Why is reddish Pluto a different colour from its three greyish moons, Charon, Nix and Hydra? Because the three are regularly resurfaced with loose material thrown up in the weak gravity of Nix and Hydra by impacts with small outer-Solar-System objects. Meanwhile, material ejected fast enough to travel as far as Pluto tends to be moving with sufficient speed to escape the gravity of the Pluto system altogether, according to Alan Stern of the Lunar and Planetary Institute in Houston, Texas. Pluto also repaints itself with an annual frost, and is partly shielded by Charon.
By assuming that the three moons are coated in the same stuff, Stern predicts that Nix and Hydra are both about 50 kilometres in diameter. NASA's New Horizons spacecraft, which Stern is in charge of, will find out for sure in 2015.