Phobos, including its irregular shape, which resembles what scientists call a rubble pile asteroid — an object formed from many pieces of rock held together loosely by gravity.
It is hard to fit everything into one explanation. Phobos’ present-day gravity field, shape, density, spectral properties, and shifting orbit must be brought together under one geophysical model. Its highly irregular shape, along with its extreme closeness to Mars, makes interpreting its gravity and inner structure especially hard.
In the paper, we investigate how a compressed mass beneath Stickney crater affects the tiny moon’s gravitational signal, moments of inertia, and libration amplitude, says Haser. Those tiny changes in Phobos’ movement could ultimately reveal how material is arranged inside the Moon.
The MMX Mission Could Solve It
The puzzle depends on something scientists still know surprisingly little about: what Phobos looks like on the inside. Researchers are now trying to narrow down the possibilities by studying subtle characteristics of the moon, including its gravity and motion.
The mystery at hand may finally get solved through Japan’s upcoming MMX sample return mission, which is designed to collect physical evidence and measurements from Phobos and bring them back to Earth. That would give scientists their first chance to study the moon’s material directly.
The mission aims to measure Phobos’ gravity with unprecedented precision, which may at last resolve whether the moon is a captured asteroid or the remnant of an ancient Mars collision. That answer could entirely change how we see the formation of Mars and its moons.
Phobos’ Doomed Future
What makes Phobos remarkable goes beyond its puzzling history; it is also what is happening to the moon right now. Its orbit sits very close to Mars, and it is gradually moving toward the planet. Scientists expect that eventually it will either break apart or crash into Mars.
“Phobos’ orbit is dynamically very special; it is very close to Mars, slowly spiraling inward, and will eventually be disrupted or impact Mars,” says Haser.
Phobos serves as both a preserved record of events from billions of years ago and a moon that keeps changing. The mission has a chance to arrive in time to witness the moon’s inward spiral, and its samples could reveal Phobos’s composition.
The researchers are already laying out their next moves. They plan to put the MMX data to work, fine-tuning their models and putting their predictions about the squashed region beneath Stickney Crater to the test. The project is far from complete, yet the path ahead is plain to see.
What the Paper Says
The researchers’ 2026 paper in The Monthly Notices of the Royal Astronomical Society (MNRAS) provides the foundation for their current work. In that paper, they described their models of Phobos’ gravity and motion and their estimates of the moon’s interior structure.
The paper’s authors are Benjamin Haser, a doctoral student in planetary science at Germany’s Universität der Bundeswehr München, and Thomas Andert, his co-author. Haser presented their recent work at the EGU general assembly in Vienna, where he described the Stickney Crater as one of the most important episodes in Phobos’ history.
This paper contributes to the wider attempt to determine Phobos’ origins. Researchers have studied the moon for many years, yet the question of its source has remained just as stubbornly unanswered.
Haser’s work on Phobos has now reached a new stage. His models of the moon’s gravity and motion have narrowed the possible explanations for its origin, and the MMX mission could finally provide the evidence to choose between them.
The question is simple: beneath the crater, what lies there? The answer may settle whether Phobos is a captured asteroid or a fragment of Mars. The mission is the instrument that could reveal it.
“Phobos’ orbit is dynamically very special; it is very close to Mars, slowly spiraling inward, and will eventually be disrupted or impact Mars.”
Source material: “A giant crater may reveal the origin of Mars’ doomed moon,” ScienceDaily.
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