Phobos, the inner moon of Mars, remains one of the Solar System’s more stubborn puzzles. Only about 22 kilometers across and racing around the planet every 7 hours and 39 minutes, it could be either a captured asteroid or the leftover debris from a giant impact that once struck the Red Planet. Researchers now argue that the answer may lie inside the moon itself; specifically beneath Stickney, the 9-kilometer crater that dominates one face of Phobos.
Stickney is more than a scar. If Phobos formed from Martian impact debris, the crater may date to roughly 4.2 billion years ago; if the moon was captured, the impact could be closer to 2.6 billion years old. Modeling presented at the European Geosciences Union and published in the Monthly Notices of the Royal Astronomical Society suggests the collision may have melted and compressed rock beneath the crater floor, leaving a denser mass that would slightly alter Phobos’s gravity field, moments of inertia, and tiny wobble, or libration. Detecting that signature would constrain how mass is arranged inside the moon.
That interior is thought to be porous, possibly ice-bearing, and loosely bound; more like a rubble-pile asteroid than a solid rock. Such a “sponge-like” structure might explain how so small a body survived an impact that ought to have shattered it. Reconciling Phobos’s irregular shape, low density, spectral properties, and decaying orbit in a single geophysical model remains difficult, because the moon sits so close to Mars that the planet’s gravity overwhelms Phobos’s own weak field.
Time is limited. Phobos is slowly spiraling inward and will eventually break apart or strike Mars. Japan’s Martian Moons Exploration (MMX) mission, targeted for launch later in 2026, is designed to fly in a challenging quasi-orbit around the moon, collect surface samples with a core drill and a NASA-contributed pneumatic sampler, and return them to Earth by about 2031. Those measurements and rocks should finally show whether Stickney hides the compressed fingerprint of Phobos’s origin.
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