Star Astronomy

The two Mars moons are known as Phobos and Deimos. The larger moon, Phobos, is oblong shaped moon which orbits Mars in just eleven hours, making it appear to rise in the west and set in the east. The oddly shaped parallel grooves on its surface appear to radiate from the largest crater on Phobos to a strange shaped area at the other end of the moon; these grooves are thought to have been formed from the impact of the largest crater. Phobos is tidally locked and therefore always presents the same face towards Mars; however, as Phobos orbits Mars faster than the planet rotates the tidal forces are decreasing its orbital radius at a very steady rate, this means that some time in the future Phobos will be broken up by the tidal forces.

The smaller of Mars moons is Deimos, also rises in the east but much slower than Phobos, it takes thirty hours to orbit Mars, it is also tidally locked to Mars. It is difficult to determine the surface detail due to its covering of thick dust which has filled the craters and covers the surface. It is believed that Phobos does not have this thick dust covering its surface because it orbits Mars much closer than Deimos and the gravitational pull takes such debris away from its surface.

Both of the Mars moons are thought to be about two million years old and comprise of materials similar to meteorites, rich in water and organic materials, known as carbonaceous chondrites. It is know that carbonaceous chondrites form it the asteroid belt and therefore it is highly probable that both Mars moons are captured asteroids which originated between Mars and Jupiter, however, it is most likely that the powerful gravity of Jupiter threw both Deimos and Phobos out near Mars where they were ‘caught’, to spending their lives orbiting Mars.

Phobos and Deimos are both very small; Phobos is just over twenty two kilometres and Deimos about twelve and half kilometres. Their small size causes their gravity to be extremely weak, due to this weakness they are unable to pull themselves into a spherical shape, and hence they are both oddly shaped moons.

Both of the Mars moons were discovered by Asaph Hall in 1877 and were given the names of Phobos, which means panic or fear and Deimos, terror and dread, who according to Greek mythology both accompanied their father Ares (known as Mars to the Romans) into battle.