ESA & Mars Express closer to decerning origin of Phobos

European space scientists are getting closer to unravelling the origin of Mars’ larger moon, Phobos. Thanks to a series of close encounters by ESA’s Mars Express spacecraft, the moon looks almost certain to be a ‘rubble pile’, rather than a single solid object. Mars' largest moon is an irregularly sized lump of space rock measuring just 27 km x 22 km x 19 km. Recent passes by Mars Express have allowed teams of European scientists to determine both the mass and volume of Phobos. Putting this data together, the teams will be able to calculate the moon's density. Eventually, this will be a new important clue to how the moon formed. Preliminary density calculations suggest that it is just 1.85 grams per cubic centimetre. This is lower than the density of the martian surface rocks, which are 2.7-3.3 grams per cubic centimetre, but very similar to that of some asteroids. The particular class of asteroids that share Phobos’ density are known as D-class. They are believed to be highly fractured bodies containing giant caverns because they are not solid. Instead, they are a collection of pieces, held together by gravity. Scientists call them rubble piles. This suggests that Phobos, and probably its smaller sibling Deimos, are captured asteroids. The only problem with this scenario is that captured asteroids have erratic obits, but Phobos and Demos have equatorial orbits that seem to contradict the asteroid conclusion. To clear up the question of where the moons came from would require samples of the moon's surface. This possibility might soon become reality because the Russians will attempt to do this with the Phobos-Grunt mission, (I couldn't make this up if I tried!) to be launched next year.