The formation of the Martian moons

Phobos (22 km across) and Deimos (12 km across), the two moons of Mars, are small and oddly shaped. Those characteristics suggest that long ago, the two satellites were asteroids that got trapped in the planet’s gravitational field. The two moons, however, orbit in the equatorial plane of Mars, a highly unlikely configuration for captured asteroids. An alternative scenario, which naturally leads to equatorial orbits, is that the moons formed from a debris disk thrown up by the impact that created Mars’s enormous Borealis basin billions of years ago. That mechanism struggles to explain how it is that two small moons formed, and why they formed relatively far from Mars. A new simulation by a team led by Pascal Rosenblatt of the Royal Observatory of Belgium addresses those challenges. In their model, in the impact’s aftermath a large moon (a few 100 km in diameter) first forms from the relatively dense disk region near the planet. Over a period of a few thousand years, gravitational interactions with the disk cause the large moon to migrate outward. As it does so, it concentrates less-dense outer-disk material in a pair of expanding orbits that are in resonance with its own orbital motion. Over the next few million years, tidal forces exerted by Mars on the large moon send the moon crashing into the Martian surface. Left behind are the two outer satellites—Phobos and Deimos. (P. Rosenblatt et al., Nat. Geosci. 9, 581, 2016, doi: 10.1038/ngeo2742 .)