Scientists believe they have found ice inside craters near Mercury's poles, which they say could reveal more about the building blocks for life on other planets.
The small planet orbits closest to the sun but rotates nearly upright, meaning some areas on its poles never see sunlight.
Nasa researchers suspect the ice and other organic material accumulated in the shadowed areas of craters at the poles after comets and asteroids delivered the material to Mercury's surface.
"The new data indicates the water ice in Mercury's polar regions, if spread over an area the size of Washington DC, would be more than two miles (3.2km) thick," said Nasa researcher David Lawrence.
He added: "If you add it all up, you have the order of 100 billion to one trillion metric tons of ice."
The conclusions were drawn from data sent back to Earth from Nasa's Messenger spacecraft, which landed on the planet in 2011.
Further study of the material could explain more about how life began on Earth, the scientists said at a news conference.
"Messenger has revealed a very important chapter in the story of how water ice and other volatile materials have been delivered to the inner planets, including Mercury," said Sean C. Solomon, principal investigator of the Messenger mission.
"It's extraordinary that this chapter is so well-preserved on the planet closest to the sun."
James L. Green, director of Nasa's planetary science division, said the finding "bodes well for a continued search for water elsewhere in the solar system".
Meanwhile, Nasa has dismissed rumours that its Mars Curiosity rover has made a significant discovery on the red planet.
Speculation had been stoked when a Nasa researcher said that data coming back from soil samples was a discovery "for the history books".
But a statement from the organisation said: "Rumours of major new findings at this early stage are incorrect."