The natural satellites of Mars are very small. PHOBOS has an elliptical shape (28 x 20 km) and its orbit is very low with a mean altitude of about 6000 km. Its surface is marked by craters. The biggest one, Stickney, has a diameter of about 10 kilometers, which represents one third of the largest dimension of the satellite. A series of grooves leading out of Stickney seem to be fractures due to the impact which created this crater.
Map of all the surface of Phobos
Phobos seen by Mars Express
It is commonly admitted that PHOBOS and DEIMOS are asteroids that were "captured" by Mars gravitation field by passing too near the planet. Another hypothesis is that these two moons could be remains of a bigger old satellite which gravitated around the red planet. But, the layer of rock and dust on their surface, their regolith, seem really different.
More documented in-situ observations of the composition of Phobos surface completed by fine analyses on samples brought back on Earth should answer this question. Moreover, it is probable that its surface received in influx of rocks ejected from the surface of Mars during asteroid impacts, which should have a distinct signature from the primitive matter.
PHOBOS GRUNT mission will enable to realise in-situ observations with an important payload which should operates on the surface for one year. Moreover, it will collect samples (about 200g) which will be sent back on Earth for very precise analyses.
The mission main objectives are to solve the following interogations:
the origine of Phobos (and probably of Deimos) in relation with the study of the primitive matter,
its evolution in relation with Mars,
the role of the asteroids impacts in the formation of the planets and in the evolution of their atmosphere, their crust and in the inventory of the volatil elements.