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The makeup of the dwarf planet Ceres—the biggest object in the asteroid belt—presents a riddle. Gravity and spectrographic data from a recent NASA mission suggest it is made mostly of ice. But its heavily dimpled and pockmarked surface seems to contradict this possibility: Ice is too soft to maintain such deep craters over billions of years.
Now a team of researchers has found a potential answer to the riddle: dirty ice. Ceres may have been an ancient, muddy ocean world that gradually froze over as the dwarf planet lost heat. If this hypothesis is correct, it could offer a peek at the potential trajectories of more distant ocean worlds such as Europa and Enceladus, moons of Jupiter and Uranus.
“The craters are telling us that it isn’t icy, but then everything else is saying it’s very icy,” says Lauren Schurmeier, an associate researcher at the University of Hawaii at Manoa who studies icy moons and was not involved in the study. “What this paper resolved was that it probably is icy, but it’s dirtier ice than we would have expected.”
It’s exciting that Ceres was probably an ocean world in its past.
Researchers from Purdue University and NASA’s Jet Propulsion Lab modeled the ways different mixtures of impurities in the ice could have yielded lasting craters in the surface of Ceres over billions of years. Their findings, published in Nature Astronomy, suggest that Ceres is very icy on the surface, with a gradient of increasing impurities going deeper into the crust.
“It gets less icy as you go deeper and deeper,” says Michael Sori, an assistant professor at Purdue University’s Department of Earth, Atmospheric, and Planetary Sciences. “That represents the ocean slowly freezing over time and trapping more and more dirt into the frozen ice.” The researchers used data from a 2018 ice impurity study, which found the rigidity of ice could be ramped up by adding as little as 6 percent impurities, making it behave much more like rock.
Ian Pamerleau, a graduate student at Purdue University and first author on the paper, plans to continue modeling Ceres’ craters, specifically its largest craters. The Occator Crater, one of the largest on Ceres, contains a salt deposit that seems to have erupted from below. This deposit could indicate the presence of briny water still lurking deep beneath the dwarf planet’s crust.
Ceres is the closest icy object to Earth, and is relatively accessible in the grand scheme of planetary exploration. If a future mission to Ceres collects samples of Occator Crater’s salt deposits and brings them back to Earth, scientists could analyze the materials brewing below the crust, and determine whether any liquid ocean remains.
“It’s exciting that Ceres was probably an ocean world in its past,” says Schurmeier. “Ceres just gets cooler the more we look at it.” 
Lead image: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA / Justin Cowart
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