NASA mission maps Ceres craters where ice can accumulate

Scientists from NASA's Dawn mission have identified permanently shadowed regions on the dwarf planet Ceres, most of which likely have been cold enough to trap water ice for a billion years, suggesting that ice deposits could exist there.
"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," said Norbert Schorghofer, a Dawn guest investigator at the University of Hawaii in the US.
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, researchers said.

The regions still receive indirect sunlight, but if the temperature stays below about minus 151 degrees Celsius, the permanently shadowed area is a cold trap - a good place for water ice to accumulate and remain stable, they said.
For the study, 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, researchers said.

Using this input, a sophisticated computer model developed at NASA's Goddard Space Flight Centre 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.

Researchers found dozens of sizeable permanently shadowed regions across the northern hemisphere.
"The largest one is inside a 16-kilometre crater located less than 65 kilometres from the north pole," they said.
Taken together, Ceres' permanently shadowed regions occupy about 1,800 square kilometres. This is a small fraction of the landscape - much less than 1 per cent of the surface area of the northern hemisphere, researchers said.
Scientists expect the permanently shadowed regions on Ceres to be colder than those on Mercury or the moon. That is because Ceres is quite far from the sun, and the shadowed parts of its craters receive little indirect radiation.

"On Ceres, these regions act as cold traps down to relatively low latitudes," said Erwan Mazarico, a Dawn guest investigator at Goddard.
"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," said Mazarico.
The situation on Ceres is more similar to that on Mercury than the moon. On Mercury, permanently shadowed regions account for roughly the same fraction of the northern hemisphere.
By researchers' calculations, about 1 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). That is enough to build up thin but detectable ice deposits over 100,000 years or so, they said.

The findings were published in the journal Geophysical Research Letters.