Phobos

## General Information

Phobos is the biggest and innermost of Mars' two natural satellites. The two moons were found in 1877 by Asaph Hall, an American astronomer. It bears the name of Phobos, the twin brother of Deimos and the son of Ares (Mars), the Greek deity of panic and fear.
The mean radius of the tiny, asymmetrical object Phobos is 11 km (7 mi). The distance between Phobos and its main body is the closest of any known planetary moon to the surface of Mars, at 6,000 km (3,700 mi). It completes an orbit of Mars in just 7 hours and 39 minutes, orbiting the planet far quicker than Mars rotates. This means that, from Mars' surface, it seems to rise in the west, travel across the sky in about 4 hours and 15 minutes, and set in the east twice a day.

### History

Astronomer Asaph Hall found Phobos on August 18, 1877, at the US Naval Observatory in Washington, D.C., at approximately 09:14 Greenwich Mean Time. A few days earlier, on August 12, 1877, at approximately 07:48 UTC, Hall had discovered Deimos, the other moon of Mars. The names of the moons, originally spelled Phobus and Deimus, respectively, were proposed by Henry Madan (1838–1901), an Eton College science master, and were based on Greek mythology, which depicts Phobos as a companion of the god Ares.

### Properties

With measurements of 27 km × 22 km × 18 km, Phobos is too small to circle under the force of its own gravity. As a result of its low mass and gravity, Phobos lacks an atmosphere. With an albedo of roughly 0.071, it is among the Solar System's least reflecting bodies. Its composition is comparable to that of the surface of Mars, and infrared spectra reveal that it contains carbon-rich material, which is characteristic of carbonaceous chondrites. Phobos is known to have high porosity and its density is too low to make it a solid rock. These findings raised the possibility that Phobos might have a sizable ice reserve. Although spectral observations show that the regolith layer at the surface is not hydrated, ice beneath it is not completely ruled out.

Though the moon is tiny, Phobos is severely cratered compared to Deimos, with one crater near the equator having a central peak. Of these, the most notable is Stickney, a massive impact crater with a diameter of about 9 km (5.6 mi) that occupies a significant amount of the moon's surface. Similar to Mimas's crater Herschel, Phobos must have been almost completely destroyed in the collision that formed Stickney.

The uneven surface is also covered with many streaks and grooves. Originally thought to have resulted from the same event that produced Stickney, the grooves are normally under thirty meters deep, between 100 and 200 meters broad, and as much as twenty kilometers in length. However, examination of data from the Mars Express mission showed that the grooves are really centered on Phobos's leading peak in its orbit, which is not distant from Stickney, rather than being radial to Stickney. Scientists believe that debris sent into orbit by impacts on Mars' surface may have been responsible for its excavation.

Although faint dust rings from Deimos and Phobos have been for some time expected, observations of these rings have not been successful. A coating of fine-grained regolith, at least 100 meters thick, covers Phobos, as demonstrated by recent photographs from Mars Global Surveyor. This layer is thought to have been formed by collisions from other bodies, however it is unclear how the material adhered to an object with nearly no gravity. It has been suggested that the unusual Kaidun meteorite, which landed on a Soviet military camp in Yemen in 1980, may have been fragmented from Phobos, although this has not been confirmed because Phobos' precise makeup is unknown.