No it isn't. There is no reason whatsoever to suppose Phobos isn't a natural phenomenon. On the Phobos blog, you can find a recent post (http://webservices.esa

No it isn't. There is no reason whatsoever to suppose Phobos isn't a natural phenomenon. On the Phobos blog, you can find a recent post (http://webservices.esa.int/blog/post/7/1085 http://webservices.esa.int/blog/post/7/1085), which says: [..] 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. In other words, the moon is pretty porous and asteroids aren't, so it isn't a captured asteroid. However, that is only one of three hypotheses. In a previous post (http://webservices.esa.int/blog/post/7/989 http://webservices.esa.int/blog/post/7/989) they state: The origin of Phobos is a mystery, in fact three scenarios are considered possible. The first is that the moon is a captured asteroid; the second is that it formed in-situ as Mars formed below it, and the third is that Phobos formed later than Mars, from debris flung into martian orbit when a large meteorite struck the Red Planet. Among other objectives, the Phobos flybys are designed to provide clues towards answering this question. So one of the three hypotheses has been ruled out. The other two are still candidates. There is no mention of any internal voids with inexplicably 'geometric' shapes