Ceres' water ice can mask massive surface salt deposits

The dwarf planet Ceres is arguably one of the most overlooked and underappreciated planetary objects in the solar system. This could be because while Ceres has the same dwarf planet designation as Pluto, Ceres is largely considered an "asteroid" since it resides in the main asteroid belt. However, along with being the largest object in the main asteroid belt, Ceres accounts for between 25% and 30% of the belt's total mass.

Despite its lack of recognition, Ceres' interior is layered like those of most rocky worlds, including Pluto, and its surface displays evidence of past water ice activity (also called cryovolcanism) with several "bright spots," which are highly reflective salt deposits. But what can these salt deposits teach scientists about Ceres' formation and evolution?

Now, a pair of researchers from the California Institute of Technology (Caltech) might have brought us closer to understanding a connection between Ceres' water ice activity and surface salt deposits.

In findings published in the Journal of Geophysical Research: Planets, the researchers presented evidence that previous estimates of the amount of salt in deposits observed by NASA's Dawn spacecraft, which explored Ceres from 2015 to 2018, might have been incorrect.

Ice may conceal more salt

For the study, researchers used a combination of Dawn spacecraft data and a series of computer models to examine the evolution of water ice and salt deposits on Ceres' surface. The primary motivation behind the study was to fill a long-standing knowledge gap between observations of surface water ice and salt deposits and what's actually happening. Specifically, this study focused on the formation of salt deposits as water ice turns directly into vapor (scientifically called sublimation) and whether data collected by Dawn provided accurate estimates of the amount of water ice and salt.

In the end, the researchers found that the salt deposits might be hidden by the water ice deposits they're mixed with. The amount of salt could be between 30% and 50% greater than previously hypothesized. Additionally, they found that landslides (scientifically called mass wasting) contribute to more surface ice being exposed, which then sublimates, leaving behind the salty bright spots observed by Dawn. They note that some water ice deposits observed by Dawn could have been recently exposed by landslides.

"We proposed a qualitative model of long-term surface evolution of Ceres' salty ices based on observations that is akin to thermal segregation [sorting] on icy satellites," the study notes. "Ice in deposits with salts sublimates more readily and may move to colder areas, resulting in a feedback that creates compositional differences between colder, icy surfaces and warmer, salt-richer regions.

"Ongoing replenishment of ice at the optical [visible] surface is required to reconcile multiple detections with geologically negligible lifetimes of ice. The finding resolves an existing paradox about how salt-rich deposits arise in the Ceres near-surface; they are likely not initially salt-rich but instead result from enrichment of salt at the optical surface by sublimation of salty ices."

Sublimation leaves salt behind

Essentially, the salt and water ice deposits are sorted based on heat and sublimation: The salt within the water ice mixture causes the water ice to sublimate faster, leaving behind the salt deposits. This creates a feedback loop that results in different proportions and types of mixtures among separate water ice and salt deposits. Additionally, surface water ice is exposed by landslides, adding to this sublimation feedback loop.

Finally, the study resolves a long-standing misunderstanding about how salt deposits form and evolve: They don't start out with large amounts of salt. Instead, they start out with low amounts of salt, but their salt content increases as water ice that also contains salt sublimates.