Nasa has found evidence of "abundant" ice and organic material beneath the surface of Mercury.
Enormous black patches of dark material have been found beneath polar regions of the solar system's innermost planet, which are permanently in shadow.
Data from the Messenger space craft suggests these patches are large deposits of water ice and other volatile organic matter.
It might seem unlikely to find organic material and ice on the planet closest to our Sun, where temperatures can reach 426 degrees Celsius during the day.
But because the planet has such a low axial tilt - less than 1% - the poles are kept in permanent darkness. As such there are pockets of Mercury where ice and other frozen material can survive.
Above: Shown in red are areas of Mercury's north polar region that are in shadow in all images acquired by MESSENGER to date.
It has been theorised for 20 years than the planet might hold ice. The Arecibo radio telescope in Puerto Rico found bright patches on Mercury in 1991, which is often evidence of frozen water.
But the latest data, published as three papers in Science Express, appears to confirm it beyond doubt.
Using Messenger's laser altimeter and neutron spectrometer, which measures hydrogen levels and the reflectiveness of a surface, scientists were able to make out water ice on the surface of the poles, as well as dark patches in warmer areas.
By firing more than 10 million laser pulses at the planet, they discerned that the dark patches were "thermally insulating layers" of organic matter, such as coal or soot, and that underneath those patches there lies yet more water ice.
The researchers say that both the ice and the organic material could have been delivered on the back of a comet or asteroid.
David Paige of the University of California, Los Angeles, said that the "organic material may have been darkened further by exposure to the harsh radiation at Mercury's surface".
So while the finds are probably not evidence that life existed on the planet, it might affect how we look for life elsewhere in our solar system or beyond. For instance, it is possible that planets like Mercury are vital for creating life in more habitable zones.
"The new observations have also raised new questions," said Sean Solomon of the Columbia University's Lamont-Doherty Earth Observatory, and lead investigator on the Messenger mission.
"Do the dark materials in the polar deposits consist mostly of organic compounds? What kind of chemical reactions has that material experienced? Are there any regions on or within Mercury that might have both liquid water and organic compounds?
"Only with the continued exploration of Mercury can we hope to make progress on these new questions."Suggest a correction