Even though it’s just one of millions of objects in the asteroid belt, Ceres is special. It’s the largest object in that group, with a diameter of about 940 kilometers, accounting for roughly one-third of the belt’s total mass. And while most smaller asteroids have an irregular shape, Ceres is nearly spherical, having scooped up all the material in its orbital neighborhood. This combination of large size and round shape led astronomers to reclassify it as a dwarf planet.
Ceres is no featureless rock. It has distinct planet-like landscapes, including craters, pits, domes, slopes formed by landslides, and even volcanoes. A prime example is Ahuna Mons, a 4-kilometer-high dome-like structure created by Ceres’ own brand of volcanism, which involves cold, molten ice slurries rather than magma. The sides of Ahuna Mons are smooth with hardly any craters, pointing to a relatively recent eruption. All this complex and varied geology reinforces the notion of Ceres being less like an asteroid and more like a planet, one that long ago may have hosted life.
Ceres had a moment in the limelight a decade ago, when NASA’s Dawn spacecraft paid an extended visit. Dawn entered orbit around Ceres on March 6, 2015, and kept up its observations until 2018, when it ran out of maneuvering fuel. Thanks to the close-up imagery and other data taken during that mission, we know that Ceres’ surface composition is fairly uniform and that ammonia and magnesium silicates, as well as carbonates, are widespread. These are mixed with a dark component, which is thought to include subsurface material dug up by meteorite impacts. Dawn also identified salts and water ice on Ceres’ surface, and many of the geological features seen in the images indicate that water altered the entire dwarf planet in the past. Organic material was also detected, though it’s unclear whether it’s indigenous or came from nearby asteroids.
Dawn data showed that Ceres’ density is quite low, suggesting a high overall water (rather than rock) content. That seemed inconsistent, however, with the general lack of shallow craters and the observation that deep craters are very well preserved. Recently, following a paper published in Nature Astronomy by Ian Pamerleau and colleagues from Purdue University and the Jet Propulsion Lab, we found a solution to this apparent mystery.
How quickly an ice-rock mixture would flow during a volcanic eruption or crater impact on Ceres depends very much on the purity of the mixture and its temperature. Simulations run by Pamerleau’s group showed that the crust could flow very slowly indeed, so that craters wouldn’t appear to change much over billions of years. The simulations, when combined with the crater data, point to a crust containing 90% ice near the surface, with the ice content gradually decreasing down to a depth of 117 kilometers. Below that, it’s solid rock.
The implication is that Ceres once had a muddy ocean that froze from the top down, until all the liquid water became ice. That would make Ceres similar in many ways to Jupiter’s moon Europa, with the exception that its ocean became completely frozen over time, while tidal forces between Jupiter and all of its large moons warm up the interiors of its icy moons enough to keep their oceans liquid.
Why does any of this matter? Because where there is (or used to be) water, there may be life. This is especially true on Europa, where the rocky mantle is thought to be in direct contact with liquid water, providing a supply of elements needed for biology to arise. Within the icy ocean worlds, environments might exist similar to hydrothermal vents on Earth’s ocean floors, which provide energy and nutrients to sustain life. Even the presence of “Lost City”-type environments — where life on Earth may have originated — is feasible within some of the icy ocean worlds.
However, astrobiologists debate whether life requires a planetary surface to be in contact with an atmosphere, allowing for cycles of wetting-drying and freezing-thawing. Studying extraterrestrial ocean worlds should help us better understand the complicated — and possibly diverse — pathways to the origin of life elsewhere in the Universe.
One thing we shouldn’t expect, however, is to find life on Ceres today. Any subsurface ocean, if it ever existed, is now completely frozen. But a “dirty” ocean, with its high mud and mineral content, could well have given rise to biology in the distant past. In fact, the catalyzing properties of certain clay minerals, known to be present on Ceres, might have been essential for early life. Remnants of Ceres’ ancient ocean can still be seen in some of the bright patches on its surface, and we should sample those patches in the future. Fortunately, fossils, even microscopic ones, tend to preserve relatively well in ice.
For all these reasons, Ceres might move up on the list of attractive targets for future exploration. Europa and Saturn’s moon Enceladus still top that list, but Europa’s ocean is locked under kilometers of ice, and Enceladus is a long way away. Ceres is the closest icy ocean world to us, and may be easier to explore. Don’t let it be forgotten.
This article Ceres: The asteroid belt’s forgotten ocean world is featured on Big Think.
The post “Ceres: The asteroid belt’s forgotten ocean world” by Dirk Schulze-Makuch was published on 06/23/2025 by bigthink.com