A list of possible explanations:
1. Phobos was an asteroid formed in the main asteroid belt and was caught by Mars's gravitational force.
2. Phobos was formed from the shrapnel caused by a collision between a space rock and Mars's surface.
3. Phobos could have formed from an earlier moon that was destroyed by Mars's gravitational force.
However, recent data from the European Space Agency's Mars Express spacecraft are making the asteroid capture hypothesis seem less likely.
Recent observations as thermal infrared wavelengths using the Planetary Fourier Spectrometer (PFS) instrument on Mars Express show a poor match between the rocks on Phobos and any class of chondritic meteorite known from Earth.
We detected for the first time a type of mineral called phyllosilicates on the surface of Phobos, particularly in the areas northeast of Stickney, its largest impact crater," said co-author Dr Marco Giuranna, from the Italian National Institute for Astrophysics in Rome.
Other observations from Phobos appear to match the types of minerals identified on the surface of Mars. Thus, the make-up of Phobos appears more closely related to Mars than to asteroids from the main belt, say the researchers.
Original article: http://www.bbc.co.uk/news/science-environment-11378762