New evidence supports the theory that Mercury - the closest planet to the Sun - has abundant water ice in permanently shadowed polar craters.
Temperatures can reach 430C on parts of Mercury's surface, but the planet barely tilts and there are pockets of its poles where the Sun never shines.
Newest data from the Messenger spacecraft strongly indicated water ice was the major constituent of deposits at Mercury's north pole, Nasa said. That ice was exposed at the surface in the coldest of the deposits.
Across most of the deposits, where temperatures were a bit too warm for ice to be stable at the surface, it was buried beneath an unusually dark material.
Findings about the ice are presented in three papers published by Science Express.
David Paige, from the University of California, and the lead author of one of the papers, said the dark material was likely to be a mix of complex organic compounds delivered to Mercury by the impacts of comets and volatile-rich asteriods.
The same objects were also likely to have delivered water to the planet.
It was estimated there could be between 100 billion and one trillion tonnes of ice on Mercury. That would be enough to encase Washington DC in a frozen black 4km deep.
Three independent lines of evidence supported the water ice conclusion, Nasa said.
One was measurements of excess hydrogen at Mercury's north pole made with Messenger's neutron spectrometer. That data indicated radar-bright deposits at Mercury's poles contained, on average, a hydrogen-rich layer more than tens of centimetres thick, beneath a superficial layer 10-20cm thick. The hydrogen content of the buried layer was consistent with nearly pure water ice.
Another piece of evidence was the first measurements of the reflectance of Mercury's polar deposits at near-infrafed wavelengths made with Messenger's Mercury laser altimeter (MLA), which fired millions of laser pulses at the planet.
The third piece of evidence was the first detailed models of the surface and near-surface temperatures of Mercury's north polar region using the actual topography of the planet, as measured by the MLA.
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