New photographs from space suggest that water occasionally flows on the frigid surface of Mars, raising the tantalizing possibility that the Red Planet is hospitable to life, scientists reported Wednesday. The new images, taken by NASA's Mars Global Surveyor before it lost contact with Earth, do not actually show flowing water. Rather, they show changes in craters that provide the strongest evidence yet that water coursed 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-based Malin Space Science Systems, which operates a camera on the Global Surveyor. The news excited 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. Scientists believe ancient Mars was awash with pools of water. And at present-day Mars' north pole, researchers have spotted evidence of water ice. But they have yet to actually see water in liquid form. "This underscores the importance of searching for life on Mars, either present or past," said Bruce Jakosky, an astrobiologist at the University of Colorado at Boulder, who had no role in the study. "It's one more reason to think that life could be there." Some researchers were skeptical that liquid water was responsible for the surface feature changes seen by the spacecraft. They said other materials such as 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 Friday's issue of the journal Science. The Global Surveyor previously spotted tens of thousands of gullies that scientists believed were geologically young and carved by fast-moving water coursing down cliffs and steep crater walls. Scientists decided to retake photos of thousands of gullies in a search for evidence of recent water activity. Two craters in the southern hemisphere that were originally photographed in 1999 and 2001 were examined again in 2004 and 2005, and the images yielded changes consistent with water flowing down the crater walls, according to the study. Scientists said five to 10 pools of water rushed down the craters in each case. In both craters, scientists found bright, light-colored deposits several hundred yards long in gullies that were not present in the original photos. They concluded that the deposits - possibly mud, salt or frost - were left there when water recently cascaded through. Edgett said a combination of factors, including the shape and color of the deposits, led the team to believe it was recent water action and not dust that slipped down the slope. He said dust would leave dark deposits. Water cannot remain a liquid on Mars for long because of subzero surface temperatures and low atmospheric pressure that would turn water into ice or gas. But scientists theorize that liquid water is being shot up to the surface from an underground source, like geysers. The Global Surveyor, managed by NASA's Jet Propulsion Laboratory, abruptly lost radio contact with Earth last month. Several attempts to locate the spacecraft, which has mapped Mars since 1996, have failed and scientists fear the mission was likely at an end. Mars formed more than 4.5 billion years ago and scientists generally believe it went through an early wet and warm era that ended after 1.5 billion to 2.5 billion years, leaving the planet extremely dry and cold. NASA's durable twin rovers have sent scientists strong evidence that the planet once had liquid water at or near the surface, based on observations of alterations in ancient rocks. The images from the Global Surveyor suggest the process is still occurring. "We're now realizing Mars is more active than we previously thought and that the mid-latitude section seems to be where all the action is," said Arizona State University scientist Phil Christensen, who was not part of the current research.