In a recent Space.com article it is revealed that Phobos' density is 1.85 grams per cubic centimeter, revealing that "the moon is not solid, but probably filled with giant caverns. Researchers call this kind of body, which is basically a clump of rocks held together by gravity, a rubble pile."

25143 Itokawa is a rubble pile asteroid, and looks its part, clearly being formed from a jumble of different asteroid hunks.

Phobos does not look like a rubble pile asteroid at all. In fact, even in deep blasted out craters we see no sign that it is composed of numerous individual pieces. It appears to be formed from one piece. Granted, Phobos is 22km wide, while Itokawa is only 500 meters wide, but it seems as if Phobos, if indeed a rubble pile, should show signs of being composed of different pieces.

Recent images indicate Phobos has a layer of regolith at least 100 meters thick, perhaps formed by impacts. It is not known how this material stuck to an object with so little gravity.

Furthermore, the low density of phobos could mean it has a large reservoir of ice within it, despite the surface regolith having no evidence of hydration.

My speculation is that Phobos is the remains of a comet, or some other sort of icy body. If such a body contained adequate ammounts of carbonaceous chondrite material, evenly interspersed with the ice (the fabled "Dirty Snowball") this material might remain while the ice sublimated until the surface of the body for hundreds of feet deep was only the carbonaceous chondrite material, with only the deep core of the body still posessing any ice. This could explain the mysterious deep regolith, the mysterious density issues of Phobos, and how despite indications of being a rubble pile, the non cratered areas of phobos appear remarkably smoothe (at least by Itokawa standards).

Just armchair speculation however.

Best wishes,

---Ben.

Not bad speculation. I don't think low density necessarily translates into vast caverns. IIRC, carbonaceous chondrite is fairly low density. So, Phobos could be a fairly solid body and still have a low density.

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Yeah, that sublimation sounds good. But if you think Phobos might have been a comet, you have to consider how it could have been captured by Mars' relatively puny gravity. Comets move very fast. As far as I know, they either whizz by, or smack into you.
Of course, there might be already be a well-recognised mechanism for this and I just don't know