Stickney’s impact on Phobos geophysical observables

ABSTRACT The origin of Phobos, the larger of the Martian moons, is one of the most researched yet unresolved questions in the context of the Martian environment. Its low albedo, low mass, and unique orbital configuration are difficult to reconcile with any single formation mechanism. While its internal structure remains poorly constrained, current estimates suggest a porous interior with possible water–ice content. Although recent space missions have provided essential data, they have not yielded precise constraints on its mass distribution and internal composition. Detailed gravitational field mapping emerges as a crucial method to address these open questions. Motivated by the hypothesis that the Stickney impact produced a localized zone of densified material, first proposed by S. Le Maistre, A. Rivoldini & P. Rosenblatt (2019), we model how such a compressed area would affect Phobos’ geodetic observables. Using a voxel-based mascon approach, we vary the distribution of compressed material beneath Stickney (flat versus deep), the assumed high-density impact material proxies (coesite versus stishovite), and its volume fraction, across four interior types and varying water + void content. For each realization, we compute gravity coefficients up to degree and order 10, the principal moments of inertia, and the libration amplitude. Within our framework, a compressed zone beneath Stickney can shift the degree-2 gravity coefficient $C_{2,0}$ towards its current estimate, explaining deviations from the homogeneous-density models. If a higher subsurface density beneath Stickney is fully responsible for the degree-2 departure from its homogeneous analogue, our results indicate that this denser region would account for 7 per cent of Phobos total volume.