NEW YORK — The changing appearance of gullies on Mars as captured by spacecraft imagery over the last seven years suggests liquid water has flowed recently in two areas and may still seep out in brief bursts, researchers said Dec. 6 .

In what is billed as “the squirting gun,” new images of previously imaged gullies show evidence of new flows that seem to have occurred in the last few years as a result of water bursting from crater walls. “We’ve had this story of ancient water on Mars. Today we’re talking about liquid water that is present on Mars right now,” researcher Kenneth Edgett, who participated in the Mars gully study, said during a press briefing at NASA’s Washington headquarters.

Using data collected by the now silent Mars Global Surveyor spacecraft, Edgett and his colleagues compared the appearance of gullies and depression-like landforms in 2000 to imagery gathered during a revisit to those same areas earlier this year. 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 water flow that occurred since the 2000 imagery was collected. The research was detailed in the Dec. 8 issue of Science.

“I think the evidence for liquid water is compelling,” said Philip Christensen, a leading Mars researcher at Arizona State University who did not participate in Malin’s study. “But I think certain questions still remain … but that’s the natural flow of science.” Those remaining questions, Christensen said, 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 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. “I think we’re really positioned to go forward with a view of Mars as a dynamic, active place.”

The news also was seen by some as an important reminder that robotic spacecraft have a critical role to play in planetary exploration. “In a time when science has been under attack within the NASA budget, this once again demonstrates the importance of science missions like Mars Global Surveyor. They are missions of discovery, where the most important findings are often the most unexpected,” said Bruce Betts, director of projects at the Pasadena, Calif.-based Planetary Society. “The fact that Mars Global Surveyor lasted so far beyond its projected lifetime has allowed this type of discovery that requires observing the same area over and over again.”

Researchers have known of gullies on Mars since 2000, when the Mars Global Surveyor’s Mars Orbiter Camera (MOC) — built by Malin Space Science Systems in San Diego — first observed the eye-catching landforms. Found mostly on slopes or ridges, the gullies sparked long-running debates about whether they had been formed from groundwater seeping out of the Martian surface or in dry landslides.

“Our level of certainty … is high, but not extremely high,” said Michael Malin, chief scientist at Malin Space Science Systems and the study’s leader. “The evidence is mostly suggestive.” Malin’s team also used the MOC instrument in their new study, which compared base images of two mid-latitude regions in Mars’ southern hemisphere taken in 1999 and 2001 to more recent images captured in the years since.

In an area known as Terra Sirenum, new light-toned deposits coating gullies in April 2005 were not present in December 2001. Similar changes 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.

Malin and his team believe some form of water, briny, acidic or slushy, may be bursting from underground sources and leaving the tell-tale signs. The result, Malin added, could resemble the sort of mudflows seen on Earth after torrential rains or flash floods.

Edgett said early estimates call for somewhere between five and 10 swimming pools’ worth of water to have formed the gully changes seen on Mars. “And if you were there, and this thing was coming down the slope, you’d kind of want to get out of the way,” Edgett said, adding that Mars’ thin atmosphere would force the water to boil off from a lack of pressure as it flowed out.

That liquid water once existed on Mars in some form has been known conclusively since 2004, when NASA’s Opportunity rover found evidence that it permeated rocks in the planet’s ancient past. Mars scientists have long associated the search for liquid water on red planet with the possibility of life, since the two are closely linked here on Earth. The existence of subsurface liquid water on Mars also could serve as a potential supply source for future exploration crews.

But determining conclusively that the gully changes seen by the Mars Global Surveyor stem from liquid water is daunting, and likely will require an up-close visit, a challenge due to the risk of contaminating a gully site with Earth microbes or other material.

“Personally, I think you’re going to have to go to one [and see],” Malin said. “It’s something that will not be trivially easy to go to, but something there’s a lot of interest in.”

Christensen said NASA’s proposed Astrobiology Field Laboratory would be a good candidate as a robotic gully visitor, because it would not only be sterilized but also capable of traversing rough terrain.

Edgett said the gully changes seen by the surveyor may be the first of many to be found by spacecraft orbiting Mars.