Until a million years ago, dwarf planet Ceres, the largest body in the asteroid belt, was the scene of cryovolcanic eruptions: below the Occator Crater, subsurface brine pushed upward; the water evaporated, leaving behind bright, salty deposits. This process is probably still ongoing. A team of researchers led by the Max Planck Institute for Solar System Research (MPS) in Germany comes to these conclusions after evaluating high-resolution camera images of Ceres from the final phase of NASA's Dawn mission. The scientific journals Nature Astronomy, Nature Geoscience, and Nature Communications today dedicate a total of seven articles to this and other results from the Dawn mission. The publications paint a picture of a unique world, in whose interior remains of a global ocean have survived to this day and whose remarkable cryovolcanism is probably still active.
Cryovolcanism has long been considered a phenomenon of the outer solar system, occurring exclusively on some icy moons of Jupiter, Saturn, Uranus, and Neptune. Churned by the enormous gravitational forces of their mother planets, these moons offer so much warmth in their interior despite the considerable distance from the Sun that the water within does not freeze completely and is sometimes ejected into space in spectacular plumes. In contrast, the asteroid belt offers a completely different setting. The many millions of larger and smaller chunks that orbit the Sun between the orbits of Mars and Jupiter are generally regarded as simply structured, waterless, and inactive bodies.
Occator Crater provides insights into Ceres' past
Today’s publications prove that this view cannot be upheld. Ceres, the largest inhabitant of the asteroid belt with a diameter of 950 kilometers, turns out to be an active body displaying a unique form of cryovolcanism. The researchers' findings are primarily based on observational data from the final phase of NASA's Dawn mission, which examined Ceres from 2015 to 2018. In its final five months on a strongly elliptical orbit, the space probe ventured within 35 kilometers of the surface - closer than ever before. The probe's scientific camera system, which was developed and built under the leadership of MPS, took unique pictures during this time.
The authors of today’s papers including researchers from the University of Münster (Germany) and the National Institute of Science Education and Research (NISER, India) paid special attention to Occator Crater, a prominent impact crater in the northern hemisphere of Ceres. With a diameter of about 92 kilometers, its dimensions exceed even those of most terrestrial craters. Even more striking is its bright white coloration, which prompted speculation about possible water deposits already during the approach to Ceres. "On closer inspection, Occator Crater has a very complex structure with elevations, a large central depression, deposits, cracks, and furrows. In all its details, this became clear only during the final phase of the mission," explains Dr. Andreas Nathues from MPS, Principal Investigator of Dawn's camera team. "From the crater's current morphology, we can reconstruct its evolution - and thus take a look at Ceres' eventful past," he adds.
The high-resolution images make it possible to determine the age of the individual crater areas. To this end, researchers analyze the number and nature of impact craterings that cover every body in the solar system. The younger the surface is, the fewer small craters it exhibits.
As it turned out, Occator Crater was formed about 22 million years ago by a large impact. As in many other impact craters on Earth and on other planets, a central peak was formed, which collapsed again after some time. About 7.5 million years ago, brine rose to the surface within the remnants of the central peak. The water evaporated and certain salts, so-called carbonates, remained. They are responsible for the prominent bright deposits we see today, called Cerealia Facula, in the center of Occator Crater. Due to the loss of material in the interior, the inner part of the crater subsided. A round depression with a diameter of about 15 kilometers formed.
In the following millions of years, activity concentrated mainly on the eastern part of the crater floor. Through cracks and furrows, brine also rose to the surface there and produced further bright deposits, the Vinalia Faculae. About 2 million years ago the center of the crater woke up again: brine rose to the surface and within the central depression a dome of bright material was formed. "The main process may have lasted until one million years ago and is probably still fading," summarizes Dr. Nico Schmedemann from the Geological Planetology Group under the lead of Prof. Harald Hiesinger of Münster University.
"What is particularly remarkable is how long Occator Crater was active and possibly still is", says Andreas Nathues. He believes this refutes theories according to which the escaped fluid is exclusively due to meltwater from the original impact. The heat generated by such an impact could not have been sustained inside the body for so many millions of years.
Instead, the results suggest that deep below Occator Crater there are remnants of a global, salty ocean. Similar to road salt in winter, the dissolved salt ensures that the brine remains liquid despite the low temperatures inside the dwarf planet. This interpretation is supported by another article published today. The scientists from NASA's Jet Propulsion Laboratory (JPL) evaluate Dawn's gravity measurements. According to their analysis, a reservoir of liquid brine lies about 40 kilometers below Occator Crater.
Possibly, that water is still escaping from Ceres. In 2014, a publication of observations with the Herschel Space Telescope provided indications of an extremely thin, water-containing and possibly only sporadically occurring exosphere. During the following Dawn mission, Andreas Nathues and his team found signs of a kind of thin haze apparently existing intermittently over the floor of Occator Crater. These pieces of the puzzle are now joined by the publication of the Dawn spectrometer team led by the Istituto di Astrofisica Spaziale e Fisica Cosmica in Italy. The researchers were able to detect salt compounds containing water in the bright, deposited material. However, the lightly bound water evaporates from Ceres’ surface quickly; the deposits can therefore not be old. “Their work is complimentary to ours, particularly our finding of a unique spectrally red nature in Cerealia Facula from our Framing Camera color data”, says Dr. Guneshwar Thangjam from NISER.
"We assume that Ceres is still occasionally cryovolcanically active," concludes Andreas Nathues. While there is some evidence that the eruptions in the early developmental phases of Occator volcanism were in part quite explosive, this process should have calmed down considerably by now. The researchers suspect that water now escapes primarily through evaporation and sublimation. "According to current knowledge, such cryovolcanism is unique in the solar system," says Nico Schmedemann. In addition, Thangjam opined, “The evidence for very recent geological activity on Ceres contradicts the general belief that small solar system bodies are not geologically active. This has paved a new direction in planetary science exploration toward understanding conditions of the very early solar system as well as having implications for habitability”.
Andreas Nathues, Nico Schmedemann, Guneshwar Thangjam et al. (2020): Recent cryovolcanic activity at Occator on Ceres, Nature Astronomy, doi: 10.1038/s41550-020-1146-8