NASA scientists have found evidence that liquid water is flowing on Mars.

Images taken by NASA's Mars Global Surveyor point to the presence of liquid water on the Martian surface, a tantalizing perspective for Martian life. Some 240,000 images were taken by the long-orbiting Mars Global Surveyor spacecraft. The images depict changes in the walls of two Martian craters that may have been caused by water flows in the past seven years.

Compared images of the Martian surface taken seven years apart found the existence of 20 newly formed craters caused by impact from space debris as well as the evidence suggesting liquid water flowing down crater walls. Debris deposits suggest that sediments were carried downhill by occasional liquid flows. "The shapes of these deposits are what you would expect to see if the material were carried by flowing water," said Michael Malin, president and chief scientist at Malin Space Science Systems. "These observations give the strongest evidence to date that water still flows occasionally on the surface of Mars," said Michael Meyer, lead scientist for NASA's Mars Exploration Program.

But liquid water cannot remain long on the planet's frigid, nearly airless surface because it would rapidly freeze or evaporate. Liquid water may seep out in some places from time to time. The two sites are inside craters in the Terra Sirenum and the Centauri Montes regions of southern Mars. "They have fingerlike branches at the downhill end, and [the deposits] easily diverted around small obstacles."

"It could be acidic water, it could be briny water, it could carry lots of sediment or be slushy, but [it appears that] water is involved," said Kenneth Edgett of Malin Space Science Systems.

Martian surface features, like ravines, have long been studied and showed that liquid water flowed on ancient Mars. Ice is known to exist in the planet's polar regions, and water vapor has been detected in the Martian atmosphere, but liquid water has not been detected. "Ten years ago Mars scientists were talking about water [on Mars] billions of years ago," said Philip Christensen, a professor at Arizona State University in Tempe.

"Five years ago they were talking about water [on Mars] millions of years ago. Today, we can honestly talk about liquid water on Mars today-that revolution in our thinking has changed how we view Mars."

But where does the Martian water come from? Is melting possible on a planet with mean surface temperatures of - 22 to -124 degrees Fahrenheit (- 5 to -87 degrees Celsius)? Or if water flows from underground, how can it stay liquid at such low temperatures?

Martian water could be highly acidic, which could decrease its freezing point to a temperature below those found on the red planet. Even so, it would likely flow only short distances before freezing in the planet's brutal cold.

Liquid water is essential in the search for extraterrestrial life. If life ever existed on Mars, liquid water is an important part of the equation. Liquid water on Mars would open the possibility for subterranean environments that might harbor living organisms. Some scientists suspect Mars once sheltered primitive, bacteria-like organisms. Yet scientists can't yet be sure that they've seen what they think they've seen and stronger confirmation is needed. The researchers first found the gullies in 2000, but this is the first time they have revealed the presence of newly deposited material seemingly carried by liquid water.

Photo credit: NASA