A mountain of salty ice
In the vast expanse of the main asteroid belt between Mars and Jupiter orbits Ceres, a dwarf planet that is the belt's largest object. Observations by NASA's Dawn spacecraft, which orbited Ceres from 2015 to 2018, revealed a surface that is not entirely dormant. The most prominent feature is a solitary mountain named Ahuna Mons, the only one on Ceres. Ahuna Mons is a cryovolcano, or "ice volcano," that extrudes a viscous, salty, muddy brine instead of molten rock.
This enormous dome-shaped mountain rises about 4 kilometers (2.5 miles) above its surroundings and is approximately 17 kilometers (11 miles) wide at its base. Its slopes are streaked with bright material identified as sodium carbonate. Scientists believe this material was left behind after salty water erupted onto the frigid surface and the ice sublimated into space. The few impact craters on its surface suggest Ahuna Mons is a relatively recent feature, with an age estimated to be no more than a few hundred million years old. This indicates that Ceres has been geologically active in its recent past.
A dynamic and evolving world
The engine for this cryovolcanism is thought to be a deep reservoir of brine, about 40 kilometers (25 miles) deep and hundreds of miles wide. The decay of radioactive elements like uranium and thorium in Ceres's interior likely provides enough heat to maintain this subsurface liquid. The high concentration of salts in the water lowers its freezing point, allowing it to remain a liquid slurry and ascend through fractures in the crust.
Ahuna Mons may be the most prominent cryovolcano, but it is not the only one. Scientists have identified at least 22 other features on Ceres that are likely older, relaxed cryovolcanoes that have slumped and flattened over hundreds of millions of years due to a process called viscous relaxation. This process is similar to how honey or putty slowly flattens under its own weight. Models suggest that on average, a new cryovolcano forms on Ceres every 50 million years. This ongoing activity, coupled with the discovery of organic molecules, water ice, and ammoniated clays, makes Ceres a complex world. The evidence shows an interior that may still hold liquid from a past global ocean, changing the view of small solar system bodies as simple, inactive rocks.