A crater-filled dwarf planet called Ceres sits in the main asteroid belt, located between Mars and Jupiter. For a long time, it was believed that Ceres was made of materials where water ice was not the main component. However, researchers from Purdue University have utilized information from NASA’s Dawn mission to reveal that more than 90 percent of Ceres’ crust might actually be ice.
Since Giuseppe Piazzi first discovered Ceres in 1801, astronomers and planetary scientists have been curious about its composition. The surface of Ceres is heavily scarred with impact craters, leading scientists to believe that the presence of these craters indicated that Ceres did not contain much ice.
Researchers at Purdue University, along with NASA’s Jet Propulsion Lab (JPL), have now suggested that Ceres is indeed a significantly icy body that may have once resembled a muddy ocean world. The findings, led by Ian Pamerleau, a PhD student, and Mike Sori, an assistant professor at Purdue, were published in Nature Astronomy. Together with Jennifer Scully, a JPL research scientist, they conducted computer simulations to understand how craters on Ceres change shape over millions of years.
“We believe there is a substantial amount of water ice near the surface of Ceres, which decreases in quantity as you go deeper,” stated Sori. “Previous notions suggested that if Ceres contained a lot of ice, craters would change quickly over time, similar to the way glaciers move on Earth or how honey flows. Yet, our simulations indicate that ice can be much more resilient under the conditions found on Ceres if even a little solid rock is mixed in.”
This team’s discovery challenges the earlier assumption that Ceres was quite dry, which stated that the ice content was less than 30%. Now, Sori’s team estimates that the surface is generally around 90% ice.
“Our conclusion is that Ceres could have been an ‘ocean world’ similar to Europa, one of Jupiter’s moons, but with a mixture of muddy ocean water,” Sori explained. “As this muddy ocean cooled over time, it formed a crust of ice with rocky materials trapped within.”
Pamerleau elaborated on how their team utilized computer simulations to model the way craters on Ceres have developed over billions of years.
“Even solid materials can flow over long periods, and ice flows more easily than rock does. Craters tend to have deep shapes that create high stress, which then gradually relaxes into a state of lower stress, leading to a shallower bowl shape due to solid-state flow,” he noted. “The initial conclusion from NASA’s Dawn mission was that the absence of shallow craters meant the crust couldn’t be very icy. Our simulations have accounted for a different way that ice can flow with just slight amounts of non-ice impurities, allowing for a rich ice crust that hardly flows even over extensive periods. We tested various crust structures in our simulations and discovered that a gradual crust with higher ice content near the surface and gradually less ice at greater depths was the best scenario to limit crater relaxation on Ceres.”
Sori, a planetary scientist focused on planetary geophysics, addresses challenges concerning planetary interiors and their relationship with surfaces. His research spans various bodies in the solar system, from the Moon and Mars to icy celestial objects located in more distant regions.
“Ceres is the largest object found in the asteroid belt, classified as a dwarf planet. Some people envision small, irregular shapes when thinking of asteroids (and indeed, most are!), but Ceres actually resembles a planet more closely,” Sori remarked. “With a diameter of about 950 kilometers, it features surface characteristics such as craters, volcanoes, and landslides.”
NASA initiated the Dawn mission on September 27, 2007. This mission became the first and only spacecraft to orbit two different solar system bodies — the protoplanet Vesta and Ceres. Although it launched in 2007, Dawn didn’t reach Ceres until 2015, and it orbited the dwarf planet until 2018.
“We employed various observations gathered from the Dawn mission to support our findings around Ceres’ ice-rich crust that resisted crater alterations. Different surface features, such as pits, domes, and landslides, hinted that Ceres’ subsurface contains ample ice,” Pamerleau explained. “Spectroscopic data also indicates there’s likely ice below the surface layer on Ceres, and gravity data suggest a density close to that of ice — specifically impure ice. We also created a topographic profile of a complex crater on Ceres to inform our simulation geometry.”
Sori remarked that due to Ceres being the largest asteroid, there were speculations about its icy characteristics based on some mass estimates derived from Earth, making it an excellent candidate for a spacecraft visit.
“The thrilling part of this research, if we’re accurate, is discovering a frozen ocean world relatively close to Earth. Ceres could provide meaningful comparisons with the icy moons of the outer solar system, like Europa and Enceladus,” Sori said. “Thus, we believe Ceres is the most reachable icy world in the universe, making it an ideal target for future spacecraft missions. Some of the bright patches seen on Ceres’ surface are remnants of its ancient, mostly frozen muddy ocean that have erupted onto the surface. This gives us a location to gather samples from a once-thriving ocean world that isn’t too difficult to reach with a spacecraft.”
This study received support through a NASA grant (80NSSC22K1062) as part of the Discovery Data Analysis Program.
