Evidence for a Past Martian Ring from the Orbital Inclination of Deimos

We numerically explore the possibility that the large orbital inclination of the Martian satellite Deimos originated in an orbital resonance with an ancient inner satellite of Mars more massive than Phobos. We find that Deimos's inclination can be reliably generated by outward evolution of a Martian satellite that is about 20 times more massive than Phobos through the 3:1 mean-motion resonance with Deimos at 3.3 Mars radii. This outward migration, in the opposite direction from tidal evolution within the synchronous radius, requires interaction with a past massive ring of Mars. Our results therefore strongly support the cyclic Martian ring-satellite hypothesis of Hesselbrock & Minton. Our findings, combined with the model of Hesselbrock & Minton, suggest that the age of the surface of Deimos is about 3.5–4 Gyr, and require Phobos to be significantly younger.