The changing appearance of gullies on Mars over the last seven years suggests that liquid water flowed recently on the Red Planet and may still seep out in brief bursts, researchers said Wednesday.

    New images of known gullies on Mars show evidence of new flows and deposits, pointing to explosive events in which some form of water burst from crater walls and ran down their slopes and support what researchers call "the squirting gun" theory.

    "We've had this story of ancient water on Mars," researcher Kenneth Edgett, who participated in the Mars gully study, said during a news briefing at NASA's Washington headquarters. "Today we're talking about liquid water that is present on Mars right now."

    Edgett and colleagues utilized images from NASA's Mars Global Survivor to sudy regions earlier this year where gullies, depressionlike landforms on the Red Planet's surface, were found in 2000.

    They found new, light-colored deposits that do not appear to have formed from landslides, but could be the work of frost, salt deposits or long-sought evidence that water flowed recently on Mars. The research is detailed in this week's issue of the journal Science.

    Christensen said the questions still to be answered include determining the source of water at the gully sites, and making in-depth spectral analyses to confirm the photographic evidence of liquid water.

    Pinning down the source of any liquid water source, be it a subsurface aquifer, ice pack or melting snow, is key, he added.

    "The great news is that NASA has the tools to do that," said Christensen, who also serves as the principal investigator for the Thermal Emission Imaging System aboard NASA's 2001 Mars Odyssey orbiter, currently circling Mars. "I think we're really positioned to go forward with a view of Mars as a dynamic, active place."

    New light-toned deposits coating gullies in April 2005 that were not present in December 2001 were found in an area known as Terra Sirenum. Similar differences were seen in a crater etched into the Centauri Montes region of Mars, which apparently changed sometime between August 1999 and February 2004.

    "I think this is pretty interesting evidence that says yes, there is subsurface water," Christensen said, adding that aquifers, snow packs and ground ice are all plausible sources for liquid Martian water. "It remains to see which ones are most plausible."

    (Agencies)

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