But Jupiter, Saturn, Uranus, and Neptune are all gas giants. Rocky planets like Mercury, Venus, Earth, and Mars, would obviously be much denser than them. I wonder how it stacks up, density-wise, to these rocky planets. Ice at the center would seem to suggest it would have a hollow core. That would be odd, given its rocky exterior.
They say here below, with 3 references, that the small moon is known to have “significant porosity” (has internal crevices). I guess that would explain the low over all density.
“Spectroscopically it appears to be similar to the D-type asteroids,[10] and is apparently of composition similar to carbonaceous chondrite material.[11] Phobos’s density is too low to be solid rock, and it is known to have significant porosity.[12][13][14]”
http://en.wikipedia.org/wiki/Phobos_%28moon%29#Physical_characteristics
Sounds like you could be right about it having some ice in its interior. But it could just as well have had empty space in those internal crevices.
“D-type asteroids have a very low albedo and a featureless reddish electromagnetic spectrum.[citation needed] It has been suggested that they have a composition of organic rich silicates, carbon and anhydrous silicates, possibly with water ice in their interiors.[citation needed] D-type asteroids are found in the outer asteroid belt and beyond; examples are 152 Atala, and 944 Hidalgo as well as the majority of Jupiter’s Trojans.
The Nice model suggests that D-type asteroids may have originated in the Kuiper belts.[1]
http://en.wikipedia.org/wiki/D-type_asteroid