December 15, 2016
At first glance, Ceres, the largest body in the main asteroid belt, may not look icy. Images from NASA’s Dawn spacecraft have revealed a dark, heavily cratered world whose brightest area is made of highly reflective salts — not ice. But newly published studies from Dawn scientists show two distinct lines of evidence for ice at or near the surface of the dwarf planet. Researchers are presenting these findings at the 2016 American Geophysical Union meeting in San Francisco.
“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,” said Carol Raymond, deputy principal investigator of the Dawn mission, based at NASA’s Jet Propulsion Laboratory, Pasadena, California.
Water ice on other planetary bodies is important because it 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.
Ceres’ uppermost surface is rich in hydrogen, with higher concentrations at mid-to-high latitudes — consistent with broad expanses of water ice, according to a new study in the journal Science.
“On Ceres, ice is not just localized to a few craters. It’s everywhere, and nearer to the surface with higher latitudes,” said Thomas Prettyman, principal investigator of Dawn’s gamma ray and neutron detector (GRaND), based at the Planetary Science Institute, Tucson, Arizona.
Researchers used the GRaND instrument to determine the concentrations of hydrogen, iron and potassium in the uppermost yard (or meter) of Ceres. GRaND measures the number and energy of gamma rays and neutrons emanating from Ceres. Neutrons are produced as galactic cosmic rays interact with Ceres’ surface. Some neutrons get absorbed into the surface, while others escape. Since hydrogen slows down neutrons, it is associated with fewer neutrons escaping. On Ceres, hydrogen is likely to be in the form of frozen water (which is made of two hydrogen atoms and one oxygen atom).
Rather than a solid ice layer, there is likely to be a porous mixture of rocky materials in which ice fills the pores, researchers found. The GRaND data show that the mixture is about 10 percent ice by weight.
“These results confirm predictions made nearly three decades ago that ice can survive for billions of years just beneath the surface of Ceres,” Prettyman said. “The evidence strengthens the case for the presence of near-surface water ice on other main belt asteroids.”
Image credit: NASA JPL-Caltech, UCLA, MPS, DLR, IDA, PSI