The National Aeronautics and Space Administration spacecraft has observed hydrated silica, or opal, spread across large regions of Mars. That, scientists said, suggests liquid water was on the planet's surface as recently as 2 billion years ago.

"This is an exciting discovery because it extends the time range for liquid water on Mars, and the places where it might have supported life," said Scott Murchie, at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

The newly discovered silicates formed where liquid water altered materials created by volcanic activity or meteorite impact on the Martian surface, scientists said. One such location is the large Martian canyon system called Valles Marineris.

"We see numerous outcrops of opal-like minerals, commonly in thin layers extending for very long distances around the rim of Valles Marineris and sometimes within the canyon system itself," said Ralph Milliken of NASA's Jet Propulsion Laboratory in Pasadena, Calif.

Milliken is lead author of an article describing the discovery in the November issue of the journal Geology.