The larger of the two moons of Mars, considerably more massive than its small fellow moon Deimos. Together the two moons are named for the twin sons of Ares (the Greek form of the name Mars): Phobos and Deimos, 'Horror' and 'Terror'.
Origins and Structure
Phobos is an irregular and elongated object (often described with some justification as 'potato-shaped') averaging about 11 kilometres in diameter. It does not appear to be composed of solid rock, but rather to be a conglomeration of loosely bound material with a thin outer crust. This material probably originated from Mars itself, thrown into orbit by an impact on the planet's surface, though the origins of Phobos and its fellow moon Deimos are not fully understood at present.
The surface of Phobos is peppered by small craters, with one immense crater dwarfing the others. This is Stickney, the remnant of a major impact event that stretches some 9 kilometres from edge to edge and dominates one end of the moon's elongated form.
Phobos follows an orbit that carries it extremely close to Mars' surface: at about 6,000 km, it comes closer to its primary planet than any other known moon. This remarkably low orbit means that Phobos travels around Mars with incredible speed: faster even than Mars itself rotates on its axis. To an observer on Mars' surface, the effect of this low, rapid orbit would be to see the small moon rise and set twice each Martian sol or 'day'.
This unusual orbit does not seem destined to remain stable in the long term. Phobos is slowly descending towards the surface of Mars, and current projections indicate that the moon will likely be broken apart as it ventures too near its parent planet, probably within a few tens of millions of years.
Long before the moons of Mars were discovered, authors had speculated about the subject. Two notable cases were Swift's Gulliver's Travels (1726) and Voltaire's Micromégas (1752), both of which included references to Mars having two moons. These were literary inventions (the actual moons would not be discovered until 1877, more than a century after these books were published) but they were curiously prescient.
For this reason, both of Mars' moons have craters named to honour Voltaire and Swift. On the smaller moon Deimos are two craters bearing their names, while Phobos has a number of surface features named for characters from Gulliver's Travels. Among these are a small crater named Limtoc inside the walls of Phobos' dominant crater Stickney, and another named for the character of Gulliver himself.