The Dynamics and Origin of the Martian Moons

The two moons of Mars, Phobos and Deimos, have long been a mystery to astronomers. Small and irregular in shape, they are usually considered to be captured asteroids. However, their near-equatorial orbits point to their likely origin in an accretion disk around Mars. It is still not clear if these satellites were accreted from Martian debris excavated by impacts, or from material derived from captured small bodies. The Martian moons' dynamics over the age of the solar system is becoming better understood now. Phobos is interior to the synchronous orbit and its orbit is decaying; it is expected that Phobos will be disrupted into a ring by the action of Martian tidal forces in the next few tens of Myr. A crucial question about the past orbital evolution of Phobos is whether it formed close to the synchronous orbit and always migrated inward. An alternative view is that Phobos is a product of an ongoing ring-moon cycle around Mars, with past generations of inner moons being significantly more massive. I will present my recent work on the orbit of Deimos, which strongly supports the view that Mars had past rings and much larger inner moons. I will also discuss the implications for the ultimate origin of Phobos and Deimos.