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.
In what is billed as "the squirting gun," 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.
"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 used images from NASA's now-silent Mars Global Survivor to revisit regions earlier this year where gullies, depressionlike landforms on the Red Planet's surface, were found in 2000.
What they found were 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.
"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. "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 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."
Researchers have known of gullies on the Mars since 2000, when Global Surveyor's Mars Orbiter Camera — 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 on whether they formed from groundwater seeping onto the Martian surface or from dry landslides.
"Our level of certainty [with] which we can address the question of whether the gully features that we're reporting on were formed by water is high, but not extremely high," study leader Michael Malin, chief scientist at Malin Space Science Systems, told Space.com.
"The evidence is mostly suggestive," he said.
Malin's team also used the camera in their new study, which compared base images of two midlatitude regions in Mars' southern hemisphere taken in 1999 and 2001 with 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, 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."
Malin and his team believe that some form of water, be it briny, acidic or slushy, may be bursting out from underground sources at the gullies and leaving the telltale 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 created 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 as it flowed out.
A watery trail
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 the Red Planet with the possibility of life, since the two are closely linked here on Earth. The existence of subsurface liquid water on Mars could also serve as a potential supply source for future Red Planet explorers.
Slideshow 12 photos
Month in Space: January 2014
But determining conclusively that the gully changes seen by Mars Global Surveyor stem from liquid water is daunting, and will likely require an up-close visit — which would be 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, adding that the contamination hurdle is daunting. "It's something that will not be trivially easy to go to, but something there's a lot of interest in."
Christensen said that 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 told Space.com that the gully changes seen by Mars Global Surveyor may be the first of many to be found by Mars-watching orbiters.
"More of these could happen if we just watch," Edgett said.