Dwarf planet Ceres (Credit: NASA/JPL-CalTech/UCLA/MPS/DLR/IDA)
In a nutshell
- Scientists discovered that Ceres has a crust containing about 90% ice near its surface, three times more than previously thought, by showing that impurities mixed with ice can prevent it from flowing and deforming over time
- This ice-rich composition suggests Ceres once had a global ocean that froze from top to bottom, trapping increasing amounts of rocky material as it solidified, making it more similar to icy moons like Europa than to other asteroids
- As the closest “ocean world” to Earth, Ceres could serve as an invaluable target for future space missions studying how such bodies evolve and potentially lose their liquid water over time
LAFAYETTE, Ind. — When Italian astronomer Giuseppe Piazzi first spotted Ceres in 1801, he couldn’t have imagined that this distant object would still be puzzling scientists over 220 years later. Fresh research from Purdue University presents compelling evidence that this mysterious body, our solar system’s largest asteroid and smallest dwarf planet, once hosted an ancient ocean that froze from the top down, leaving behind a surprisingly ice-rich crust.
At roughly 585 miles across, about the size of Texas, Ceres orbits the Sun in the asteroid belt between Mars and Jupiter. While most asteroids are irregular chunks of rock, Ceres is massive enough that its own gravity has pulled it into a spherical shape. This makes it unique among the millions of objects in the asteroid belt and earned it classification as a dwarf planet alongside Pluto in 2006.
The study, published in Nature Astronomy, analyzed data from NASA’s Dawn mission, which orbited Ceres from 2015 to 2018. This data revealed a world far more complex than anyone expected. The spacecraft spotted mysterious bright spots in craters, evidence of possible recent geological activity, and signs of what might be a buried ocean. However, these findings created a scientific puzzle. While several lines of evidence pointed to an ice-rich interior, the dwarf planet’s heavily cratered surface suggested it couldn’t contain much ice at all. Pure water ice tends to flow and deform over geological time, much like glaciers on Earth, which should cause impact craters to gradually smooth out and disappear.
“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,” explains study author Mike Sori, assistant professor at Purdue University, in a statement. “However, we’ve shown through our simulations that ice can be much stronger in conditions on Ceres than previously predicted if you mix in just a little bit of solid rock.”
This discovery dramatically revises our understanding of Ceres’ composition. While previous estimates suggested Ceres contained less than 30% ice, the new research indicates its surface may be closer to 90% ice. The team, led by Purdue University Ph.D. student Ian Pamerleau along with Sori and Jennifer Scully from NASA’s Jet Propulsion Laboratory, used sophisticated computer models to reach this conclusion.
Beyond just measuring Ceres’ surface features, the Dawn mission provided crucial data about the dwarf planet‘s composition through various instruments. Its gamma ray and neutron detector revealed high amounts of hydrogen, a key component of water, in the upper surface layers. The spacecraft’s gravity measurements suggested the crust has a density very close to that of water ice with some impurities mixed in.
The surface of Ceres also holds clues to its watery past. The most prominent feature is Occator Crater, home to the brightest of Ceres’ mysterious bright spots. These spots are now believed to be deposits of salt left behind when water from the interior erupted onto the surface and evaporated. Similar features appear in several other craters, suggesting that liquid water has played an active role in shaping Ceres’ geology.
“Even solids will flow over long timescales, and ice flows more readily than rock,” says Pamerleau. “Craters have deep bowls which produce high stresses that then relax to a lower stress state, resulting in a shallower bowl via solid state flow.”
Their simulations revealed that just a small amount of non-ice impurities mixed into the ice would allow a very ice-rich crust to maintain its shape over billions of years. This new understanding of how impure ice behaves under Ceres’ conditions led the team to propose a new model for the dwarf planet’s structure. Rather than having distinct layers of ice and rock, they suggest Ceres has a gradual transition from very ice-rich material near the surface to more rocky material at depth. This structure likely formed as an ancient ocean slowly froze from top to bottom, trapping increasing amounts of rocky material in the ice as the freezing progressed.
“To me the exciting part of all this, if we’re right, is that we have a frozen ocean world pretty close to Earth,” says Sori. “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.”
Sori adds that Ceres’ relatively accessible location makes it “the most accessible icy world in the universe,” potentially making it an invaluable target for future space missions. This discovery adds Ceres to a growing list of solar system bodies that either currently have or once had oceans. Unlike the deep liquid oceans believed to exist beneath the icy shells of moons like Europa and Enceladus, Ceres’ ocean has completely frozen. However, its relatively close location in the asteroid belt makes it an ideal laboratory for studying how ocean worlds evolve and potentially lose their liquid water over time.
The presence of a frozen ancient ocean on Ceres also raises intriguing questions about the potential for past habitability. The combination of liquid water, organic compounds (which Dawn detected on the surface), and mineral salts could have created conditions conducive to the chemistry necessary for life, at least during the period when the ocean was liquid.
Paper Summary
Methodology
The researchers used computer models to simulate how craters of different sizes on Ceres would change shape over billions of years. They tested three different scenarios for Ceres’ crustal structure: one uniform throughout, another with two distinct layers, and a third where the composition gradually changed from top to bottom. The simulations incorporated data from NASA’s Dawn mission, including actual crater shapes, gravity measurements, and surface composition data. They also used recent laboratory findings about how ice behaves when mixed with other materials. Their models considered factors like surface temperature (which varies by latitude) and different mixtures of ice and rock to predict how craters would deform over time.
Results
The simulations showed that a crust containing about 90% ice near the surface, gradually transitioning to rockier material at depth, best matched all the observations from Dawn. Importantly, they found that adding just 6% of impurities to the ice made it much more resistant to flow, explaining how an ice-rich Ceres could maintain its cratered surface over billions of years. This structure suggests Ceres once had a global ocean that froze gradually from the surface downward, trapping increasing amounts of impurities as it solidified.
Limitations
The computer models were limited to simulating craters up to 40 kilometers in diameter, as larger craters tend to have more complex shapes that couldn’t be accurately represented in their two-dimensional simulations. The researchers note that while their proposed structure matches observations, other configurations might also be possible. Additionally, the exact composition of the impurities mixed with the ice remains uncertain, though they likely include minerals, salts, and organic compounds detected by Dawn.
Takeaways
This research resolves a long-standing puzzle about how Ceres could be ice-rich while maintaining ancient surface features. It suggests Ceres formed through the freezing of an ancient, muddy ocean, making it more similar to outer solar system ocean worlds than previously thought. The finding that small amounts of impurities can dramatically strengthen ice may have implications for understanding other icy bodies in the solar system. The study also identifies Ceres as potentially the most accessible ocean world for future exploration.
Funding and Disclosures
The research was funded by NASA’s Discovery Data Analysis Program (DDAP) grant 80NSSC22K1062. The authors declared no competing interests. The study utilized data from NASA’s Dawn mission, which was the first spacecraft to orbit two extraterrestrial destinations – the protoplanet Vesta and dwarf planet Ceres.
Publication Information
Published in Nature Astronomy, Volume 8, November 2024, pages 1373-1379. The paper, titled “An ancient and impure frozen ocean on Ceres implied by its ice-rich crust,” was authored by I. F. Pamerleau, M. M. Sori from Purdue University’s Department of Earth, Atmospheric, and Planetary Sciences, and J. E. C. Scully from NASA’s Jet Propulsion Laboratory at the California Institute of Technology.
