The data and photos collected by the Mars Express satellite could help unwrap some of the mystery about the moon, the ESA said. Three scenarios are possible, one, that the moon is actually a captured asteroid. The second is that it formed at the same time as Mars formed below it. The third is that Phobos formed later than Mars, out of debris flung into Martian orbit when a large meteorite struck the red planet, according to the ESA.
From the ESA Web site describing the photos:Phobos is an irregular body measuring some 27 × 22 × 19 km. Its origin is debated. It appears to share many surface characteristics with the class of 'carbonaceous C-type' asteroids, which suggests it might have been captured from this population. However, it is difficult to explain either the capture mechanism or the subsequent evolution of the orbit into the equatorial plane of Mars. An alternative hypothesis is that it formed around Mars, and is therefore a remnant from the planetary formation period. Mars Express so far has taken photos on March 7, 10 and 13. According to ESA, Mars Express will continue to fly past Phobos until the end of March.