Our Moon is believed to have been formed when a Mars sized body struck an early Earth and blasted out material into orbit, leaving it with such a recoil that even billions of years after the material collected into a body, our satellite is still edging nervously away from us.
The origins of other satellites around other planets have long been less clear. An example in point are the irregularly shaped Phobos and Deimos, pieces of rubble in orbit of Mars. Now research carried out comparing the minerals present on Phobos seem to suggest that it too was once a part of the planet it now orbits.
Analysis of the composition of the satellite shows more similarities to Mars than to asteroids in the belt between Mars and Jupiter (where another hypothesis suggested the body might have originated). Further to this, minerals associated with the presence of water and silicates have been seen. As something as small as Phobos would’ve lost heat too quickly alone in the early solar system to have had enough time for such reactions to take place, this suggests that the satellite contains material once part of a bigger and wetter body, or had a presently unknown internal heat mechanism. Phobos is also a lot spongier than an asteroid surviving capture by Mars would likely be as well as being in a more circular, more equatorial orbit than chance would suggest (a characteristic shared by Deimos).