Ceres: The First Known Ice Dwarf Planet
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Ceres is the only dwarf planet inside the orbit of Neptune. Like the traditional planets, the asteroid is differentiated. This means the heavier, rocky elements settled into a core while lighter ices and rock rose to the mantle and crust. The asteroid’s surface is a cosmopolitan mix of rock, some water-ices, and hydrated minerals such as clay and carbonates, some of which account for its famous bright spots. And although not much ice is directly observable on the surface, the mass of near-surface rocks (within a few meters of the surface) weighs in at 10% water. This much near-surface ice supports earlier theories that as Ceres differentiated, water separated from rock and metal, forming an ice-rich outer crust. Ceres’ surface also contains iron-rich clays, but the core of the dwarf planet is probably composed of rock and metal, like its terrestrial cousins (Fig. 4.1).