Exploring the Recycling Model of Phobos Formation: Rubble-pile Satellites*

Phobos is the target of the return sample mission Martian Moons eXploration by JAXA that will analyze in great detail the physical and compositional properties of the satellite from orbit, from the surface, and in terrestrial laboratories, giving clues about its formation. Some models propose that Phobos and Deimos were formed after a giant impact giving rise to an extended debris disk. Assuming that Phobos formed from a cascade of disruptions and reaccretions of several parent bodies in this disk, and that they are all characterized by a low material cohesion, Hesselbrock & Minton showed that a recycling process may happen during the assembling of Phobos, by which Phobos's parents are destroyed into a Roche-interior ring and reaccreted several times. In this paper, we explore the recycling model in detail and pay particular attention to the characteristics of the disk using 1D models of disk/satellite interactions. In agreement with previous studies, we confirm that, if Phobos's parent bodies are gravitational aggregates (rubble piles), then the recycling process does occur. However, Phobos should be accompanied today by a Roche-interior ring. Furthermore, the characteristics of the ring are not reconcilable with today's observations of Mars' environment, which put stringent constraints on the existence of a ring around Mars. The recycling mechanism may or may not have occurred at the Roche limit for an old moon population, depending on the internal cohesion. However, the Phobos we see today cannot be the outcome of such a recycling process.