The American probe Dawn, after visiting the asteroid Vesta, is now entering orbit around the asteroid Ceres, a few months before New Horizons arrives at Pluto, thus becoming the first probe to visit a dwarf planet. Ceres was thus discovered to be spherical and to have formed not in the Main Asteroid Belt, where it resides, but likely beyond the orbit of Neptune. This discovery is supported by the enormous amount of ammonia detected, unavailable in this part of the Solar System but abundant in the Kuiper Belt. Furthermore, Ceres is active, containing a large amount of water that is occasionally ejected from the surface, then evaporates or sublimates, leaving behind expanses of extremely white salt. This water perhaps comes from a salty underground ocean, which could constitute, or have constituted in the recent geological past, an entire layer of the stratified dwarf planet. One crater where this bright, clear spot is evident in its center is Occator Crater. A 5-kilometer-high ice volcano, Ahuna Mons, has also been identified.
