In a long-running debate over whether Mars ever had long-lasting rivers, the latest images supporting the "yes" side have been put forth.

Pictures from NASA's Mars Global Surveyor orbiter show eroded ancient deposits of transported sediment long since hardened into interweaving, curved ridges of layered rock. Scientists interpret some of the curves as traces of ancient "meanders" made in a sedimentary fan as flowing water changed its course over time.

"Meanders are key, unequivocal evidence that some valleys on early Mars held persistent flows of water over considerable periods of time," said Michael Malin of Malin Space Science Systems, San Diego, which supplied and operates the spacecraft's Mars Orbiter Camera.

"The shape of the fan and the pattern of inverted channels in it suggest it may have been a real delta, a deposit made where a river enters a body of water," Malin said. "If so, it would be the strongest indicator yet Mars once had lakes."

Most scientists now agree that Mars has abundant water ice, at both its poles and under the dusty surface across much of the rest of the planet.

If Mars did have liquid water for lengthy periods, then conditions would might been favorable to the development of life.

The fan covers an area about 8 miles (13 kilometers) long and 7 miles (11 kilometers) wide in an unnamed southern hemisphere crater, downslope from a large network of channels that apparently drained into it billions of years ago, according to a NASA press release.

"This latest discovery by the intrepid Mars Global Surveyor is our first definitive evidence of persistent surface water," said Jim Garvin, NASA's Lead Scientist for Mars Exploration. "It reaffirms we are on the right pathway for searching the record of Martian landscapes and eventually rocks for the record of habitats. Such localities may serve as key landing sites for future missions, such as the Mars Science Laboratory in 2009."

No liquid water has been detected on Mars, although one of the previous discoveries from Mars Global Surveyor pictures suggests some gullies have been cut in geologically recent times by the flow of ephemeral liquid water.

Some researchers, however, remain unconvinced what those features show. One aspect to the debate suggests some other substance, and not water, may have carved the canyons of Mars. Other scientists have argued the flows -- whatever material involved -- were brief and catastrophic rather than sustained.

Malin and his colleague, Ken Edgett based their analysis also on information from Mars Global Surveyor's laser altimeter and from cameras on Mars Odyssey and NASA's Viking Orbiter.

"Because the debris in this fan is now cemented, it shows that some sedimentary rocks on Mars were deposited by water," Edgett said. "This has been suspected, but never so clearly demonstrated before."

Images Credit: NASA/JPL/Malin Space Science Systems

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