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Hydrogen on Ceres
NASA’s Dawn spacecraft determined the hydrogen content of the upper yard, or meter, of Ceres’ surface. Blue indicates where hydrogen content is higher, near the poles, while red indicates lower content at lower latitudes. (NASA / JPL-Caltech / UCLA / MPS / DLR / IDA / PSI Photo)

Readings from instruments aboard NASA’s Dawn orbiter support the view that a treasure trove of frozen water lies just beneath the surface of the dwarf planet Ceres.

Researchers reported those findings today at the American Geophysical Union’s fall meeting San Francisco, as well as in two papers published by Nature Astronomy and Science.

The findings are based on hydrogen readings from Dawn’s gamma ray and neutron detector, or GRaND, as well as from the spacecraft’s cameras and infrared mapping spectrometer.

GRaND’s science team found ample evidence of hydrogen on Ceres, with higher concentrations at mid- to high latitudes. That’s consistent with the presence of broad expanses of water ice within a yard (meter) of the surface.

“On Ceres, ice is not just localized to a few craters. It’s everywhere, and nearer to the surface with higher latitudes,” GRaND principal investigator Thomas Prettyman, who’s based at the Planetary Science Institute in Arizona, said in a NASA news release.

GRaND’s readings suggest that there’s a porous mixture of rocky materials on Ceres’ surface, with ice filling the pores. The ice makes up about 10 percent of the mixture by weight.

Prettyman and his team had their paper published in Science. The Nature Astronomy paper focused on a different line of evidence: images from Dawn that show deposits of bright material at the bottom of about 10 shadowed craters out of the hundreds identified in Ceres’ northern hemisphere.

This bright material is different from the bright spots for which Ceres is most famous. Those once-mysterious spots are now thought to consist primarily of bright-colored, highly reflective salts.

Dawn’s infrared mapping spectrometer confirmed that the bright material lying within a partially sunlit crater was ice.

Such findings suggest that frozen water can build up at the bottoms of cold, dark craters on Ceres. Such “cold traps” have been spotted on Mercury and on Earth’s moon as well. In those cases, scientists say that the ice was most likely delivered by cosmic impacts, and that stray water molecules found their way to the cold traps.

Dawn’s researchers haven’t yet made up their mind about the scenario on Ceres.

“We are interested in how this ice got there, and how it managed to last so long,” said the University of Hawaii’s Norbert Schorghofer, a co-author of the Nature Astronomy paper. “It could have come from Ceres’ ice-rich crust, or it could have been delivered from space.”

The prevalence of water is intriguing for several reasons. First, it provides clues as to Ceres’ origins.

“These studies support the idea that ice separated from rock early in Ceres’ history, forming an ice-rich crustal layer, and that ice has remained near the surface over the history of the solar system,” the Dawn mission’s deputy principal investigator, Carol Raymond of NASA’s Jet Propulsion Laboratory, said in today’s release.

Another angle has to do with the fact that water is an essential ingredient for life as we know it.

“By finding bodies that were water-rich in the distant past, we can discover clues as to where life may have existed in the early solar system,” Raymond said.

Looking ahead, water ice is considered a valuable resource for space settlement and in-space resupply. Ceres may well be an attractive way station if humanity ever makes it to the main asteroid belt, more than 230 million miles beyond Earth’s orbit. And maybe not just Ceres.

“”The evidence strengthens the case for the presence of near-surface water ice on other main-belt asteroids,” Prettyman said.

Dawn was launched in 2007 and orbited the asteroid Vesta in 2011 and 2012. Then it headed onward to enter orbit around Ceres in 2015. The spacecraft began its extended mission phase in July and is currently flying in an elliptical orbit that takes it more than 4,500 miles from Ceres.

Check out this video of today’s briefing at the AGU meeting:


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