Ceres: a dwarf planet that has it all

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 NASA Dawn ‘extension’ mission show signs of cryovolcanism there. Long after the shock pool was formed, salt solutions flowed out of the depths and organic compounds were deposited.

It is the largest of the smallest: about 950 kilometers in diameter, the dwarf planet Ceres is the most massive piece in the asteroid belt between Mars and Jupiter. Data from NASA’s Dawn probe has already impressively demonstrated just how amazingly complex this celestial body is: it orbited Ceres from 2015 to 2018 and provided detailed images. Scientists focused primarily on craters. The most conspicuous “Occator” in the northern hemisphere of Ceres. Research into its structures has already provided exciting clues about the hidden features of the dwarf planet. Because salty remains of an underground brine were discovered inside, which supposedly until recently reached the surface in cryo-volcanic processes. In addition, there are already signs that Ceres has an extremely complex chemistry: traces of exposed carbon – so-called organic compounds – have been discovered in the ‘Ernutet’ crater.

Detailed view of the Urvara Crater

“The large shock structures on Ceres give us access to deeper layers,” says Andreas Nathues of the Institute for Solar System Research. Max Planck in Göttingen. In current research, he and his colleagues have now focused on another visible scar on the dwarf planet: 170 kilometers in diameter, “Urvara” is Ceres’ third largest crater. The results are based on previously invaluable images and spectroscopic data from the Dawn mission. They were created during the ‘extension’: after completion of the planned activities, there was still fuel for risky maneuvers. Astronomers have directed Dawn into highly elliptical orbits to bring the spacecraft as close as possible to the Ceres surface at some points. The result was images of the Urvara crater showing structures only a few meters in size.

According to the researchers, the images show many terraced escarpments that surround the shock pool. A little further from the center of the crater, a mountain range about 25 kilometers long and three kilometers high is particularly prominent, characterized by rugged cliffs, boulder fields, but also clearly smooth terrain and other structures. Some patches of bright material similar to those in Occator Crater are of particular interest.

Cryovulcanism and organic compounds

First, scientists determined the age of different areas: to do this, they counted the small impact marks in the crater area: because older surfaces had more time to ‘accumulate’ impacts from smaller pieces from space, they had more craters than younger ones. According to the results, the most original areas of the Urvara crater are about 250 million years old – at that time it was formed by an asteroid impact. “However, our assessments have shown that the different areas of the crater differ greatly in age,” says co-author Nico Schmedemann of the Institute of Planetology at the Westfälische Wilhelms-Universität Münster. “The age difference is up to 100 million years. This indicates that there were processes at work that were active long after the actual formation of the crater “- explains the scientist.

Younger surfaces within the crater thus include vast, smooth and dark areas. There are also structures that scientists identify as sinkholes, possibly caused by an underground gas leak. Further traces of the former geological activity also emerged from the analysis of the spectrographic records. Especially the bright spots were in the spotlight. The characteristics of the light reflected from the surface allowed to draw conclusions about their mineralogical composition.

Salty wet underground?

As it turned out, these are salts. Of particular interest, on a slope west of the central mountain range, the researchers found a relatively young salt deposit combined with previously unknown organic compounds. “The organic compounds that are visible in the Urvara Crater in the Southern Hemisphere are very different from those in the Ernutet Crater areas in the Northern Hemisphere,” emphasizes co-author Guneshwar Thangjam of India’s National Institute of Education and Research in Bhubaneswar.

Research from the Urvara crater sheds 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 fully understand yet, and which leaves room for interpretation. However, there is evidence that the brine was involved and it emerged from within and triggered further processes, ”says Nathues. The current results also support an exciting theory that was developed earlier from Dawn data: Ceres may have once had a deep ocean containing organic compounds. The remains may even have survived to this day in liquid reservoirs some 40 kilometers deep. Since the high salt content might have kept them from freezing, that was the assumption.

By further evaluating the data, scientists now wish to devote themselves to more detailed research of organic compounds. “The question of the origin and formation of organic matter on Ceres remains open. The answers could have an impact on our understanding of Ceres’ full geological history, potentially referring to questions in astrobiology, ”concludes Thangjam.

Source: Max Planck Institute for Solar System Research, Nature Communications, doi: 10.1038 / s41467-022-28570-8

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