Ceres Bright Spot is Salty

Spectral observations by NASA’s Dawn spacecraft reveal that the dwarf planet’s mysterious beacon is made of salt.

A patchwork of bright splashes inside Occator Crater on Ceres continues to puzzle planetary scientists. NASA's Dawn spacecraft recorded this composite view in August 2015.
NASA / JPL / UCLA / MPS / DLR / IDA

Planetary scientists say they’ve finally solved the mystery of Ceres’ bright spot. The spot, a comparatively reflective blotch in the crater Occator, has been a mesmerizing topic of debate for several years now. JPL even ran a public pollasking people what they thought the spot was — with options of volcano, geyser, rock, ice, salt, and “other” — and has had more than 190,000 respondents. As of last month, the two most popular options were ice (about 28%) and other (38%).

Unfortunately for those voters, the winning answer is salt.

Scientists have suspected salts for a while. (Ice can’t survive on Ceres’ surface: it’ll sublimate.) Last December, scientists with NASA’s Dawn mission suggested the bright Occator splash — which isn’t the only one: there are many, lesser bright spots across the dwarf planet — contained magnesium sulfate. But Maria Cristina De Sanctis (National Institute of Astrophysics, Rome) and fellow Dawn scientists report June 29th in Nature that infrared spectral data instead reveal a different salt, sodium carbonate, with some ammonia mixed in.

This enhanced-color image from NASA's Dawn spacecraft reveals subtle differences in the bright material in Ceres' crater Occator. The close-up view reveals a dome in a smooth-walled pit in the crater's bright center (left center of image). Numerous linear features and fractures crisscross the dome's top and flanks. Click here for a larger version.
NASA / JPL-Caltech / UCLA / MPS / DLR / IDA / PSI / LPI

We’ve only detected sodium carbonate on Earth and in the plumes of Saturn’s moon Enceladus. It arises when liquid water interacts with certain minerals. The team thinks brine must have welled up from below the surface, then the water would have evaporated away, leaving the salts behind. Where that liquid water came from scientists don’t know: the impact that created Occator some 80 million years ago might have heated some subsurface ice, or perhaps liquid water deeper down rose up through the crater’s fractures. It’s even possible that the liquid water would boil near the surface, creating geysers, Mikhail Zolotov (Arizona State University) speculates in a perspective piece in the July Nature Geoscience. Or maybe the salt was deposited beneath the surface and exhumed by later craters. There are plenty of options to go around.

Although not on the surface, there’s a fair amount of water ice inside Ceres. A second paper by Michael Bland (U.S. Geological Survey) and colleagues in Nature Geosciencelooks at how deeply big craters dig. The more water ice in the subsurface, the faster these craters will fade away, because an ice-rich surface would “relax” with time as the ice flowed (think glacier movements). Based on the craters, the scientists conclude that Ceres’ shallow subsurface is 30% to 40% ice by volume, with the rest a mixture of rocks, salts, and other stuff.

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