Once thought to be a unique feature of Earth, evidence has been found to suggest that water does, or once did, exist on the Moon, Venus, Mars, Pluto, Enceladus, Titan, Charon and Ceres. Two new studies have focused on the lattermost of these, using data gathered by NASA's Dawn mission to determine that the dwarf planet's salty, icy crust could be the remains of an ancient global ocean – and there may still be some liquid water locked up deep beneath the surface.
Since Dawn entered Ceres' orbit in March 2015, the probe has spotted ice in dark craters at the poles, examined the mystery of its bright spots, and detected clear signs of organic molecules. Now two new studies of the dwarf planet's internal composition and structure suggest it was once covered in a global ocean. If that's the case, the missing water is locked in the planet's crust, which is made up of ice, salts and hydrated materials, while some traces may still exist in a liquid state deeper down.
"More and more, we are learning that Ceres is a complex, dynamic world that may have hosted a lot of liquid water in the past, and may still have some underground," says Julie Castillo-Rogez, co-author on both studies.
The first of the new studies aimed to determine the internal structure and composition of the dwarf planet, using shape and gravity data gathered by Dawn. The scientists compared models of Ceres' gravity with what Dawn observed, and found discrepancies over four major landmarks: three craters called Occator, Kerwan and Yalode, and a large mountain named Ahuna Mons. That suggests these landmarks are the result of gravity anomalies, and lends weight to the idea that Ceres is currently geologically active.
Studying Ceres's crust, the team discovered something strange: its density is close to that of ice, but it's far too strong for ice to be the main component. And that's where the second study comes in: that team modeled how the dwarf planet's surface would have changed over time, to determine what its crust is made of.
According to the model, the crust is most likely a mixture of ice, salts, rock and clathrate hydrates – a crystalline solid where water molecules surround and trap gas molecules. Altogether, this combination is about as dense as plain old ice but can be up to 1,000 times stronger, explaining both the crust's composition as well as the fate of the ancient ocean.
But that's not the whole story. The topographical model also revealed that Ceres' surface was once more mountainous, but these features have smoothed out over the last few billion years. For that to occur, the crust must be sitting on top of a slushy layer, which likely contains some liquid water.
Source: NASA JPL