Phobos is heavily cratered. The most prominent surface feature is Stickney crater, named after Asaph Hall's wife, Angeline Stickney Hall, Stickney being her maiden name. Like Mimas's crater Herschel on a smaller scale, the impact that created Stickney must have almost shattered Phobos. Many grooves and streaks also cover the oddly shaped surface. The grooves are typically less than 30 m deep, 100 to 200 m wide, and up to 20 km in length, and were originally assumed to have been the result of the same impact that created Stickney. Analysis of results from the Mars Express spacecraft, however, revealed that the grooves are not in fact radial to Stickney, but are centered on the leading apex of Phobos in its orbit (which is not far from Stickney), and must have been excavated by material ejected into space by impacts on the surface of Mars. The grooves thus formed as crater chains, and all of them fade away as the trailing apex of Phobos is approached. They have been grouped into 12 or more families of varying age, presumably representing at least 12 Martian impact events.

The unique Kaidun meteorite is thought to be a piece of Phobos, but this has been difficult to verify since little is known about the detailed composition of the moon.

Orbital characteristics

Phobos's unusually close orbit around its parent planet produces some unusual effects.

As seen from Phobos, Mars would appear 6 400 times larger and 2 500 times brighter than the full Moon appears from Earth, taking up a quarter of the width of a celestial hemisphere.

Phobos orbits Mars below the synchronous orbit radius, meaning that it moves around Mars faster than Mars itself rotates. Therefore it rises in the west, moves comparatively rapidly across the sky (in 4 h 15 min or less) and sets in the east, approximately twice a day (every 11 h 6 min). Since it is close to the surface and in an equatorial orbit, it cannot be seen above the horizon from latitudes greater than 70.4°.

As seen from Mars' equator, Phobos would be one-third the angular diameter of the full Moon as seen from Earth. Observers at higher Martian latitudes would see a smaller angular diameter because they would be significantly further away from Phobos. Phobos' apparent size would actually vary by up to 45% as it passed overhead, due to its proximity to Mars' surface: for an equatorial observer, for example, Phobos would be about 0.14° upon rising and swell to 0.20° by the time it reaches the zenith. By comparison, the Sun would have an apparent size of about 0.35° in the Martian sky.

Phobos' phases, in as much as they could be observed from Mars, take 0.3191 days (Phobos' synodic period) to run their course, a mere 13 seconds longer than Phobos' sidereal period.

Solar transits

An observer situated on the Martian surface in a position to observe Phobos would see regular transits of the moon across the Sun. Phobos is not large enough to cover the Sun's disk, and so cannot cause a total eclipse. Several of these transits have been photographed by the Mars Rover Opportunity. During the transits, Phobos's shadow is cast on the surface of Mars, which has been photographed by several spacecraft.

Future destruction

Phobos' low orbit means that it will eventually be destroyed: tidal forces are lowering its orbit, currently at the rate of about 20 meters per century, and in 11 million years it will either impact the surface of Mars or (more likely) break up into a planetary ring. Given Phobos' irregular shape and assuming that it is a pile of rubble (specifically a Mohr-Coulomb body), it has been calculated that Phobos is currently stable with respect to tidal forces. But it is estimated that Phobos will pass the Roche Limit for a rubble pile when its orbital radius drops by a little over 2 000 km to about 7 100 km. At this distance Phobos will likely begin to break up forming a short lived ring system around Mars. The rings themselves will then continue to spiral slowly into Mars.

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, albedos and densities very similar to those of C- or D-type asteroids. Based on this similarity, one hypothesis is that both moons may have been captured into Martian orbit from the main asteroid belt. Both moons have very circular orbits which lie almost exactly in Mars' 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 likely 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. Landis has pointed out that the capture could have occurred if the original body was a binary asteroid that separated due to tidal forces. The main alternative hypothesis is that the moons accreted in the present position. Another hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large planetesimal.
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