NASA’s Dawn probe has mapped all the craters on Ceres that can accumulate ice, which have likely been on the dwarf planet for a billion years.

Ceres has enough mass to hold onto water molecules, and those molecules are basically permanently ice because of the incredible coldness on Ceres, according to a NASA statement.

The pits of the crater, where there is no direct sunlight, traps the water, and scientists had predicted that would be the case on Ceres. But this is the first time they have actually mapped out where those cold traps are.

The biggest one is at the bottom of a 10-mile-wide crater about 40 miles south of the north pole on Ceres. There are about 695 square miles of terrain that is permanently shadowed from the sun in all.

This allows water to stay on the dwarf planet — otherwise it would evaporate. On the moon and Mercury, where it also gets very cold, only a few permanently shadowed regions very close to the poles get cold enough for ice to be stable on the surface.

“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.”

“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.”