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, including University of Hawaii astronomer Norbert Schörghofer, show two distinct lines of evidence for ice at or near the surface of the dwarf planet. These findings, which verify predictions made by scientists formerly at UH, are being presented at the 2016 American Geophysical Union meeting in San Francisco, California.
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 coming 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 a 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.”
More than a quarter century before the Dawn spacecraft arrived at Ceres, Fraser Fanale and James Salvail, who at the time were at the University of Hawaii, made predictions about how close to the surface ice would be. “In a big picture view, Fanale & Salvail were right,” said UH astronomer Norbert Schörghofer, who is a co-author of the Science study. Ice has remained near the surface of Ceres over the history of the solar system.
Ice in Permanent Shadow
A second study, published in the journal Nature Astronomy [http://www.nature.com/natastron] by Thomas Platz and colleagues, including Norbert Schörghofer, focused on craters that are persistently in shadow in Ceres’ northern hemisphere. Earlier this year, a study led by Schörghofer already identified persistently shadowed regions. In the new work, scientists closely examined hundreds of these cold, dark craters called “cold traps” — at less than minus 260 degrees Fahrenheit (110 Kelvin). They are so chilly that very little of the ice turns into vapor in the course of a billion years. Researchers found deposits of bright material in some of these craters. In one crater that is partially sunlit, Dawn’s infrared mapping spectrometer confirmed the presence of ice.
This suggests that water ice can be stored in cold, dark craters on Ceres. Ice in cold traps has previously been spotted on Mercury and, in a few cases, on the Moon. All of these bodies have small tilts with respect to their axes of rotation, so their poles are extremely cold and peppered with persistently shadowed craters. Scientists believe impacting bodies delivered this ice to Mercury and the Moon, or it was generated by solar wind.
“We are interested in how this ice got there and how it managed to last so long,” said co-author Norbert. “It could have come from Ceres’ ice-rich crust, or it could have been delivered from space.”
Regardless of its origin, water molecules on Ceres have the ability to hop around from warmer regions to the poles. Those that leave the surface would fall back onto Ceres, and could land in cold traps.
