The small world in the asteroid belt once had an underground ocean, remnants of which may still exist. This assumption is now supported by test results from the Urvara crater in the south of the dwarf planet Ceres: data from the “extension” of NASA’s Dawn mission show traces of cryovolcanism there. Long after the formation of the impact basin, saline solutions rose from the depths and organic compounds were deposited.
It is the biggest of the little ones: with a diameter of around 950 kilometers, the dwarf planet Ceres is the most massive chunk in the asteroid belt between Mars and Jupiter. The data from NASA’s Dawn probe have already impressively demonstrated how amazingly complex this celestial body is: it orbited Ceres between 2015 and 2018 and provided detailed images. The scientists focused primarily on the craters. The most conspicuous is “Occator” on the northern hemisphere of Ceres. Studies of its structures have already provided exciting clues to hidden features of the dwarf planet. Because in its interior, saline remains of an underground brine were discovered, which apparently reached the surface through cryovolcanic processes until very recently. In addition, there were already indications that Ceres has an extremely complex chemistry: signs of exposed carbon-containing – so-called organic compounds – were discovered in the “Ernutet” crater.
Detailed view of Urvara crater
“The large impact structures on Ceres give us access to the deeper layers,” says Andreas Nathues from the Max Planck Institute for Solar System Research in Göttingen. In the current study, he and his colleagues have now targeted another prominent scar on the dwarf planet: With a diameter of 170 kilometers, “Urvara” is the third largest Ceres crater. The results are based on previously unevaluated images and spectroscopic data from the Dawn mission. They arose during their “extension”: after the planned actions had ended, the remaining fuel was still enough for risky manoeuvres. The astronomers steered Dawn on strongly elliptical orbits in order to bring the probe as close as possible to the surface of Ceres in some places. This resulted in images of the Urvara crater that show structures only a few meters in size.
As the researchers report, the images show multiple terraced escarpments that enclose the impact basin. A little further away from the center of the crater, a mountain range about 25 kilometers long and three kilometers high is particularly noticeable, which is characterized by rugged cliffs, boulder fields, but also noticeably smooth areas and other structures. Of particular interest are some patches of bright material similar to those in Occator Crater.
Cryovolcanism and organic compounds
First, the researchers determined the age of the different areas: To do this, they counted small impact marks in the crater area: Since older surfaces had more time to “accumulate” impacts from smaller chunks from space, they have more craters than younger ones. According to the results, the most original areas in the Urvara crater are about 250 million years old – at that time it was formed by the impact of an asteroid. “However, our evaluations showed that different areas of the crater are very different in age,” says co-author Nico Schmedemann from the Institute for Planetology at the Westfälische Wilhelms-Universität Münster. “The age difference is up to 100 million years. This indicates that processes were at work there that were active long after the crater actually formed,” explains the scientist.
The younger surfaces within the crater thus include the extensive smooth and dark areas. There are also structures that the researchers identify as sinkholes, which were probably formed by underground gas leakage. Further indications of the former geological activity also emerged from the analysis of the spectrographic recordings. The bright spots in particular were the focus of attention. The characteristics of the light reflected from the surfaces allowed conclusions to be drawn about their mineralogical composition.
Salty wet underground?
As it turned out, these are salts. What was particularly interesting was that on a slope to the west of the central mountain range, the researchers found a comparatively recent deposit of salts in combination with organic compounds that had not previously been known. “These organic compounds, which are evident in Urvara crater in the southern hemisphere, are very different from those in areas in Ernutet crater in the northern hemisphere,” emphasizes co-author Guneshwar Thangjam from India’s National Institute of Science Education and Research in Bhubaneswar.
The survey results from Urvara crater shed further light on the complex features of Ceres. “All in all, the Urvara crater presents us with an extremely complex picture that we do not yet fully understand and that leaves room for interpretation. However, there is some evidence that a saline brine was involved, which rose from the interior and set further processes in motion,” says Nathues. The current results also support an exciting theory that developed previously based on the Dawn data: Ceres may once have had a deep ocean containing organic compounds. Remnants may even have survived to this day in liquid reservoirs some 40 kilometers deep. Because the high salt content could have saved them from freezing, so the assumption.
Through further data evaluations, the scientists now want to devote themselves to more detailed research into the organic compounds. “The question of the origin and formation of organic matter on Ceres remains open. Answers could have implications for our understanding of the full geological history of Ceres, potentially making connections to questions in astrobiology,” concludes Thangjam.
Source: Max Planck Institute for Solar System Research, Nature Communications, doi: 10.1038/s41467-022-28570-8