Scientists are hunting a hidden dense patch inside Mars’ doomed moon to settle where it came from

A crater nine kilometers across sits on the surface of a moon barely twenty-two kilometers wide, and inside that scar planetary scientists suspect a clump of unusually dense rock is hiding. Locating that hidden mass beneath Stickney Crater, the dominant feature on Mars’ inner moon Phobos, could help settle a question that has outlasted several spacecraft missions already: whether Phobos is the reassembled wreckage of a captured asteroid or the debris of a catastrophic strike on Mars itself. Benjamin Haser, a doctoral researcher in planetary science at Germany’s Universitat der Bundeswehr Munchen, and co-author Thomas Andert laid out the search in a paper published this year in the Monthly Notices of the Royal Astronomical Society.

Phobos races around Mars once every seven hours and thirty-nine minutes, closer to its planet than any other moon in the solar system orbits its own, and that proximity is slowly killing it. The moon is spiraling inward and is expected eventually to break apart or collide with Mars, a fate that leaves researchers a narrowing window to read its interior before gravity finishes the job. Haser and Andert argue that any compressed material hiding under Stickney would leave a measurable signature in Phobos’ gravity field, in its moments of inertia, and in the slow wobble the moon exhibits as it orbits, known as libration.

Two Rival Stories for a Ten-Mile-Wide Moon

Planetary scientists have never fully settled how Phobos formed. One camp holds that a large object struck Mars long ago, blasting debris into orbit that clumped first into a disk and then into the planet’s two small moons, Phobos and Deimos. A rival camp argues the moons began as ordinary asteroids that wandered too close to Mars and were captured by its gravity, a story supported by their irregular shapes and their spectral properties, which resemble carbon-rich asteroids more than they resemble Mars itself.

The two theories point to very different ages for Stickney Crater. Under the giant-impact scenario, the collision that carved the nine-kilometer-wide crater may have happened around 4.2 billion years ago, not long after Phobos itself formed from that debris disk. Under the captured-asteroid scenario, the Stickney impact likely landed much later, closer to 2.6 billion years ago, long after the moon had already settled into orbit around Mars.

The Dense Patch Stickney May Have Left Behind

An impact large enough to gouge a crater nearly half the width of the moon itself should, by ordinary physics, have shattered a body as small as Phobos. Haser has proposed that survival was possible only if Phobos carried an unusually low and evenly distributed density at the time, something closer to a loosely packed sponge than a solid rock, able to absorb the shock rather than fracture under it.

That same collision would have generated intense heat where it struck, hot enough to melt and compress material directly beneath the crater floor. Haser and Andert’s model suggests that compressed pocket never fully rebounded, leaving a small, denser region under Stickney that stands apart from the porous, possibly ice-bearing material thought to fill much of the rest of the moon’s interior. The gravitational signature of that buried patch, faint as it would be, is what the current search is designed to detect.

Reading a Gravity Field Mars Will Not Let Go Of

Mapping that signature is unusually hard because Phobos’ own gravity is faint next to the pull of the planet it circles. Its irregular shape and its extreme closeness to Mars distort measurements that work cleanly around larger, rounder worlds, so a dense patch beneath Stickney would show up only as a subtle shift in the moon’s moments of inertia and in the size of its libration.

Reconciling all of that with Phobos’ known shape, density, and slowly decaying orbit inside one coherent physical model remains, by Haser’s own account, the harder half of the problem. Confirming a buried dense mass would not by itself prove which origin story is correct, but it would rule out models that assume a uniformly porous moon throughout, narrowing the field of remaining explanations considerably.

A Sample-Return Mission Built to Settle the Question

Japan’s Martian Moons Exploration mission, known as MMX, is scheduled to launch in late 2026 and is designed to orbit Phobos closely enough to attempt something no earlier mission has managed: collecting physical samples from its surface. Because Phobos offers no stable orbital environment of its own, the spacecraft will fly a series of carefully managed approaches rather than settle into a conventional orbit.

MMX carries two collection systems. One is a core sampler reaching roughly two centimeters into the surface; the second, contributed by NASA, is a pneumatic sampler that fires pressurized gas at the ground to loft loose material into a collection container. The samples MMX gathers are expected to reach Earth by the middle of 2031, years after Haser and Andert’s gravity-based model will likely have already narrowed the possibilities those samples are meant to confirm.

What continues to puzzle Haser is not simply which raw materials make up Phobos, but what single internal structure could account for the moon’s gravity, its shape, its density, and its spectral fingerprint all at once. Distinguishing between asteroid capture, impact debris, or some hybrid of the two depends, in his assessment, on solving that structural question first, well before any sample ever reaches a laboratory on Earth.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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