Scientists with NASA's Dawn mission have identified, on the dwarf planet Ceres, permanently shadowed regions most of which likely have been cold enough to trap water ice for a billion years.The findings suggest that that ice deposits could exist in these regions even now."The conditions on Ceres are right for accumulating deposits of water ice," said Norbert Schorghofer, a Dawn guest investigator at the University of Hawaii at Manoa. "Ceres has just enough mass to hold on to water molecules, and the permanently shadowed regions we identified are extremely cold -- colder than most that exist on the moon or Mercury," Schorghofer noted.The findings appeared online in the journal Geophysical Research Letters.Permanently shadowed regions 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.
The regions still receive indirect sunlight, but if the temperature stays below about minus minus 151 degrees Celsius, the permanently shadowed area is a cold trap -- a good place for water ice to accumulate and remain stable. Cold traps were predicted for Ceres but had not been identified until now."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," Chris Russell, principal investigator of the Dawn mission, based at University of California, Los Angeles, pointed out."Some observations indicate Ceres may be a volatile-rich world that is not dependent on current-day external sources," Russell said.In this study, the researchers studied Ceres' northern hemisphere, which was better illuminated than the south. Images from Dawn's cameras were combined to yield the dwarf planet's shape, showing craters, plains and other features in three dimensions. Using this input, a sophisticated computer model developed at NASA's Goddard Space Flight Center, Greenbelt, Maryland, was used to determine which areas receive direct sunlight, how much solar radiation reaches the surface, and how the conditions change over the course of a year on Ceres.The researchers found dozens of sizeable permanently shadowed regions across the northern hemisphere. Taken together, Ceres' permanently shadowed regions occupy about 1,800 square kilometers. This is a small fraction of the landscape -- much less than one percent of the surface area of the northern hemisphere.