Strange bright spots on the dwarf planet Ceres are giant salt pans caused by the evaporation of water from a subsurface ocean, scientists have confirmed.

The new findings, reported in the journal Nature, solve an enigma which has intrigued both scientists and the public ever since the strange spots were first seen by NASA's Dawn spacecraft.

"We were definitely surprised to see the bright spots," said one of the study's authors, Dr Vishnu Reddy of the Planetary Science Institute in Tucson, Arizona.

"These bright spots are consistent with briny water coming up from the sub-surface which sublimates [a process where a solid evaporates into a gas without passing through a liquid phase] leaving behind [salts] on the surface," Dr Reddy said.

"We had some hints that Ceres might have a subsurface ocean but the presence of salts on the surface was a surprise."

The dwarf planet Ceres has a diameter of about 950 kilometres and is the largest object in the main asteroid belt between Mars and Jupiter.

The general surface of Ceres is as dark as fresh asphalt, however it is covered in over 130 bright spots — some as white as ocean ice. Almost all are associated with impact craters.

Using data from Dawn's spectral framing camera, the authors examined the largest of the bright spots on the floor of the giant 90-kilometre-wide, four-kilometre-deep Occator crater, finding hydrated magnesium sulphates.

Dr Reddy and colleagues found the level of hydration decreased with distance from the spot, supporting the idea of an underground source.

"We're still not sure how or why the water is getting exposed onto the surface," Dr Reddy said.

"There is an almost global network of cracks on the surface of these craters and we think that might be a way for water in a subsurface ocean to make its way to the surface and sublimate."

The authors also examined a thin misty haze that regularly forms above the spot during daylight.

They concluded the fog was caused by the condensation of small particles of ice and salt, said lead author Dr Andreas Nathues of the Max Planck Institute in Germany.

"Water ice, salts and recent sublimation phenomena are signs of a geologically active planetary surface," Dr Nathues said.

The researchers said the findings could help us understand how our solar system — and particularly Earth — evolved.

"A significant number of objects in the inner solar system are now known to have water ice or briny water on them, and all these ingredients are needed for life on Earth as we know it," Dr Reddy said.

"So the ingredients for life could have come from all these small bodies and they contributed a lot of material to Earth early in the history of the solar system both in terms of organics and water, so we could be here because of asteroids hitting the Earth."

Ceres a long-distance visitor

Meanwhile, a second study reported in Nature found minerals on the surface of Ceres indicates the dwarf planet may have formed in the outer solar system and migrated inwards to its present orbit.

Scientists led by Dr Maria Cristina De Sanctis of the Istituto Nazionale di Astrofisica in Rome used the Visible-Infrared Mapping Spectrometer aboard the Dawn spacecraft to obtain new spectral data indicating the widespread presence of substances called ammoniated phyllosilicates across the surface of Ceres.

Ammonia ices are found in the cold outer reaches of the solar system. Their incorporation into Ceres' surface during its formation suggests that the dwarf planet formed out there, before migrating into the main asteroid belt.

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