A lighter core for Phobos?

Abstract

Context. The origin of the Martian moons Phobos and Deimos is still poorly understood, and is the focus of intense debate.\nAims. We demonstrate that a stratified internal structure of Phobos is compatible with the observed gravity coefficients.\nMethods. We fit previously derived C20 and C22 Phobos gravity coefficients derived from the combined MEX Doppler-tracking data from the close flybys in +2010 and 2013 with respect to the corresponding coefficients of a core–mantle stratification model of Phobos, with two opposite cases: a core denser than the mantle, and a core lighter than the mantle.\nResults. Only the case with a core lighter than the mantle fits at the 3σ level the previously reported observed second degree and order coefficient C20, but a homogeneous Phobos cannot be strictly ruled out at the 3σ level.\nConclusions. This possible loosening of the core density might be the result of a displacement of material toward the surface, may be caused by centrifugal forces acting on a loosely packed rubble-pile structure, and/or by a hot-then-cold in-orbit accretion process. These two hypotheses are by no means exhaustive.