How the mass beneath Stickney affects Phobos geophysical properties

Despite extensive study, the origin of the Martian moon Phobos remains still an unresolved question. Linking its orbital configuration, spectral features and geodetic observables to one formation mechanism remains a challenging task. Determining and understanding Phobos’s gravitational field is a fundamental step toward constraining its interior and, consequently, its origin. Current estimates suggest a porous interior with possible water-ice content and a denser mass concentration in its equatorial region.

This study investigates Phobos’s geophysical observables under different impact scenarios at the Stickney crater to assess whether such events could account for the observed offset in its degree-two gravity coefficients relative to the homogeneous case. We model two impact geometries, representing different mass distributions along Stickney and assume that the impact produced one of two types of molten rock, each tested for three volume fractions. Each scenario is tested on layered, rubble-pile and Perlin-Noise-based interior,  with and without impact. All interiors contain the same volume of water-ice and porosities, and we normalize the mass of its rocky component to conserve Phobos total mass.

Our results indicate that if Phobos is a based on one of the proposed interior, the offset in the degree-two coefficients is caused by the compressed mass beneath Stickney. This provides a more straightforward explanation than the suggestion that Phobos consists of a light core and dense crust.