The origin of Phobos is debated. While its density, lower than the density of the Martian surface rocks, make it appear to belong to D-class asteroids, the moon 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 following evolution of its orbit into the equatorial plane of Mars. An alternative hypothesis is that it formed in its present position, and is therefore a remnant from the planetary formation period.

Credits: ESA/ DLR/ FU Berlin (G. Neukum)

During Mars Express’ fly-bys of Phobos, the MaRS radio science experiment on Mars Express detected the gravitational influence of the moon on the trajectory of the spacecraft, so allowing the determination of the moon’s mass with unprecedented accuracy (1.072 1016 kg, or about one billionth the mass of the Earth). The 3D model of the body built thanks to HRSC data, combined with the mass measurement, is key to calculate the mean density of Phobos. In turn, this can tell scientists a lot about the moon’s composition (how much rock or ice may be present, and how porous the structure may be), helping to close in on Phobos’ origin.

Credits: ESA/ DLR (K.Willner)/ FU Berlin (G. Neukum)