After arriving at the dwarf planet Ceres in 2015, NASA’s Dawn spacecraft revealed a bizarre world where ice used to flow and erupt like lava. In the last phase of the mission, which ended in 2018, Dawn’s orbit swept within 35 kilometers (22 miles) of Occator Crater, a 92-kilometer-wide feature dotted with bright deposits of sodium carbonate and other minerals. The mission team’s analysis of the data, published on Monday in a set of seven papers in three of Nature’s journals, paints the fullest picture yet of the history of Occator Crater and substantiates earlier suspicions that a reservoir of liquid water exists beneath the surface of Ceres. The extent of that supposedly watery region, however, remains unknown.
“This is the last set of papers that really present the mission data and provide the team’s best interpretation,” says Carol Raymond, a researcher at NASA’s Jet Propulsion Laboratory (JPL) and Dawn’s deputy principal investigator. “Now the data are out in the wild, and everybody will be able to take a look at them. I’m sure we’re going to learn a lot more in the future.”
Early in Dawn’s mission, researchers surmised that the minerals in the deposits had been carried to the surface by liquid water that subsequently evaporated. The youngest of these deposits formed a mere four million years ago, whereas the impact that created Occator Crater occurred around 20 million years ago. This discrepancy left scientists unsure whether the water started as ice that was then melted by the heat of the impact or already existed in liquid form in a subsurface reservoir.
To find out, the team measured how the speed of the spacecraft changed throughout its orbit to build a detailed map of Ceres’s gravitational field, which in turn revealed regions of high and low density in the crust. Then Raymond and her colleagues combined the density map with models of how heat would travel in an ice-rich crust to reconstruct the aftermath of the Occator impact. The scenario they uncovered, Raymond says, “opens up a new way to think about the geology” of icy bodies such as Ceres, which is the largest object in the asteroid belt. The energy of the impact created a “melt chamber” of liquid water near the surface, as well as fractures in the crust. These fractures, Raymond says, connected the melt chamber to a deep reservoir of liquid water that already existed about 35 kilometers underground. Over the course of millions of years, brine rose through the network of fractures as the melt chamber gradually refroze. Upon reaching the surface, the water rapidly boiled away in the near-vacuum conditions, leaving behind the sodium carbonate and other salts.
High-resolution data also allowed researchers to study the composition of the bright deposits more closely. Subtle gradations in the reflected light and thermal glow from the central dome in Occator Crater revealed the presence of a mineral called hydrohalite—essentially table salt with ice trapped in its crystal structure. The mineral is common in Earth’s sea ice but had never before been detected elsewhere in the solar system. The team calculated that once exposed on the surface of Ceres, the ice in the hydrohalite would disappear in roughly 100 years, leaving behind solid sodium chloride (which would not be detected by Dawn’s instruments). Thus, the presence of hydrohalite indicated that the brine continues to rise to the surface today. “We nailed the fact that there is ongoing geologic activity,” says Raymond, who is a co-author of all seven studies.
In a Nature Astronomy commentary accompanying the new research, Julie Castillo-Rogez, a planetary scientist at JPL and a co-author of six of the papers, hails Ceres as an “ocean world.” “Liquid inside Ceres ... is something we had been suspecting since the very early results of the mission,” she says. “But now we have more of a smoking gun.” Yet other scientists say that calling the dwarf planet an ocean world is an overstatement, given the current data.
“I remain pretty skeptical about a subsurface ocean on Ceres,” says Jim Zimbelman, a geologist at the Smithsonian Institution who was not involved with the research. “Modeling is great, but I will need to see some unequivocal geophysical evidence that a recent ocean existed, let alone still [is] there today.” Mikhail Zolotov of Arizona State University, who was consulted for one of the seven studies but was not directly involved in the work, welcomes the high-resolution gravity data but dismisses the conclusion that Ceres hosts a brine reservoir as “wishful thinking.” In 2009 he proposed that the world’s crust is highly porous, with very little ice, an explanation that he still considers more plausible than the Dawn team’s conclusion. “The interpretation of many data is not unique,” he says. “We don’t need ice to explain what we see.”
Lindy Elkins-Tanton, principal investigator of NASA’s upcoming mission to the asteroid 16 Psyche, who was not part of the Ceres studies, says she finds the evidence of a brine reservoir on the world “convincing” but doubts that it is large enough to be considered an ocean. For her, the most important new finding is that “the heat for the ongoing geologic ‘life’ of a small body can come from impact,” she says. “It actually doesn't have to be the primordial heat of that body.” Similar geologic process on other asteroids and moons, Elkins-Tanton says, could give rise to the chemical reactions thought to precede biological life.
Having put the final stamp on Dawn’s reconnaissance of Ceres, scientists are now considering a follow-up mission. On Monday Castillo-Rogez submitted a report on the prospect to NASA as part of the agency’s Decadal Survey on Planetary Science and Astrobiology, which will identify research priorities for the period of 2023–2032. She and others hope to land a spacecraft in Occator Crater to investigate the composition of the brine in more detail. The project will face competition from proposals for first-time visits to other bodies in the asteroid belt. “The challenge is to come up with the science goal that is really going to grab both NASA and the scientific community’s attention to the point that they’re willing to forego going elsewhere,” Zimbelman says.
For Joseph O’Rourke, a planetary scientist at Arizona State University, who was not a member of the Dawn team, the possibility of a brine layer makes Ceres an “absolutely compelling destination for future exploration.” Elkins-Tanton agrees that learning more about the brine would be valuable. But in her view, other minutiae of Ceres’s geology discussed in the new papers hold limited relevance for scientists’ broader understanding of the solar system. “If you’re interested in the really specific geology of small worlds like Ceres, there’s a million questions you could ask and a million things you could do,” she says. “To me, the interest in all of science really relies on the big questions only.”
Whether NASA sends another spacecraft to Ceres or not, planetary scientists concur that Dawn was a groundbreaking mission. As the first probe to orbit two objects beyond Earth (it visited the asteroid Vesta before continuing to Ceres), Dawn was “revolutionary from the engineering side,” Zimbelman says. And from the science side, O’Rourke says, “it shows how you can go someplace new and discover amazing things, which is really what planetary science is all about.”