Mineral evidence for a water environment capable of supporting life has been discovered on Mars, scientists said.
Deposits of carbonate, formed in neutral or alkaline water, were spotted by NASA's Mars Reconnaissance Orbiter, the scientists told a meeting of the American Geophysical Union in San Francisco.
"Obviously this is very exciting," said John Mustard of Brown University in Rhode Island. "It's white -- it's a bulbous, crusty material."
Carbonate is formed when water and carbon dioxide mix with calcium, iron or magnesium. It dissolves quickly in acid, so its discovery counters the theory that all water on Mars was at one time acidic.
"It would have been a pretty clement, benign environment for early Martian life," said Bethany Ehlmann, a graduate student at Brown University who led the study published in the journal Science.
"It preserves a record of a particular type of habitat, a neutral to alkaline water environment."
Carbonates on Earth like chalk or limestone sometimes preserve organic material, but scientists have found no such evidence on Mars.
The 3.6 billion-year-old carbonate was discovered in bedrock at the edge of a 1,490-km-wide crater.
Carbonate previously had been found in minuscule amounts in soil samples provided by the Phoenix Mars Lander, Martian dust and Martian meteorites on Earth. But this is the first time scientists have found a site where carbonate formed.
The deposits are about the size of football fields and are visible in images taken by the Mars Reconnaissance Orbiter.
The deposits appear to be limited, but the neutral or alkaline water environment may once have been more widespread, said Scott Murchie, a scientist at Johns Hopkins University in Baltimore.
Phyllosilicates, which form under similar conditions to carbonate but do not dissolve in acidic environments, are abundant on Mars.
"There were these different water environments in early Mars history, (which) increases the possibilities that life started," said Richard Zurek of NASA's Jet Propulsion Laboratory in California.