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Slush on Ceres widens hopes for water worlds

Even before NASA’s Dawn probe mapped Ceres in detail in 2015, scientists suspected that the dwarf planet was a water world. Now they’ve traced Ceres’ upwellings of salty slush and mud to reservoirs deep beneath the surface.

The details came out this week in a package of papers published by Nature Astronomy, Nature Geoscience and Nature Communications. The findings serve as a fitting coda to an 11-year mission that almost didn’t happen, but ultimately succeeded in solving many of the mysteries surrounding Ceres as well as its sister asteroid Vesta.

For example, consider the bright spots in Ceres’ Occator Crater, which some have likened to “alien headlights.” The reflectivity is due to a crust of sodium carbonate, a salt left behind by the evaporation of briny water that percolated up to the surface.

Gravity readings gathered during the latter days of Dawn’s mission led scientists to conclude that the brine came from a reservoir that’s 25 miles deep and hundreds of miles wide.

In one of the bright spots, known as Cerealia Facula, Dawn’s instruments detected a concentration of hydrated salt compounds. The fact that those compounds are still hydrated suggest that they must have reached the surface relatively recently — perhaps within the past few centuries. That suggests that the transfer of liquid material from Ceres’ deep reservoir is continuing.

“For the large deposit at Cerealia Facula, the bulk of the salts were supplied from a slushy area just beneath the surface that was melted by the heat of the impact that formed the crater about 20 million years ago,” Dawn principal investigator, Carol Raymond, explained in a news release. “The impact heat subsided after a few million years; however, the impact also created large fractures that could reach the deep, long-lived reservoir, allowing brine to continue percolating to the surface.”

Dawn’s scientists saw additional evidence for Ceres’ active, slush-based geology in the presence of conical hills reminiscent of earthly features known as pingos. On Earth, pingos are formed when pressurized groundwater freezes beneath the surface and pushes up the soil above. Similar geological structures have been observed on Mars.

On icy moons such as Europa, Enceladus and Titan, geological activity is primarily driven by gravitational interactions with their parent planets. The fact that Ceres is geologically active, even though its crust is not being flexed by a nearby planet, widens the possibilities for finding slush or liquid water deep within ice-rich worlds in the main asteroid belt, the Kuiper Belt and elsewhere.

The data-gathering phase of Dawn’s mission ended in 2018, and the dead spacecraft is now silently circling Ceres. But decades or centuries from now, the scientific findings resulting from the mission just might guide explorers to new kinds of interplanetary watering holes.

By Alan Boyle

Mastermind of Cosmic Log, contributing editor at GeekWire, author of "The Case for Pluto: How a Little Planet Made a Big Difference," president of the Council for the Advancement of Science Writing. Check out "About Alan Boyle" for more fun facts.

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