December 8, 2006
Before-and-After Photos Hint at Occasional Water Flows on Mars
By Warren E. Leary
Pictures of Martian gullies taken several years apart strongly suggest that water still flows at least occasionally on the surface of the planet, scientists have announced.
If water is present, that would raise the possibility of microbial life: with water and some form of steady heat, bacteria can grow even in hostile environments.
The new images were taken by NASA's Mars Global Surveyor spacecraft, which fell silent last month after almost 10 years of observing the planet from orbit. They show light-colored deposits in two gullies within crater walls that were not there in pictures taken in 1999.
The before-and-after pictures show what appear to be deposits of materials left by bursts of water flowing down the sides of the gullies, said Michael Malin, chief scientist for the spacecraft's camera system.
"The shapes of these deposits are what you would expect to see if the material were carried by flowing water," Malin said during a televised news conference held here at NASA headquarters. The deposits appeared to have been left by a liquid mixed with dirt or other material that flowed down a slope for hundreds of yards before dispersing, he said.
"We think water was the fluidizing material here," said Malin, president of Malin Space Science Systems of San Diego, the operator of the spacecraft's camera.
Even though the evidence is circumstantial, the possibility of liquid water on Mars is exciting, Malin said. It raises the questions of where the water comes from and whether it could be used as a resource for future explorers. For instance, water could be broken down into hydrogen and oxygen, which can be used as rocket fuel, or the oxygen could be used for astronauts to breathe.
Kenneth Edgett, another scientist with Malin's company, said that whenever the surface of Mars was disturbed by spacecraft on the ground or by strikes from space rocks, it appeared as a dark color. The light tone of the material in the flow patterns suggests that it came from minerals or other chemicals that were in the water, he said.
The researchers said they believed that underground water slowly made its way to the surface, perhaps accumulating under a crust of ice until being released in a sudden burst by a quake or a meteor strike.
When freed, Edgett said, the water shoots out as if from a squirt gun and immediately begins evaporating or boiling away as it flows downhill.
The flow appears to be from a thick, sediment-laden material that moves like a mudslide that occasionally courses around obstacles and diverts into finger-shaped marks as it disperses at the bottom. Water in the flow would evaporate or turn into ice crystals within hours or days, leaving sediment along its path.
Edgett said that the photographed flow patterns traveled down slopes for 450 to 550 meters, or 1,500 to 1,800 feet, and each probably carried the equivalent of "5 to 10 swimming pools of water."
In a paper to be published Friday in the journal Science, the researchers said pictures of the same areas taken in 2004 and 2005 clearly showed changes that occurred since initial images taken in 1999. While some scientists have suggested that past flow evidence seen in craters might be from landslides or rocks falling down slopes, geological properties of the crater areas strongly suggest that the changes were caused by a fluid, probably water, the researchers said.
Philip Christensen, a professor of geological science at Arizona State University who was not involved with the research, said he had been skeptical of previous claims about signs of flowing water on present-day Mars but found the new evidence "reasonable and plausible."
"I'm about 80 percent convinced that this is evidence of water flow happening now, and I admit I have been a skeptic," he said in an interview. "We're now realizing Mars is more active than we previously thought."
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