NASA: Mars' missing water may flow under snow

Water flowed across Mars in recent times and could be flowing today, one of NASA's principal Mars investigators said yesterday.

Arizona State University geologist Phil Christensen, using detailed photos from the Mars Odyssey spacecraft, also may have unraveled one of the planet's biggest mysteries: how water could have carved fresh gullies without vanishing into the atmosphere or freezing in temperatures that reach 150 degrees below zero.

His solution: Blankets of snow that can last tens of thousands of years and likely persist today insulate water only a few inches underground. Those snow remnants, Christensen said, could offer a "wonderful abode for life" and should be a target for the National Aeronautics and Space Administration's quest to learn whether life could have existed on Mars.

"If you were to land on (a snow remnant) and take out a shovel, you would be shoveling snow," Christensen said at NASA headquarters. "If life ever existed on Mars, I can't think of a more exciting place to look."

After decades of disheartening findings suggesting that any water would exist miles below the surface, yesterday's announcement resurrects hope that Mars is not a barren, lifeless orb, but may contain refuges for life near the surface accessible to robot explorers.

"The depressing thing was we were talking about drilling a kilometer or two which is very expensive and difficult," said Lynn Rothschild, a biologist and expert on extreme life forms at NASA Ames Research Center at Moffett Field, Calif. "The real key is finding liquid water."

On Earth, primitive life forms such as "extremophile" bacteria have been discovered in acid baths, deep ocean trenches and under Antarctic ice. In the Sierra Nevada, fuchsia-colored algae can flourish in harsh mountain environments, creating "watermelon snow" in winter and then going dormant in summer, Rothschild said.

There is no guarantee that life exists on Mars, but astrobiologists note that the planet appears to offer the three ingredients required: a heat source (the sun and geothermal energy), organic chemicals and water. "You've expanded the envelope of places on Mars life could conceivably be alive even today," Rothschild said.

When photos of the mysterious gullies first were splashed onto front pages in 2000, they overjoyed scientists who'd sought evidence that water to support current or past life existed on Mars. The implications of the images, taken by the Mars Global Surveyor, were considered so important that NASA briefed the White House before releasing them.

A news leak set off a media frenzy. But ecstatic scientists soon grew frustrated. They could not explain how water could have flowed on the parched, frozen world.

Theories on what carved the deep, miles-long gullies abounded: leaking underground aquifers inexplicably perched high within crater rims; pressurized geysers that somehow didn't freeze; high-pressure bursts of carbon-dioxide vapors strong enough to carve rock; invisible volcanic heat sources below the surface.

Scientists knew the gullies were fresh because they were not pockmarked with craters like much of the battered planet. Age estimates range from 1 million years — almost yesterday in geologic time — to literally yesterday.

Creative as they were, none of the theories really fit. "They were getting pretty exotic and stretching a bit," said David Senske, a Mars geologist at the Jet Propulsion Laboratory in Pasadena, Calif. Most perplexing of all, the gullies were on the coldest, shadiest sides of craters, not the warmer sides, where melting water might be expected. Any water would freeze instantly, if it did not evaporate into the thin atmosphere.

Christensen's snow theory explains why the features are on the colder side: That's where the most snow would accumulate. His findings are to be published today in the journal Nature.

Like many Mars scientists, he believes snow has come and gone repeatedly on the planet, at times blanketing the midlatitudes where the gullies have been found and at other times leaving only ice caps at the frozen poles. The midlatitude regions of Mars are thought to alternate between very, very cold and just plain cold on a 100,000-year cycle related to how the planet tilts toward the sun.

Models show that Martian ground probably is frozen solid for more than half a mile down because of the low temperatures. But when snow falls in the midlatitudes, it acts as insulation or a "miniature greenhouse" and keeps the ground warm enough a few inches below the surface to allow water to exist, Christensen said. This meltwater then carves the gullies as it runs down slope, he said.