At 940 kilometres across, Ceres is so big that it contains roughly one-third of all the mass in the asteroid belt — and it is technically both an asteroid and a dwarf planet.
Asteroids, considered to be dry and barren, which most of them are but except the Solar System’s biggest asteroid — Ceres — which is full of water, NASA’s Dawn spacecraft discovered on December 15th.
Today, the water is either frozen as ice, filling pore spaces deep inside Ceres, or locked inside hydrated minerals at the surface. But billions of years ago, early in Ceres’s history, heat left over from the Solar System’s formation probably kept the asteroid warm inside. This allowed the water to churn and flow, helping to separate Ceres into layers of rock and ice.
“We know the water and the rock have separated and interacted over time,” said Carol Raymond, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, at a meeting of the American Geophysical Union in San Francisco on 15 December.
Hydrogen levels were richest in the middle to high latitudes, with the greatest concentrations — up to 30% water — present at the north pole. Around the equator, frozen water has probably sublimated into space and dried out Ceres’s surface. An astronaut there would have to dig down about 1 metre to find frozen water, whereas at the north pole, a visitor “would just swipe and find the ice table”, he says.
Similarly to the Moon and Mercury, the airless Ceres apparently manages to trap frozen water in dark areas on its surface
The discovery adds to a growing awareness of Ceres as an active, wet world that pushes the boundary of what it means to be a planet. Today it sports a 4-kilometre-high ice volcano and bright spots of salt mixed with ice and rock.