Dawn maps Ceres' permanently icy craters
Water ice has built up over a billion years, NASA scientists say. Bill Condie reports.
Some craters near the poles of dwarf planet Ceres are cold traps that have probably accumulated ice over a billion years or more, NASA scientists have found. Ceres is the largest object in the asteroid belt between Mars and Jupiter but is only 945 kilometres in diameter. Nevertheless, it is just large enough to have enough gravity to hold on to water molecules.
The icy regions lie in areas that are permanently shadowed regions that do not receive direct sunlight. They are typically located on the crater floor or along a section of the crater wall facing toward the pole. Even if they receive indirect light, as long as they remain at a temperature below about minus 150 °C water ice accumulates and remains stable.
The largest icy region is inside a 16-kilometre-wide crater located less than 65 kilometres from the north pole. Taken together, Ceres' permanently shadowed regions occupy about 1,800 square kilometres, less than 1% of the surface area of the dwarf planet.
"While cold traps may provide surface deposits of water ice as have been seen at the moon and Mercury, Ceres may have been formed with a relatively greater reservoir of water," said Chris Russell, principal investigator of the Dawn mission, based at the University of California, Los Angeles.
"Some observations indicate Ceres may be a volatile-rich world that is not dependent on current-day external sources."
"On the moon and Mercury, only the permanently shadowed regions very close to the poles get cold enough for ice to be stable on the surface," says Erwan Mazarico, a Dawn guest investigator at NASA's Goddard Space Flight Center in Maryland.
The scientists calculate that one out of every 1,000 water molecules generated on the surface of Ceres will end up in a cold trap during a year on Ceres (1,682 days).