The search for water on Mars -- and the implications that its discovery would have for the planet’s ability to sustain life -- has long captivated planetary scientists and amateur space enthusiasts alike. While the Martian landscape contains gulley-like structures that could have been made by flowing water, scientists have never seen direct evidence of liquid water on the planet. Indeed, the only water detected on the planet thus far is frozen solid.

Malin’s team studied images of thousands of gullies taken over nine years by the Surveyor’s Mars Orbital Camera and found before-and-after evidence that water has flowed through two gullies within the past few years. When photographed in 2004-05, both gullies contained brightly-coloured streaks with “finger-like” endings (see figure: “Alluvial flow”). These streaks were not present in images taken in 1999 and 2001, which the researchers claim is clear evidence that water flowed down the gullies some time in the intervening years (see figure "Before and after").

“The shapes of these deposits are what you would expect to see if the material were carried by flowing water,” said Malin. “They have finger-like branches at the downhill end and [the flow is] easily diverted around small obstacles.”

Malin and co-workers believe that the water wells up from beneath the ground through cracks created by meteor impacts, rather like springs here on Earth. The researchers estimate that the amount of water flowing down one gully was about five to ten swimming pools’ worth. Both gullies were located in Mars’ southern hemisphere at about 37 degrees latitude, where daytime temperatures could go above zero degrees Celsius making liquid water possible. However, the water would also evaporate and freeze as it flows in Mars’ thin atmosphere and often very cold temperatures.

Earlier analyses of images from the Mars Orbital Camera led Malin’s team to suggest that liquid water had flowed on Mars early on its history. “We can now honestly start talking about water flowing on Mars today,” said Philip Christensen of Arizona State University at a press conference at NASA headquarters in Washington, DC. Other scientists, like James Rice, also at Arizona State, are more cautious: “I’m not convinced we’re seeing modern fluid flow”, he said.