Water Ice In Eternal Polar Night on Ceres

The American Dawn space probe has been orbiting the asteroid Ceres between Mars and Jupiter since March 2015. Thanks to the two identical onboard cameras from the Max Planck Institute for Solar System Research (MPS), the Framing Cameras, the dwarf planet has been almost completely mapped. In a current study, a team headed by scientists from the MPS reports on Ceres’ most northerly regions, where the Göttingen cameras have performed a very special feat: they have succeeded in taking photos of water ice deposits in places ruled by almost eternal darkness.

View of the North Pole: The colours show the varying height of Ceres’ landscape. The numbers refer to ten craters where the Framing Cameras built in Göttingen at the Max Planck Institute for Solar System Research have discovered water ice.

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.

Credits: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

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.”

Clues to Ceres’ inner life

Concentrations of iron, hydrogen, potassium and carbon provide further evidence that the top layer of material covering Ceres was altered by liquid water in Ceres’ interior. Scientists 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.

Ice in permanent shadow

A second study, led by Thomas Platz of the Max Planck Institute for Solar System Research, Gottingen, Germany, and published in the journal Nature Astronomy, focused on craters that are persistently in shadow in Ceres’ northern hemisphere. Scientists closely examined hundreds of 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 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 Norbert Schorghofer of 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, including the Herschel Space Observatory’s observations of water vapor at Ceres in 2012-13. 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.

‘Bright spots’ get names

This video shows a flyover of the intriguing crater named Occator on dwarf planet Ceres. Occator is home to Ceres’ brightest area.