The dynamical evolution and origin of the Martian moons

Abstract

The satellites of Mars are found today on relatively circular, un-inclined orbits that lie close to their primary. Assuming that the secular acceleration of Phobos observed today is caused by solid body tides, the orbital evolution equations can be integrated backwards in time. Doing this indicates that the original orbits of Phobos and Deimos were both close to the synchronous orbit and were of low inclination; the eccentricity of Phobos' orbit may have been moderate in the past but Deimos' was always near zero. Integrating forward shows that Phobos will crash onto Mars in less than 10 8 years. Spacecraft measurements of low densities and albedos for Phobos and Deimos, as well as Phobos' reflectance spectrum suggest that the martian moons are carbonaceous chondritic in composition, that is, derived from material presumably formed much further away from the Sun than Mars. Two origin scenarios are described that produce satellites, composed of chondritic material, in fairly regular orbits. The first is the usual accretion theory, except that the composition of the satellites is determined by the last material brought into the system and that matter is not compacted since the central pressures are low. The second suggests that the martian satellites are the remnants of an interplanetary meteoroid which was captured and fragmented upon its collision with a primordial circumplanetary nebula surrounding Mars; further interaction with the nebula regularized the orbits. This mechanism appears to be successful in also explaining the irregular satellites found in the outer solar system. The rotations of Phobos and Deimos are tidally locked to be synchronous with their orbital periods; the longest axes of the satellites point approximately along the Mars-to-satellite lines while the shortest are normal to the satellite orbital planes. These configurations are a natural outcome of the action of tides on the satellites; their adoptions take place with characteristic timescales of ∼10 6 years for Phobos and ∼10 8 years for Deimos. Phobos is found to exhibit free librations of ∼5° about its synchronous position due to a near-resonance between its natural libration frequency and the frequency at which it is being forced. The current rotation states of the martian moons say nothing about their origin.