Icarus http://doi.org/p26 (2013)


Little is known about the origin and internal make-up of the moon Phobos and its relationship with its host planet Mars. A reanalysis of observations from the Mars Express spacecraft's 2008 flyby of Phobos, along with observations from a closer approach in 2010, suggests a porous and unhomogeneous moon that accreted close to Mars.

Martin Pätzold at the Universität zu Köln, Germany, and colleagues used the 2010 flyby data to further refine estimates of the mass and density of Phobos. The calculated density is too low for Phobos to be composed of solid rock. The density can, however, be explained by the presence of a loosely consolidated interior, consistent with models that suggest a porous jumble of rock and ice. Such a high porosity is inconsistent with the hypothesis that Phobos is an asteroid that was captured by the gravitational pull of Mars.

Instead, Phobos, like the Earth's Moon, may be a second-generation Solar System object that accreted from material orbiting in a disk around its primary planet.