Ceres' salty history hints at bright spot origin

Dawn's happy snaps also hint at underground water ice

When the Dawn mission's first photographs arrived from Ceres, everyone from scientist to conspiracist got excited about the bright spots. Now, scientists say the spots are a kind of salt that hint at subsurface water ice on the object.

An analysis led by boffins from the Max Planck Institute concludes that the strong reflections in the 130-plus bright spots (most of which are impact craters) are caused by the magnesium sulfate called hexahydrite.

In its announcement, NASA says the salt-rich areas were left behind when water ice sublimated in the past. Asteroid strikes then pierced the surface to expose the bright patches.

That opens another tantalising possibility: “The global nature of Ceres' bright spots suggests that this world has a subsurface layer that contains briny water-ice”, according to lead author Andreas Nathues of the Max Planck Institute.

Nathues' team concentrated their analysis on the largest of Ceres' bright spots, the 90 km-wide, 4 km-deep Occator crater, examining photographs taken by Dawn's spectral framing camera.

The boffins also found that the hydration fell with increasing distance from the centre of the crater, which supports the idea that there's underground water-ice - something previously offered as a hypothesis for the bright craters.

A second study by the Dawn mission team suggests Ceres also has clays rich in ammonia. On its own, they say, ammonia would have evaporated.

That could mean that instead of forming in its current orbit in the asteroid belt, Ceres could have been born in the outer solar system; or, alternatively, collisions with stuff drifting in from the outer solar system could have brought the ammonia.

The paper in Nature describing Ceres' salt pans is here, and the paper associating ammonia with Ceres' possible origins is here. ®

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