They are both. The moons of Mars are probably captured asteroids from the asteroid belt which have taken up permanent orbit around Mars for probably over the last billion years or so. This is a photograph of Phobos at a resolution of 12 meters. By 2000 we have obtained images from several asteroids such as Ida, Eros and Mathilda, and they all look very similar to Phobos. If Phobos were located anywhere else in the solar system, we would call it an asteroid.
The actual mechanics of how Phobos and Deimos were 'captured' into orbit remains a rather complicated, and in some ways, mysterious problem in celestial mechanics. Ordinarily, it is impossible for a single body to be captured into orbit around another body in the so-called '2-body' problem. It is somewhat easier is multiple bodies are involved because then the necessary surplus of angular momentum can be carried away by the ejected body leaving the other behind with just the right amount of kinetic energy and angular momentum. As for Mars, the origin of Phobos and Deimos as captured satellites remains a bit peculiar.
Copyright 1997 Dr. Sten Odenwald
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