Water and Ice Helped Form Features in Ceres’ Occator Crater
NASA’s Dawn spacecraft gave scientists extraordinarily close-up views of the dwarf planet Ceres, which lies in the main asteroid belt between Mars and Jupiter. By the time the mission ended in October 2018, the orbiter had dipped to less than 22 miles (35 kilometers) above the surface, revealing crisp details of the mysterious bright regions inside Ceres’ Occator Crater.
Analysis of new high-resolution images of the surface, along with gravity data that gives insights into the dwarf planet’s deep interior, are presented in a suite of seven papers published Aug. 10, 2020 in Nature Astronomy, Nature Geoscience and Nature Communications. The findings suggest that Ceres is an ocean world and has been geologically active in the very recent past.
“Occator Crater is young – about 20 million years – and its interior is complex. There are dramatic bright spots and fractures, there are extensive flows of material, a central pit and dome structure, and many small-scale features, like hills and knobs,” said Planetary Science Institute Senior Scientist Hanna Sizemore, a co-author on five of the seven papers. Other PSI co-authors on the Nature papers are Lucille Le Corre, Jian-Yang Li, Thomas Prettyman and David O’Brien.
Many of the features inside Occator require the movement of water and/or ice slurries to form, and this activity must have gone on for a long time after the impact that created the crater. A big question for the Dawn science team was: Does water come from deep in the dwarf planet, in a volcanic process? Or did the impact only create shallow melt that refroze to create the features we see? The end-of-mission data suggests that both may have happened.
“The gravity data tells us there is probably a deep reservoir of brine – salty water – about 40 kilometers below Occator. Some of that deep water likely erupted on the surface and contributed to bright spots and other features in Occator. On the other hand, our image-based analysis of the large flows and small hills inside the crater suggests that shallow sheets of muddy impact melt flowed around the inside of the crater, and formed interesting features as they refroze,” Sizemore said. “Both of these are exciting, because it means there were transient and long-lived sources of water in this area, and extensive mixing. Liquid water and mixing are always exciting for astrobiology.”
Earlier, scientists had figured out that the bright areas in Occator were deposits made mostly of sodium carbonate, a compound of sodium, carbon and oxygen. They likely came from liquid that percolated up to the surface and evaporated, leaving behind a highly reflective salt crust. But what they hadn’t yet determined was where that liquid came from.
Some of the evidence for recent liquids in Occator Crater comes from the bright deposits, but other clues come from an assortment of interesting conical hills. These features are reminiscent of terrestrial pingos – small ice hills formed by frozen pressurized groundwater in Earth’s polar regions. Similar features have also been spotted on Mars, but their observation on Ceres is the first evidence that frost heaving may occur on a dwarf planet. Evidence for shallow processes like frost heave, combined with evidence for deep interior activity, will make Ceres an interesting area of study for years to come.
Sizemore’s research was funded by a grant to PSI from NASA’s Dawn at Ceres Guest Investigator Program.
