Phobos atlas maps where loose regolith ends up before MMX collects its 10 grams

Self-gravity on a body this small does not simply win. "Phobos evolves in a highly dynamical environment where surface-material motion is controlled by the combined effects of self-gravity, time-dependent Martian tides, and inertial forces," Herreros and Charnoz wrote ^[7]. In that low-gravity regime, they wrote, the displacement of loose material "cannot be inferred from topographic slope alone" ^[8].

They began with a digital terrain model that carries the grooves, lineaments and landslides, the sparsely cratered ground, the dominant Stickney Crater and the moon's separate spectral units ^[10]. Onto that they mapped acceleration vectors, and where the vectors lined up in particular patterns they marked dynamical features ^[11]. Those first products "provide information only about the direction of the local acceleration with respect to the local topography," the authors wrote ^[12]. So the centrifugal and tidal terms went in next ^[13], on the argument that mobility on Phobos answers "not only by local topography but also by orbital and rotational forces (centrifugal, tidal, and Coriolis)" ^[9].

MMX is to gather about 10 grams ^[3]. Phobos is roughly 11 km in diameter ^[4]; treated as a sphere of that size, its surface covers about 380 square kilometres ^[18]. Ten grams is one point on that, and phys.org's account of the letter says the material MMX collects has likely flowed across the surface in the past ^[21]. Earlier work had marked the zones where material is likely to be dislodged and stopped there, without saying where the dislodged material goes ^[14].

What the atlas does not tell you is when. Herreros and Charnoz argue that redistribution has to be modelled as "the dynamical motion of surface material under the combined action of gravity, tides, rotation" ^[20], and their maps give the direction of that motion ^[12]. A direction field carries no rate, so nothing in the atlas dates a grain's journey.

The origin question is what the provenance work is for. Phobos may be a rubble pile with a thin crust that Martian tides are slowly pulling apart, a captured asteroid, a moon that coalesced from debris thrown into orbit by an impact on Mars, or a hybrid of those routes ^[15]. Ten grams can bear on that only with context, and decoding what a few grams contains depends on understanding the object it came from ^[16]. About four years will separate MMX's arrival at Phobos from the sample's arrival on Earth ^[19].