Phobos and Deimos ("Fear" and "Terror" in Greek) are the two small moons of Mars. Phobos, the larger of the two at roughly 28 x 20 x 18 km, has too low a density to be solid rock and is in a decaying orbit around Mars. Its distance from Mars is decreasing by 1.8 m per century and in 50 million years or so it will likely break up from gravitational tidal forces forming a ring of material around the planet. As you can see, it's heavily pocked and streaked from meteorite impacts.
In the past, Phobos and Deimos were thought to be captured asteroids due to their asteroid-like irregular shapes and small size. Mars also lies close to the asteroid belt although the low density of the moons suggest an ice and rock composition found in objects further out in the solar system (trans-Neptunian objects).
New data from the Mars Express spacecraft, a European Space Agency mission, indicates that Phobos (and by analogy Deimos) may have formed from accretion of material tossed into orbit by a collision of an asteroid with the surface of Mars (kind of like how our moon formed but on a smaller scale). Examining Phobos at thermal infrared wavelengths with the Planetary Fourier Spectrometer (PFS) revealed a composition that didn't really match any known asteroids (but was similar to Mars) and a group of minerals called phyllosilicates were also detected.
Phyllosilicates are essentially the micas and clay minerals and generally form in the presence of water. Phobos is too small to ever have had liquid water but Mars was at one time wet. These minerals may well have formed on the Martian surface a few billion years ago and found their way into the moon by an impact on Mars.
In addition, the spacecraft was able to very accurately measure the mass of Phobos from its gravitational perturbation of the spacecraft's orbit and came up with a density for the moon of 1.86 ± 0.02 g/cm3. Given that the average density of silicate rocks is around 2.7 g/cm3, this is a low density. This low density can be explained if Phobos is full of voids (anywhere from 25-45% of its interior) and this is consistent with the collision-accretion model of its formation. Phobos is basically a rubble pile held together by gravity.
The porosity of Phobos would also explain why it didn't break up when crater Stickney formed (see that huge "dimple" in the image above?). This internal structure allowed the moon to absorb all the energy from that impact without blasting apart.
This new data was presented by Marco Giuranna, a researcher from the Istituto di Fisica dello Spazio Interplanetario at the European Planetary Science Congress running from September 19-24 in Rome.