Phobos, the larger of the two satellites of Mars, orbits the planet just 6000km above its surface. Measuring 27x21x19km, Phobos is dominated by the great crater Stickney, which is 8km across. Parallel grooves radiate from Stickney & may be evidence of the severe fracturing caused by the impact which formed the crater & almost blew the moon apart. Phobos is composed of a black material that reflects only 3-5% of the sunlight striking it. Phobos orbits Mars in 7.7 hours in a synchronous orbit, which means that the same hemisphere is always facing the planet.