Images from NASA’s Dawn probe have revealed a dark, cratered world whose brightest area is made of reflective salts — not water ice. But several new studies show distinct lines of evidence for ice at or near Ceres’ surface.
This false-color image shows the dwarf planet Ceres. Image credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA.
According to a study published in the journal Science, the uppermost surface of Ceres is rich in hydrogen, with higher concentrations at mid-to-high latitudes — consistent with broad expanses of water ice.
“On Ceres, ice is not just localized to a few craters. It’s everywhere, and nearer to the surface with higher latitudes,” said study lead author Dr. Thomas Prettyman, a researcher at the Planetary Science Institute and principal investigator of Dawn’s gamma ray and neutron detector (GRaND).
“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,” said co-author Dr. Carol Raymond, deputy principal investigator of the Dawn mission, based at NASA’s Jet Propulsion Laboratory
The team used the GRaND instrument to determine the concentrations of hydrogen, iron and potassium in the uppermost yard (or meter) of Ceres.
Rather than a solid ice layer, there is likely to be a porous mixture of rocky materials in which ice fills the pores, the authors found. The GRaND data show that the mixture is about 10% 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. The evidence strengthens the case for the presence of near-surface water ice on other main belt asteroids,” Dr. Prettyman said.
Concentrations of iron, hydrogen, potassium and carbon provide further evidence that the top layer of material covering Ceres was altered by liquid water in the dwarf planet’s interior.
Planetary researchers theorize that the decay of radioactive elements within Ceres produced heat that drove this alteration process, separating Ceres into a rocky interior and icy outer shell.
Separation of ice and rock would lead to differences in the chemical composition of Ceres’ surface and interior.
Because meteorites called carbonaceous chondrites were also altered by water, scientists are interested in comparing them to Ceres. These meteorites probably come from bodies that were smaller than Ceres, but had limited fluid flow, so they may provide clues to Ceres’ interior history.
The Science study shows that Ceres has more hydrogen and less iron than these meteorites, perhaps because denser particles sunk while brine-rich materials rose to the surface.
Alternatively, Ceres or its components may have formed in a different region of the Solar System than the meteorites.
Dr. Prettyman, Dr. Raymond and their colleagues reported their other findings Dec. 15 at the 2016 American Geophysical Union Fall Meeting in San Francisco, CA.
Another study, published in the journal Nature Astronomy, focused on craters that are persistently in shadow in Ceres’ northern hemisphere.
Lead author Dr. Thomas Platz, a researcher at the Max Planck Institute for Solar System Research in Germany, and co-authors examined hundreds of cold, dark craters called ‘cold traps’ — at less than minus 260 degrees Fahrenheit (minus 162 degrees Celsius, or 110 degrees Kelvin), they are so chilly that very little of the ice turns into vapor in the course of a billion years.
They found deposits of bright material in 10 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 may have delivered ice to Mercury and the moon. The origins of Ceres’ ice in cold traps are more mysterious, however.
“We are interested in how this ice got there and how it managed to last so long,” said co-author Dr. Norbert Schorghofer, from the University of Hawaii.
“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. A tenuous water atmosphere has been suggested by previous research.
Water molecules that leave the surface would fall back onto Ceres, and could land in cold traps.
With every hop there is a chance the molecule is lost to space, but a fraction of them ends up in the cold traps, where they accumulate.
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T.H. Prettyman et al. Extensive water ice within Ceres’ aqueously altered regolith: Evidence from nuclear spectroscopy. Science, published online December 15, 2016; doi: 10.1126/science.aah6765
T. Platz et al. 2016. Surface water-ice deposits in the northern shadowed regions of Ceres. Nature Astronomy 1, article number: 0007; doi: 10.1038/s41550-016-0007
Carol A. Raymond. Exploration of an Ancient Ocean World: Dawn at Ceres. 2016 AGU Fall Meeting, abstract # P41C-01
Kynan Hughson et al. Ice under cover: Using bulk spatial and physical properties of probable ground ice driven mass wasting features on Ceres to better understand its surface. 2016 AGU Fall Meeting, abstract # P43C-2117
Margaret E. Landis et al. Behavior and Stability of Ground Ice on Ceres: Modeling Water Vapor Production. 2016 AGU Fall Meeting, abstract # P43C-2119
Paul Schenk et al. Impact crater morphology and the Central Pit/Dome of Occator: Ceres as an Ice-rich Body. 2016 AGU Fall Meeting, abstract # P41C-03
This article is based on a press-release from NASA.
