Actually, both Phobos and Deimos are gigantic, alien space ships. It says so in Donald A. Wollheim's 1955 sci-fi book "The Secret of the Martian Moons."

Phobos and Deimos are inhabited by Vegans from the star Vega (last time I read the book was '58, so I might be wrong about this), who are fleeing from the Martians who are returning to Mars after interstellar space exploration. The Vegans don't realize tht they have accidentally fled right to the home planet of the Martians, who are due to arrive here any day. Of course, when that happens, the Vegans will ignite the big engines and flee once again. Ah, but what of Earth?

Forget freedom. The Martians are way too advanced for us. We don't stand a chance.
 

A direct link to the May, 2010 article:

http://onlinelibrary.wiley.com/doi/10.1029/2009GL041829/abstract

Volume 37, Issue 9, May 2010, T. P. Andert, P. Rosenblatt, M. Pätzold, B. Häusler, V. Dehant, G. L. Tyler and J. C. Marty

Article first published online : 7 MAY 2010, DOI: 10.1029/2009GL041829

We report independent results from two subgroups of the Mars Express Radio Science (MaRS) team who independently analyzed Mars Express (MEX) radio tracking data for the purpose of determining consistently the gravitational attraction of the moon Phobos on the MEX spacecraft, and hence the mass of Phobos. New values for the gravitational parameter (GM = 0.7127 ± 0.0021 × 10−3 km3/s2) and density of Phobos (1876 ± 20 kg/m3) provide meaningful new constraints on the corresponding range of the body's porosity (30% ± 5%), provide a basis for improved interpretation of the internal structure. We conclude that the interior of Phobos likely contains large voids. When applied to various hypotheses bearing on the origin of Phobos, these results are inconsistent with the proposition that Phobos is a captured asteroid.