Table 2 Observational pros (P) and cons (C) for hypotheses for the origin of the Martian moons

Low surface reflectance with reddening spectra lacking clear silicate absorption features

P

This resembles D-type asteroids, and so is regarded as strong grounds for the asteroid capture theory

N

No absorption feature indicative of ejected Martian source materials is identified so far. Note that this might be masked by strong shock alteration or mixing of materials from the impactor

Small orbital eccentricity (e) and inclination (i) relative to the Martian equatorial plane

C

It seems difficult, especially for Deimos, for these orbits to evolve from a large e and random i through tidal friction alone

P

This is naturally explained if the giant impact introduces most of the spin angular momentum of Mars associated with the generation of a Moon-forming debris disk

Low-mass objects orbiting within and beyond the co-rotation radius (r_c) with Martian rotation

N

Tidal evolution theory predicts Phobos should be formed just inside r_c, making the conditions for capture severe. A similar difficulty exists to explain Deimos’s orbital radius unless some frictional medium such as a gas envelope extended to a large distance

C

The formation of Deimos outside of r_c requires a large disk mass, resulting in the formation of a massive inner moon. Tidal friction could eventually cause such a moon to fall onto Mars but no geologic evidence for such an impact is yet identified. Repeated tidal destruction and reaccumulation of the large inner moon might have produced Phobos