By simply measuring the gravitational influence of Phobos on the Mars Express spacecraft itself (which is a function of Phobos’ shape and density), ESA was able to determine that Phobos is incredibly light, about 1 billionth the mass of Earth, and has a much lower density (1.85 grams per cubic centimeter) than Martian surface rocks (.7-3.3 grams per cubic centimeter). Theoretically, this places Phobos more correctly in the D-Class category of asteroids, which are believed to be highly fractured and cavernous. One explanation for this is that Phobos (and perhaps its sister moon, Deimos) might be a “rubble pile,” essentially a bunch of big chunks that crashed into each other that are held together by gravity. According to this idea, Phobos was formed from material somehow ejected from Mars itself eons ago that somehow assembled themselves into a coherent body. However, there is no suitable explanation for how such a “rubble pile” could manage to find itself in a near perfect equatorial orbit around Mars, a condition that is highly unlikely to have occurred just by chance, to say the least. Given that its composition and density is so different from Mars’ own, it seems more likely that Phobos came from somewhere else, perhaps somewhere in the nearby asteroid field that lies between Mars and Jupiter. The only issue that remains is the question of whether this placement is a natural condition – again, highly unlikely – or whether it was placed there in the distant past by some unseen and unknown force.
There is however one last scenario under which Phobos might turn out to be a natural object, albeit one that resides in an impossibly perfect equatorial orbit. According to Dr. Tom van Flandern’s exploded planet hypothesis, all the remnant chunks of an exploded planet (like his theorized Planet V in the orbit between Mars and Jupiter) would be expected to have orbiting companions for at least a significant portion of their lifetimes. True to form, Phobos in close up shows the tell tale-signs of just such a post explosion condition; it is criss-crossed with dozens of race track signatures which indicate orbiting companions which have spiraled into the mother body (Phobos). As van Flandern has pointed out, these types of race track patterns are a sure sign of orbiting companions.