title: The Peeling Moon subtitle: Phobos will be stripped, not shattered date: 2026-02-26 paper: Agrusa & Michel, arXiv:2602.21912

Phobos is falling toward Mars. Its orbit is sub-synchronous — it orbits faster than Mars rotates — so tidal dissipation drains its orbital energy, spiraling it inward. At some distance, the tidal forces from Mars will exceed whatever holds Phobos together, and it will come apart. The standard assumption was that Phobos would disrupt catastrophically — a threshold distance where the moon's internal stresses exceed its bulk strength, and the whole body fails simultaneously. Agrusa and Michel find a different failure mode: surface stripping. For a moon with low cohesive strength (comparable to what spacecraft have measured on asteroids like Bennu and Ryugu), the surface material gets peeled away by tides before the interior fails. Phobos doesn't shatter. It peels. The distinction matters because it changes the disruption distance. Previous studies estimated Phobos would survive until close to Mars. Agrusa and Michel argue those estimates overestimated Phobos's strength. With realistic bulk strengths — drawn from actual measurements of small bodies, not theoretical estimates — Phobos begins losing surface material beyond 2 Mars radii. That's farther from Mars than most predictions placed the disruption. The physics of why peeling precedes shattering is geometric. Tidal stress is strongest at the surface, where the distance gradient between near-side and far-side is steepest relative to the local gravity. The interior is held together by the weight of overlying material (overburden pressure). Even if the bulk material is weak, the interior is compressed. The surface isn't. So the surface fails first — material lifts off, enters orbit around Mars, and begins forming a debris ring. Then something interesting happens. Once the surface stripping starts, the moon loses mass, which reduces its self-gravity, which makes the remaining surface even more vulnerable to tides. The stripping accelerates. Agrusa and Michel speculate that this runaway might transition into collisional erosion — the stripped debris impacts the remaining body, breaking off more material. The end state might not be a clean tidal disruption at all but a grinding process: the moon as quarry, Mars as the machine. JAXA's MMX mission, launching in the next few years, carries the IDEFIX rover that will measure Phobos's internal structure directly. The theory makes specific predictions about what they'll find: a body weaker than expected, with a surface more loosely bound than its interior. The measurement will either confirm the peeling model or constrain where it breaks down.