For decades, scientists have scoured Mars in search of water, a basic ingredient for primitive life.
The best clues were always tied to the ancient past, a time when the Red Planet was warmer and wetter.
But on Wednesday, scientists made a startling announcement: Water could still course through the frigid surface.
Crisp photographs taken by NASA's old Mars Global Surveyor before it lost contact with Earth last month do not actually show flowing water. But scientists said before and after pictures of two gullies showing feature changes provide the strongest signs yet that water flowed through them as recently as several years ago, and is perhaps doing so even now.
"This is a squirting gun for water on Mars," said Kenneth Edgett, a scientist at San Diego's Malin Space Science Systems, which operates a camera on the Global Surveyor, one of six spacecraft focused on Mars.
Most noticeable, each of the gullies has a fresh coat of fine, pale sediment, which scientists say appears to be either water frost or salts left behind by briny water.
Flow patterns around rocks or other obstacles, clearly visible in those sediments, are exactly as would be expected from viscous water - a slush of water and sediment akin to a mud flow - and are different than would be expected if wind or other forces had been at work, the researchers said. The gullies, each one about one-quarter of a mile long, also have new delta-like drainage fingers splaying from their bases.
The discovery is an exciting one for scientists who hunt for extraterrestrial life. If the finding is confirmed, they say, all the ingredients favorable for life on Mars are in place: liquid water and a stable heat source.
In all of its Mars exploration missions, NASA has pursued a "follow the water" strategy to determine if the planet once contained life or could support it now.
Spacecraft missions previously detected water in the forms of vapor and ice on the planet, and scientists say ancient Mars was once awash with pools of water.
Some researchers, however, were skeptical that liquid water was responsible for the surface feature changes seen by the spacecraft. They said sand or dust can flow like a liquid and produce similar results.
"Nothing in the images, no matter how cool they are, proves that the flows were wet, or that they were anything more exciting than avalanches of sand and dust," Allan Treiman, a geologist at the Lunar and Planetary Institute in Houston, said in an e-mail.
The findings will appear in the journal Science.
Information from the Associated Press and Washington Post was used in this report.