Mars

Mars has two tiny satellites, Phobos (27 x 19 km) and Deimos (15 x 11 km). Studies of both show that they are chondritic asteroids that have been captured by Mars gravity (but were originally formed in the main belt of asteroids located beyond Mars). Both are in 1:1 spin orbit couples with Mars, meaning that the same face of these small moons faces Mars all the time. Both satellites have densities of about 2 gm/cm3, indicating that they are internally fractured as well as being made of rather light mineral and organic compounds.

The most common superficial form on these satellites is the impact crater. Carter Stickney on Phobos is the largest such feature, about 6 mi (10 km) in diameter. Crater Hall, also on Phobos, is the second largest. A ridge between these craters is named Kepler. Both satellites are covered by a thick layer of broken debris called regolith (or "soil"). Regolith on the sides of impact craters can be seen in layers, suggesting the regolith is impact related (ejected fragments).

Boulders can be seen lying on the side of Deimos. Linear grooves occur in regolith on both satellites, and the linear grooves are "beaded" (a feature that suggests finer soil particles are draining down into open spaces in the underlying fractures within the satellites). Both satellites are inside the Roche limit of stable orbit, meaning that eventually both satellites will impact on the surface of Mars. Apparently, this sort of thing has happened before as there are now some well documented elongate (low angle) impact craters on Mars that appear to have been formed by objects in orbits similar to Phobos and Deimos.