Ending a debate that began more than a decade ago, NASA announced today that water does indeed flow on Mars. The agency is calling the discovery one of its most significant to date.

"We had evidence that water had flowed across the surface of Mars billions of years ago," said Ken Edgett of the Malin Space Sciences System. "But we can say that liquid water is present on Mars right now."

Scrutinizing before-and-after photographs obtained by the Mars Global Surveyor (see below), NASA identified evidence of new deposits in two of Mars' tens of thousands of gullies--formations that scientists say have formed over the last seven years and could have only been formed by a flowing liquid. Their morphology, color and location led to the conclusions NASA revealed today.

While scientists believe water is the best explanation for the new findings in the Red Planet's southern hemisphere, they noted that other instruments already exploring the planet, such as the Mars Rover, might be needed to confirm the liquid water discovery.

"These fresh deposits suggest that at some places and times on present-day Mars, liquid water is emerging from beneath the ground and briefly flowing down the slopes," said Michael Malin of Malin Space Science Systems. "This possibility raises questions about how the water would stay melted below ground, how widespread it might be and whether there's a below-ground wet habitat conducive to life. Future missions may provide the answers."

Perhaps the most intriguing part of the new photographs' findings is the possibility that the Red Planet could create an environment for sustainable life, but more direct questions related to today's announcement abound.

"The source of this water is a wonderful scientific debate," said Arizona State University's Philip Christensen. Possible sources include crown water, snow melt or ice, but other instruments already exploring the planet, such as the Mars Rover, might be necessary to confirm the finding.

The Mars Global Surveyor was launched on Nov. 7, 1996, making it the longest-running craft to explore Mars. In recent months, contact between the surveyor and NASA's Jet Propulsion Agency has been sketchy. Last week, the agency announced that the mission might be coming to an end.

In addition to revealing evidence that liquid water flows on Mars, the agency unveiled images from the Mars Global Surveyor that are helping scientists understand the rate of steroid impact on the planet's surface.

Between 1999 and 2006, 20 new craters—ranging in diameter from seven to 468 feet—formed over 30 percent of Mars' surface. Scientists say the data support theories that terrain with fewer impact craters remains relatively young, and will further enable researchers to determine the age of the planet's surfaces. --Emily Masamitsu