Scientists say U.S. satellite images of Mars provide strong evidence that liquid water still flows on the red planet, suggesting an environment that could be hospitable to life. But new pictures also show other signs that Mars could be a dangerous place for life, specifically visiting astronauts. VOA's David McAlary has details from Washington.

Picture showing signs of flowing water on Mars

The images come from a spacecraft that orbited Mars for a decade, the Mars Global Surveyor, which the U.S. space agency recently lost contact with. Researchers compared some its final pictures of the Martian surface with ones it took seven years ago and have documented the formation of new craters and possible evidence for liquid water flowing in gullies.

Previous findings have suggested that water flowed on Mars millions of years ago, but the new images published in the journal Science indicate it still runs on what was thought to be only a dusty, barren planet.

"We've had this story of ancient water on Mars. Today, we are talking about liquid water that is present on Mars right now," said Kenneth Edgett of Malin Space Systems, one of the scientists who revealed the surprise findings at a NASA news briefing in Washington. "We have been able to image gullies repeatedly, the same ones over and over again, and in doing that, we found something I really didn't expect. That is, changes that indicate that material had actually flowed through a gully and made a deposit that would indicate by its shape that a liquid was involved."

Edgett's partner, Michael Malin, says they believe the liquid is water from underground judging by the way it flowed around obstacles and spread into numerous branches. Furthermore, he says, it is lighter in tone than the material around it.

"These things being bright is extremely unusual for Mars. Wherever Mars' surface is disturbed, it is always dark. Yet this disturbance is bright. That means there is something associated with this flow that caused it to be brighter. So I have advocated that this is frost that is forming as that water comes out, meets the cold air, and reprecipitates," he said.

A scientist not involved in the research, Philip Christensen of Arizona State University, says the evidence is compelling, but must be confirmed by other research. He says if the fluid is water, the finding overturns a long held scientific notion that Mars became dry after an early warm and wet period.

"Five years ago these gentlemen were talking about water millions of years ago, and I think today we can honestly talk about liquid water on the surface of Mars today. That revolution in our thinking truly has changed how we view Mars and I think how we should think about exploring Mars," he said.

But sending astronauts to explore Mars someday might be threatening because of the apparent continual bombardment by space rocks. Michael Malin says he found new craters in the Mars Global Surveyor pictures and calculated that rocks crashing from space form 12 of them per year.

"Is this a hazard for astronauts that we have to consider? The answer is probably yes. It's probably a low hazard, but it is one we need to think about in terms of these objects hitting Mars at a fairly substantial rate," he said.