"This underscores the importance of searching for life on Mars, either present or past," said Bruce Jakosky, an astrobiologist at the University of Colorado at Boulder, who had no role in the study. "It's one more reason to think that life could be there."

Some researchers were skeptical that liquid water was responsible for the surface feature changes seen by the spacecraft. They said other materials such as sand or dust can flow like a liquid and produce similar results.

"Nothing in the images, no matter how cool they are, proves that the flows were wet, or that they were anything more exciting than avalanches of sand and dust," Allan Treiman, a geologist at the Lunar and Planetary Institute in Houston said in an e-mail.

The Global Surveyor previously spotted tens of thousands of gullies that scientists believed were geologically young and carved by fast-moving water coursing down cliffs and steep crater walls. Scientists decided to retake photos of thousands of gullies in a search for evidence of recent water activity.

Two craters in the southern hemisphere that were originally photographed in 1999 and 2001 were examined again in 2004 and 2005, and the images yielded changes consistent with water flowing down the crater walls, according to the study.

Scientists said five to 10 pools of water rushed down the craters in each case. In both craters, scientists found bright, light-colored deposits several hundred yards long in gullies that weren't present in the original photos. They concluded that the deposits - possibly mud, salt or frost - were left there when water recently cascaded through.

Edgett said a combination of factors, including the shape and color of the deposits, led the team to believe it was recent water action and not dust that slipped down the slope. He said dust would leave dark deposits.

Water cannot remain a liquid on Mars for long because of subzero surface temperatures and low atmospheric pressure that would turn water into ice or gas. But scientists theorize that liquid water is being shot up to the surface from an underground source, like geysers.

Mars formed more than 4.5 billion years ago and scientists generally believe it went through an early wet and warm era that ended after 1.5 billion to 2.5 billion years, leaving the planet extremely dry and cold.

"We're now realizing Mars is more active than we previously thought and that the mid-latitude section seems to be where all the action is," said Arizona State University scientist Phil Christensen, who was not part of the current research.

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