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takyon writes:

The Dawn spacecraft has found evidence of magnesium sulphate (epsom salts) on Ceres, causing the mysterious bright spots:

The US space agency's Dawn satellite continues to return remarkable images from the dwarf planet Ceres. Now just 385km above the surface (lower than the space station is above Earth), the probe has revealed new features inside the mini-world's Occator Crater. This is the 92km-wide depression that has multiple bright spots of what are thought to be exposed salts.

[...] "The intricate geometry of the crater interior suggests geologic activity in the recent past, but we will need to complete detailed geologic mapping of the crater in order to test hypotheses for its formation." Scientists think the bright spots are deposits of epsom salts (magnesium sulphate), the trace remains of briny water-ice that at one time became exposed on the surface.

With no atmosphere on the dwarf planet, the water content would have rapidly vaporised, leaving only the magnesium sulphate spots. Ceres likely has quite a lot of buried water-ice. This idea is being investigated by the satellite's GRaND instrument, which senses neutrons and gamma rays produced by cosmic ray interactions with surface materials. It is a means to understand the chemistry of the top metre or so of Ceres' rocky "soil".

Lunar and Planetary Science Conference.

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