Moons of Mars - Phobos and Deimos

Mars has two small moons, Phobos and Deimos, which are thought to be captured asteroids. Both satellites were discovered in 1877 by Asaph Hall, and are named after the characters Phobos (panic/fear) and Deimos (terror/dread) who, in Greek mythology, accompanied their father Ares, god of war, into battle. Ares was known as Mars to the Romans.

Color image of Phobos obtained by Mars Reconnaissance Orbiter on March 23, 2008.

Diameter: 22.2 km (27×21.6×18.8)

Semi-major axis: 9377 km

Orbital period: 7.66 h

Color image of Deimos taken by the Mars Reconnaissance Orbiter on February 21, 2009.

Diameter: 12.6 km (10×12×16)

Semi-major axis: 23 460 km

Orbital period: 30.35 h



Origin

The origin of the Martian moons is still controversial. Phobos and Deimos both have much in common with carbonaceous C-type asteroids, with spectra, albedo, and density very similar to those of C- or D-type asteroids. Based on their similarity, one hypothesis is that both moons may be captured main-belt asteroids.

Both moons have very circular orbits which lie almost exactly in Mars's equatorial plane, and hence a capture origin requires a mechanism for circularizing the initially highly eccentric orbit, and adjusting its inclination into the equatorial plane, most probably by a combination of atmospheric drag and tidal forces, although it is not clear that sufficient time is available for this to occur for Deimos.

Capture also requires dissipation of energy. The current Mars atmosphere is too thin to capture a Phobos-sized object by atmospheric braking. Geoffrey Landis has pointed out that the capture could have occurred if the original body was a binary asteroid that separated under tidal forces.

Phobos could be a second-generation Solar System object that coalesced in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as Mars.

Another hypothesis is that Phobos and Deimos were formed by ejected materials, after a collision between Mars with a large planetesimal, similar to the prevailing theory for the origin of Earth's moon.

Observations of Phobos in the thermal infrared suggest a composition containing mainly phyllosilicates, which are well known from the surface of Mars.