The bright spots dotting the surface of dwarf planet Ceres have been one of the biggest mysteries of the solar system in 2015. However, scientists now appear to have a better idea what these mysterious spots are, thanks to data from NASA's Dawn spacecraft that has been circling the dwarf planet since March.

Based on observations made by Dawn spacecraft, scientists reported that the bright spots found on Ceres are made up primarily of hydrated magnesium sulfates. Magnesium sulfate is popularly known as Epsom salt and is notably used here on Earth for treating a range of ailments such as joint inflammation and sore feet.

Andreas Nathues and his colleagues at the Max Planck Institute for Solar System Research in Germany counted 130 bright spots on the surface of Ceres. Using Dawn's Framing Camera, whose data provide information on the composition of Ceres based on reflectance characteristics, Nathues' team identified the material as a kind of magnesium sulfate called hexahydrite.

The whitish patches have intrigued scientists since they were discovered. The spots are mostly linked to impact craters and are much brighter compared to the surface of Ceres as a whole. Ceres' surface is about as reflective as a freshly poured asphalt while the brightness of the spots ranges from that of a concrete to the reflectivity of the ocean ice.

Experts have earlier posited that the bright spots are composed of either water ice of some type of salt, but data from Dawn's framing camera appear to support the salt hypothesis. 

The closest match for the center of the brightest spot in the Occator Crater, which has the most famous collection of the bright spots, supports the idea that it is composed of hexahydrite albeit the composition seems to shift into less-hydrated kinds of magnesium sulfate at greater distances from the center.

"These unusual areas are consistent with hydrated magnesium sulfates mixed with dark background material, although other compositions are possible," the researchers wrote in their study, which was published in the journal Nature on Dec. 10.

"We conclude that Ceres must have accreted material from beyond the 'snow line,' which is the distance from the Sun at which water molecules condense."

The researchers added that the salty spots may have been left behind after water-ice sublimated off the surface of the dwarf planet transforming from solid to gas. This idea could mean that there are a lot of briny ice lying beneath the surface of the extraterrestrial world.

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