Phobos

Phobos is the larger of Mars’ two natural satellites, an irregular, potato-shaped moon with a mean diameter of about 27 kilometers. It is dark in appearance and has a low surface gravity, reflecting a porous composition similar to carbonaceous asteroids. Phobos is believed to have a mass on the order of 10^16 kilograms.

Discovered in 1877 by American astronomer Asaph Hall, Phobos was named after the Greek mythological personification of fear, a companion of Mars that accompanied the god in battle. Its smaller companion is Deimos, named for dread or panic. Both moons show ancient, heavily cratered surfaces and share a generally similar, dark, rocky composition.

Phobos orbits Mars at a distance of roughly 6,000 kilometers above the planet’s surface, completing an orbit every about 7.65 hours. It is tidally locked to Mars, keeping the same face toward the planet as it travels along an almost equatorial path with very little tilt. The orbit is slowly decaying due to tidal interactions with Mars, and it is expected to either crash into Mars or break apart to form a planetary ring in tens of millions of years.

Surface features include the prominent Stickney crater, about 9 kilometers in diameter, surrounded by a system of long grooves and rolls of regolith. The origin of Phobos is debated, but the prevailing view is that it is a captured asteroid, with a composition akin to carbonaceous meteorites, though some models allow a origin from debris created by a past Martian impact.

Phobos has been studied by Mars orbiters such as Viking, Mars Global Surveyor, and Mars Express, with additional data from Phobos 2. The Russian Phobos-Grunt mission (2011) attempted a sample return, and JAXA’s MMX mission aims to visit Phobos for return of samples.