For centuries, scientists have wondered what lies beneath the cratered surface of Ceres, the largest object in the asteroid belt. The heavily cratered and battered surface seemed to suggest otherwise—after all, an icy surface should theoretically flow and smooth out over time, leaving fewer impact marks.
But new research from Purdue University and NASA’s Jet Propulsion Lab (JPL) is challenging these long-held beliefs. The study, published in Nature Astronomy, suggests that Ceres is far icier than once thought and may have once been a muddy ocean world.
To reach this conclusion, the researchers used computer simulations to model how craters on CeresCeres might deform over billions of years. Their findings revealed that Ceres’ crust is likely composed of about 90% ice near its surface, far more than the previously estimated 30%.
“We think that there’s lots of water-ice near Ceres’ surface, and that it gets gradually less icy as you go deeper and deeper,” Mike Sori, assistant professor with the Department of Earth, Atmospheric, and Planetary Sciences at Purdue University explained in a recent statement.
A Brief History of Ceres
Ceres holds a unique place in the history of astronomy and planetary science. Discovered in 1801 by Italian astronomer Giuseppe Piazzi, Ceres was the first asteroid ever identified, and it remains the largest object in the asteroid belt between Mars and Jupiter. Initially classified as a planet, Ceres was later redefined as an asteroid and, more recently, a dwarf planet due to its spherical shape.
With a diameter of approximately 950 kilometers, Ceres has fascinated scientists for over two centuries, offering a window into the early solar system’s formation and evolution. As a protoplanet, it contains water-ice and other elements that could provide clues about the presence of water on other celestial bodies. NASA’s Dawn mission, which launched in 2007, became the first spacecraft to orbit two extraterrestrial bodies—Vesta and Ceres. It reached Ceres in 2015 and continued orbiting until 2018, providing valuable data about its surface and structure. This data, including observations of pits, domes, and landslides, suggested that Ceres might contain a lot of ice just beneath its surface. Additionally, spectrographic data indicated the presence of ice below the top layer of soil, while gravity data hinted at a density similar to that of impure ice.
“We used multiple observations made with Dawn data as motivation for finding an ice-rich crust that resisted crater relaxation on Ceres,” Ian Pamerleau, a PhD student with Purdue University’s Department of Earth, Atmospheric, and Planetary Sciences and the paper’s first author elaborated. “We even took a topographic profile of an actual complex crater on Ceres to construct the geometry for some of our simulations.”
Finding the Ice Beneath the Surface
Historically, scientists believed that if Ceres had a high ice content, its craters would relax and flatten over time, similar to how glaciers flow on Earth. However, this new study’s simulations demonstrated that ice can be far stronger in Ceres’ conditions than previously thought, especially when mixed with small amounts of solid rock.
“People used to think that if Ceres was very icy, the craters would deform quickly over time, like glaciers flowing on Earth, or like gooey flowing honey,” Sori said. “But we’ve shown that ice can be much stronger if you mix in just a little bit of solid rock.”
By modeling how craters on Ceres change shape, the researchers accounted for how even solids flow over very long timescales.
“Even solids will flow over long timescales, and ice flows more readily than rock,” Pamerleau said. “Our computer simulations account for a new way that ice can flow with only a little bit of non-ice impurities mixed in, which would allow for a very ice-rich crust to barely flow even over billions of years.”
An Ocean World
Based on the new simulations, the researchers believe Ceres was once an “ocean world” like Europa, one of Jupiter’s moons, but with a twist.
“Our interpretation of all this is that Ceres used to be an ocean world, but with a dirty, muddy ocean,” said Sori.
As this muddy ocean froze over time, it left behind an icy crust with traces of rocky material trapped inside it. This discovery means that Ceres could be much more similar to the icy moons in our outer solar system than previously thought.
The Ancient Oceans of Ceres
The discovery of Ceres as an icy, ocean-world-turned-dwarf-planet could have implications for future space missions.
According to Sori, if their findings are correct, “We have a frozen ocean world pretty close to Earth. Ceres may be a valuable point of comparison for the ocean-hosting icy moons of the outer solar system, like Jupiter’s moon Europa and Saturn’s moon Enceladus.”
Because Ceres is relatively accessible compared to other icy bodies in our solar system, it represents an attractive target for exploration.
“Some of the bright features we see at Ceres’ surface are the remnants of Ceres’ muddy ocean, now mostly or entirely frozen, erupted onto the surface,” Sori explained. “We have a place to collect samples from the ocean of an ancient ocean world that is not too difficult to send a spacecraft to.”
Kenna Hughes-Castleberry is the Science Communicator at JILA (a world-leading physics research institute) and a science writer at The Debrief. Follow and connect with her on X or contact her via email at kenna@thedebrief.org