Evidence Found for Flowing Water on Mars.

[Subhash]

WASHINGTON — An orbiting spacecraft has sent back new evidence for the presence of water on Mars.

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Colorful rock layers in Mars' Mawrth Vallis channel system, which holds clay-like minerals probably formed in the presence of water.

Scientists have long debated whether water flowed on the red planet, with evidence increasing in recent years. The presence of water would raise the possibility of at least primitive life forms existing there.

Images from a camera aboard NASA's Mars Reconnaissance Orbiter show alternating layers of dark- and light-toned rock in a giant rift valley.

Within those deposits are a series of linear fractures, called joints, that are surrounded by "halos" of light-toned bedrock, according to researchers from the University of Arizona.

Their findings, published in Thursday's edition of the journal Science, were being presented at the annual meeting of the American Association for the Advancement of Science in San Francisco.

Lead author Chris H. Okubo said the "halos" indicate areas where fluids, probably water, passed through the bedrock. Minerals in the fluid strengthen and bleach the rock, he said, making it more resistant to erosion than other areas.

"On Earth, bleaching of rock surrounding a fracture is a clear indication of chemical interactions between fluids circulating within the fracture and the host rock," Okubo and co-author Alfred S. McEwen reported in the paper.

The researchers also said that layered outcrops can indicate cycles with materials deposited by regular episodes of water, wind or volcanic activity.

Just last December, scientists reported evidence that water may be flowing through Mars' frigid surface. Images from Mars Global Surveyor showed changes in craters that provide the strongest evidence yet that water moved through them as recently as several years ago, and is perhaps doing so even now.

The Surveyor had 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 in a search for evidence of recent 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.