Asteroids and the Mars Satellites
Apparently, the asteroids and their ancestors should be planetesimals similar to those ones that formed the other planets of the solar system.
However, these bodies were gravitationally disturbed until they assumed tilted and noticeably elliptical orbits. Two processes could have caused this disturbance:
The biggest asteroid, Ceres, reaches a diameter of about 930 km and represents more than 25% of the combined mass of all the asteroids. The shapes of the asteroids unveil their violent history and vary between nearly spherical forms and irregular or elongated forms. The rotation periods vary between some few hours and several days. Satellites orbit some asteroids, as it happens with Ida and Dactyl.
Composition and Geological Differentiation
There are several types of asteroids according to their compositions. The chondrites are asteroids whose content of non-volatile elements reflects the proportions that should prevail in the primordial nebula. Most chondrites are rich in carbon. Others are clearly enriched with determined elements.
The presence of these elements at the surface of the asteroids would have been manifested through the geological modification of the materials, occurred during the planetary evolution processes. One of those processes would have been the differentiation, which consisted in the melting and physical segregation of the materials. Just as in the inner planets, the heavier materials would have been sunk to form the core, while the lighter ones emerged to form the crust. It remains a mystery the fact that some asteroids seem to have been kept unchanged, while other asteroids of similar sizes and in nearby orbits clearly seem to have been modified.
The asteroids that are composed by geologically modified minerals are divided in classes like the irons, the stony-irons and the achondrites. The composition of the asteroids would have been determined by their positions in the primitive solar nebula. The asteroids that are rich in high temperature minerals are essentially placed in zones close to the Sun, while the asteroids that have a composition more similar to the comets' are essentially placed at the outer regions.
The asteroid Ida and, to the right, its satellite Dactyl (NASA - JPL)
One of the last pictures taken from the asteroid Eros, in February 2001. The image is only 12 metres across (NEAR Project - JHU/APL)
The Mars Satellites
The satellites of Mars have tiny dimensions are irregularly shaped. Phobos, the biggest among them, displays perpendicular axis of 27 x 21 x 19 km. It is thought that, in remote times, Phobos and Deimos belonged to the asteroid belt, both displaying similar characteristics with the asteroids classified as carbonic chondrites.
It's possible that Mars further captured them, but it is also possible that they were members of the planetesimals cluster that ended up forming the red planet.
The Phobos satellite (NASA)