Quaoar: a giant rock in the icy Kuiper Belt?

Wesley Fraser and Mike Brown (Caltech) used the Hubble Space Telescope to place an upper limit on the size of Quaoar, which is a giant Kuiper Belt Object that inhabits the outer part of the Solar System. The Kuiper Belt is the swarm of icy comets that orbit just beyond Neptune, and these bodies represent the debris that was left over when the giant planets formed. The new upper limit on Quaoar's diameter is D<1100km, which Fraser reported at the recent DPS planetary science conference; see his abstract for details. Combining this size limit with Quaoar's known mass (about one-fifth Pluto's) yields a density that is at least 3.5 gm/cm^3. This result is a bit of a surprise, since Quaoar's density is much greater than that of water-ice, while bodies in the outer Solar System are generally expected to be composed mostly of water ice. However, internal heating in a sufficiently large body can temporarily melt it soon after its formation, which would then cause its rocky component to settle to its center before its watery outer layer freezes out. Afterwards, an impact with another large Kuiper Belt may have stripped the young Quaoar of its icy mantle, which would leave its rocky core exposed, and may account for its high density. Although this scenario is rather speculative, it is also quite plausible.