By Rick Weiss
Washington Post Staff Writer
Thursday, December 7, 2006
Photographs of the Martian surface taken by an orbiting spacecraft have revealed powerful evidence that liquid water occasionally flows on the Red Planet's surface, an unexpected finding that suddenly increases the odds that the planet may harbor some kind of life.
Scientists have long known that water exists on Mars as polar ice and atmospheric vapor. But a core requirement for life is water in liquid form, a commodity that has been seemingly absent on that cold and ruddy planet.
Thousands of dry gullies scar the face of Mars, indicating that surface water once flowed there. But until now it has been impossible to tell whether those gullies last contained water millions of years ago or much more recently -- periodically fed, perhaps, by aquifers that might persist underground to this day.
Now a comparison of photographs taken several years apart by NASA's Mars Global Surveyor has found that two gullies, at least, apparently experienced flash floods between photo shoots.
"Water seems to have flowed on the surface of today's Mars," said Michael Meyer, lead scientist for NASA's Mars Exploration Program, speaking at a news conference yesterday. "The big question is how does it happen, and does it point to a habitat for life?"
The global surveyor went into orbit around Mars in the fall of 1997 and, during its longer-than-expected nine-year life, mapped the planet's surface with more than 240,000 images before going dark last month.
In 2000, scientists from Malin Space Science Systems in San Diego, which developed and operated the camera equipment on the surveyor, reported the discovery of many gullies that they said clearly had been carved by water. The timing of that erosion remained uncertain until two of them -- photographed in 1999 and 2001 -- were re-shot in 2004 and 2005.
As described at yesterday's event and in an article appearing in tomorrow's issue of the journal Science, both looked radically different on the second pass. Most noticeable, each has a fresh coat of fine, pale sediment, which scientists say appears to be either water frost or salts left behind by briny water.
Flow patterns around rocks or other obstacles, clearly visible in those sediments, are exactly as would be expected from viscous water -- a slush of water and sediment akin to a mud flow-- and are different from what would be expected if wind or other forces had been at work, the researchers said. The gullies, each about a quarter-mile long, also have new delta-like drainage fingers splaying from their bases.
Malin scientist Kenneth Edgett said the team had calculated that each flow probably involved about as much water as would fill five to 10 swimming pools.
"If you were there . . . you'd probably want to get out of the way," Edgett said. All the more so, he said, because at the low atmospheric pressures found on Mars, much of that water would be bubbling and boiling -- even though it would not be hot.
"It would be like, 'Wow, there's this thing coming at me!' " Edgett said.
No one knows where the water may have come from, or how much of it could be feeding other gullies on the planet. One theory is that stores of water are being kept warm enough underground to remain in the liquid state, with some of it flowing slowly and steadily through cracks that open into the sides of craters. As water approaches the surface, the thinking goes, it freezes, forming an ice dam. But periodically those dams burst, sending a mini-flood foaming down a slope.
Water would be especially likely to remain in liquid form underground if it were salty or, to an even greater extent, if it were acidic from certain dissolved minerals -- factors that allow water to remain fluid at subfreezing temperatures.
Scientists said they may soon have a chance to test that proposition. That is because the Mars Reconnaissance Orbiter, which began its studies of the planet last month, has a high-resolution, near-infrared camera that can analyze the chemical composition of the new pale sediments.
The orbiter's first priority is to map sites near the icy north pole where NASA's Phoenix lander -- to be launched next August -- is scheduled to touch down in 2008. But after that, Meyer said, he suspects a look at the gullies will be a new high priority.
Other scientists said it would be important to gather more evidence to rule out alternative explanations for the gully changes, including flows of dry granular material or frozen carbon dioxide. But several agreed that they could think of no better explanation than water for what they were seeing.
"I think the evidence for this recent water is compelling," said Philip Christensen, a geoscientist at Arizona State University in Tempe who oversaw one of the global surveyor instruments and is involved in the Mars Exploration Rover missions. "I think today we can honestly talk about liquid water on Mars."
In their report in Science, the team members also described several new craters that appeared during the surveyor mission, again revealed by "before" and "after" photographs. By extrapolating from those data, they conclude that Mars is pocked by about a dozen new craters every year.
That is important, scientists said, because researchers have used the density of craters on planets to estimate the age of surface features by presuming a certain frequency of new impacts.
With the new hard data in hand, they were able to verify that the "crater clock" they have been using is essentially correct -- and that the number of meteorites falling from the Martian sky is not insignificant, said Michael C. Malin, president of Malin Space Science Systems.
"That raises a question: 'Is this a hazard for astronauts that we have to consider?' " Malin said. "The answer," he asserted, "is yes."