Deposits formed in Martian gullies during the past seven years suggest that liquid water exists on Mars today. Researchers have observed two downhill tracks of light-colored material that were not present in images taken by the Mars Global Surveyor spacecraft before 1999.

These tracks follow narrow ravines present on the downward sloping walls of craters, and they branch and flow around obstacles like a liquid or watery sediment would, researchers report in the December 8 issue of Science. "The Mars exploration program has been following the water since the mid-'90s, and now we have found it," said the program's lead scientist, Michael Mayer of NASA, at a press conference today.

In 2000 Michael Malin and Kenneth Edgett of Malin Space Science Systems in San Diego first reported the existence of gullies in crater slopes, fueling speculation that actively flowing liquid water could have carved out the relatively young channels.

Since that time the two researchers and their colleagues were busy using the Mars Orbital Camera (MOC) on the Surveyor craft to capture images of thousands of gullies at hundreds of sites, looking for signs of active water. They were rewarded with two new deposits, they report.

In one example, an MOC image from April 2005 shows a deposit on the northwest wall of a crater that was not present in the previous image from December 2001. In subsequent views of the deposit, the sun's light is coming in at different angles, but the light-colored material remains, suggesting it is not a trick of the light or the result of erosion.

Similarly, images of another crater from February 2004 show the beginnings of a second deposit, which has grown in subsequent images [see above]. Finding additional examples might be tricky, given that NASA lost contact with the Mars Global Surveyor last month after nearly ten years of operation.

Although liquid water would quickly freeze and evaporate in the planet's cold, thin atmosphere, flows of water could persist long enough to carve gullies about 500 meters long, according to NASA simulations reported last year. Researchers have speculated that flowing groundwater or melted ground ice could have formed gullies, which mostly occur in craters, pits and other depressions.

"We think what's actually happening is that there's ice formed in the rock and it's acting as a dam, and then water builds up behind that ice dam," Malin says in a NASA statement. "When the pressure of that water gets high enough, it breaks through the dam and out comes a lot of water and the debris that was forming the dam."

Nonwatery explanations for the deposits such as dust or liquid carbon dioxide are unlikely, says planetary scientist Martha Gilmore of Wesleyan University. The craters are not obviously dusty, and carbon dioxide would not stay liquid at Martian atmospheric pressure, she explains.

"Our mission at Mars is to find water because of its connection to life," Gilmore says. "But we also want to understand the history of the climate on the planet, and what this is telling us is Mars's climate is more variable than we had imagined. This is not a dead planet."

In a second finding that highlights the living nature of the Martian surface, Malin and his colleagues observed 20 new impact craters formed in the last seven years in one region of the planet. The craters, which measure two to 150 meters across, should give researchers a more accurate way to date some of the planet's geologic features.