One of the most striking features on the 27 km diameter, irregularly shaped Phobos is the presence of grooves over most of its surface. The grooves seem to radiate in all directions from the giant Stickney crater (left) and converge on the opposite side of the satellite at a region close to the Stickney antipode. The grooves are best developed near Stickney, where some measure 700 m across and 90 m deep. However, most of the grooves have widths and depths in the 100 - 200 m and 10 - 20 m range respectively.
Phobos is the larger of the two Martian satellites and orbits closest to the planet at just under 6000 kilometres from the surface. Scientists speculate that Phobos and its companion moon Deimos were once passing asteroids that were pulled in by the gravity of Mars.