Since we have put a probe in orbit about Ceres, we have a very good idea of its average density, which is more than twice that of water. (JPL says 2.162 g/cm3) So there must be a good part of it that isn't water ice.
Since we have put a probe in orbit about Ceres, we have a very good idea of its average density, which is more than twice that of water. (JPL says 2.162 g/cm3) So there must be a good part of it that isn't water ice.