That, in any case, is what the prevailing thinking has been. Now, however, it appears that thinking may be wrong. Last week NASA released a flurry of new images from the Mars Global Surveyor spacecraft that suggest that even today, water may be flowing up from the Martian innards and streaming onto the Martian surface--dramatically increasing the likelihood that at least part of the planet is biologically alive. "If these results prove true," says Ed Weiler, associate administrator of NASA's Office of Space Science, "[they have] profound implications for the possibility of life."
Finding liquid water on Mars' surface has never been easy--mostly because it simply can't exist there. The modern-day Martian atmosphere has barely 1% the density of Earth's, and the planet's average temperature hovers around a paralyzing -67[degrees]F. In an environment as harsh as this, any water that did appear would either vaporize into space or simply flash-freeze where it stood. What scientists studying Martian history have always looked for instead are clues that the planet's ancient water left behind--tracks where vanished rivers once flowed, basins where vanished seas once stood.
The 65,000 or so images the Surveyor orbiter has beamed home in the nearly three years it has been circling Mars are full of this kind of expected hydro-scarring. But a handful of the pictures took scientists by surprise. In general, the older a Martian formation is, the more likely it is to have been distorted over the eons--smoothed by the planet's periodic windstorms or gouged by the occasional incoming meteor. A few of the newly discovered water channels, however, look as fresh as the day they were formed, leading astonished researchers to conclude that that day may have been remarkably close to the present one. Says Weiler: "The water could have flowed perhaps a million years ago, perhaps 10,000 [years ago], perhaps yesterday."
If the pristine nature of the formations was unexpected, their unlikely location was even more so. Planetologists have long assumed that if underground water was going to bubble up on Mars, it would have to be somewhere in the comparatively balmy equatorial zones, where temperatures at high noon in midsummer may approach a shirtsleeves 68[degrees]F. Almost all the new channels, however, were spotted at the planet's relative extremes--north of 30[degrees] north latitude and south of 30[degrees] south--and all were carved on the cold, shaded sides of slopes.
Paradoxically, this finding may increase the chances that the gullies are water related. Any water that appeared on the sunny sides of hills would be likely to evaporate almost instantly. Moisture that seeped out in shadow would form a temporary ice rind that would last only until the pressure of upwelling water behind it caused it to burst. When it did, there would be a sudden downward gush that would leave precisely the kind of clean-cut channel Surveyor spotted. If features like these were discovered on Earth, says Michael Malin, principal investigator for the Surveyor's camera system, "there would be no question water was associated with [them]."
For a beleaguered NASA, the new findings couldn't have come at a better time. After the recent spectacular flops of two unmanned Mars probes, the agency's entire planetary-exploration program came under fire. The possibility of a wet Mars, however, suggests that not only might the planet be home to indigenous life, it could also more easily support human life. Visiting astronauts would need water for a variety of purposes--including manufacturing air and perhaps even rocket fuel. Pumping up what's on site rather than hauling supplies from home could dramatically slash the cost of a mission. All this, NASA hopes, will increase the odds that a hitherto reluctant Congress will green-light future Mars missions--both unmanned and manned. "In exploring Mars," says NASA program scientist Jim Garvin, "we have always used a follow-the-water approach." That approach, it appears, may be starting to pay off.