Phobos

Phobos, the largest Martian, gouged and nearly shattered by a giant impact crater and beaten by thousands of meteorite impacts, is on a collision course with Mars.

Phobos, named after a messenger of the Roman god of war, is the larger of Mars' two moons and 27 by 22 by 18 km in diameter. It orbits Mars three times a day, and is so close to the planet's surface that in some locations on Mars it cannot always be seen.

Phobos is nearing Mars at a rate of 1.8 meters every hundred years; at that rate, it will either crash into Mars in 50 million years or break up into a ring. Its most prominent feature is the six-mile crater Stickney, its impact causing streak patterns across the moon's surface. Stickney was seen by Mars Global Surveyor to be filled with fine dust, with evidence of boulders sliding down its sloped surface.

Phobos and Deimos appears to be composed of C-type rock, similar to blackish carbonaceous chondrite asteroids. Observations by Mars Global Surveyor indicate the surface of this small body has been pounded into powder by eons of meteoroid impacts, some of which started landslides that left dark trails marking the steep slopes of giant craters.

Measurements of the day and night sides of Phobos show such extreme temperature variations that the sunlit side of the moon rivals a pleasant winter day in Chicago, while only a few kilometers away, on the dark side of the moon, the climate is more harsh than a night in Antarctica. High temperatures for Phobos were measured at -4 degrees Celsius (25 degrees Fahrenheit) and lows at -112 Celsius (-170 degrees Fahrenheit). This intense heat loss is likely a result of the fine dust on Phobos' surface, unable to retain heat.

Phobos has no atmosphere. It may be a captured asteroid, but some scientists show evidence that contradicts this theory.

Phobos data
Discovery	08-16-1877 by Asaph Hall
Mean distance to Mars	9,270 km
Diameter	27 × 22 × 18 km
Escape velocity	0.011 km/s
Orbital period	7 hrs 39 min
Orbital eccentricity	0.0151
Orbital inclination	1.1°
Stickney Crater Diameter	9 km

Information from: SSE-JPL-NASA,Wikipedia and David Darling's encyclopedia

Deimos

Named after the Roman god of dread, Deimos is the smaller of Mars' two moons. Only 15 by 12 by 11 km in size, Deimos whirls around Mars every 30 hours.

Like Phobos, Deimos is a small "lumpy", heavily cratered object. Its craters are generally smaller than 2.5 kilometers in diameter, however, and it lacks the grooves and ridges seen on Phobos. Typically when a meteorite hits a surface, surface material is thrown up and out of the resulting crater. The material usually falls back to the surface surrounding the crater. However, these "ejecta deposits" are not seen on Deimos, perhaps because the moon's gravity is so low that the ejecta escaped to space. Material does appear to have moved down slopes, however. Deimos also has a thick regolith, perhaps as deep as 100 meters, formed as meteorites pulverized pounded the surface.

Deimos is a dark body that appears to be composed of C-type surface materials, similar to that of asteroids found in the outer asteroid belt.

Deimos data
Discovery	08-10-1877 by Asaph Hall
Mean distance to Mars	23,460 km
Diameter	10 × 12.2 × 11 km
Mean density	2.2 g/cm³
Escape velocity	0.0069 km/s
Orbital period	30.312 hours
Axial period	synchronous
Orbital eccentricity	0.00033
Orbital inclination	0.93°
Albedo	0.07

Information from: SSE-JPL-NASA,Wikipedia and David Darling's encyclopedia