Mars, the only planet to which a manned flight is presently envisioned, is a red, frigid wasteland, similar to some of our
stony deserts (for example the Strzelecki Desert in Australia). Estimates of the amount of water outgassed from Mars, based on the composition of the atmosphere, range from 6 to 150 m, but numerous erosional and depositional landscapes and several indicators of ground ice suggest that at least 500 m of water have outgassed (Carr, 1987). Some possible sources of the surface runoff include, for example, volcanic interactions with ground ice, geothermal melting of ground ice, eruption of water under pressure from confined aquifers or cometary impacts. Many valley networks resemble terrestrial drainage patterns formed by slow erosion rather than surface runoff. It implies significant groundwater resources, estimated between 1.2 x 107and 6 x 107 km3 (Risner, 1989). At the surface, some 2.3 x 106 to 9 x 106 km3of water ice can be found at the poles, under the layer of solid
carbon dioxide, which sublimates at summer. The carbon dioxide atmosphere, which Mars is losing at the rate of 1-2 kg s-1 contains 10 μm of water, close to saturation at night temperatures (200 K). Hydrological cycle models indicate that Mars has an active but nearly static hydrological
cycle, dominated by water
recharge in the ice-covered poles. Groundwater flows play a major role in this system. Significant progress in understanding of these phenomena is expected through the Martian Surface and Atmosphere Through Time (MSATT) project (NASA, 1989).
In June of 2000, researchers at Malin Space Science Systems released pictures from NASA's Mars Global Surveyor showing freshly carved gullies that
appeared to be created by recently flowing water in several dozen locations. But the Malin scientists admitted their conclusion presented a perplexing problem: Logic dictated the freezing temperatures at Mars should have
prevented what they saw from happening.
In December of 2000, images from Mars Global Surveyor showed layered features that suggested ancient lakes. The layering had been seen by the Mariner missions of the 1970s, but
never in such detail. Scientists said the discovery would be fascinating if it held up, but they admitted it would be heavily debated.
In 2001, an extensive analysis of Mars data led a team of geologists to conclude that
a vast, ancient reservoir of water in one part of Mars had sculpted huge gorges and left water trapped in numerous subsurface reservoirs. That adds to the case for ancient water and helps explain where some of it might have
And fresh analysis in late 2002 of dark streaks spotted racing down canyon walls, first noted in the Viking era, suggested to other researchers that water is still running on Mars, at least in brief spurts.
European Space Agency scientists put out one of the first images from their new Mars Express orbiter. It provided a high-resolution 3D view of a channel they think was carved by a river long ago. At the bottom of the channel is
a dark region, not a shadow but some sort of deposit that's been laid down.
"There is no other phenomena that we know of that would produce these effects," said Gerhard Neukum of Germany's Free University of
Berlin. "It is fair to say we are sure this is evidence of once-flowing water on the surface of Mars."
Other studies, however, have held that if water sculpted Mars, it probably did so in brief, hellish periods
- not the sort of conditions prone to fostering the development and sustenance of life. Supporting this less glamorous scenario, research found a relative dearth of limestone structures called carbonates that should have been
left behind if there were ever serious lakes or oceans.
Craddock said the lack of carbonates might be easily explained. They're sometimes hard to detect on Earth, he said, so scientists might simply not be seeing them on
Mars when looking down from orbiting spacecraft. Also, Mars' thin atmosphere lets in more ultraviolet radiation, which is known to destroy carbonates.
If scientists can find sites that once held water - ideally places
that were recently wet - then those might be good locations to send new missions designed to search for evidence of past or present life, such as fossils or chemical signatures of biological activity.
Water would also be
useful to future manned missions, both for drinking and to generate hydrogen, which could be used for fuel on a return flight.
With all the excitement of late, and some media hype, it might seem almost inevitable that
Mars was once a life-bearing planet. Most mainstream scientists are very cautious on this point, however. Some are very doubtful.
"I think it's very unlikely that Mars ever had life," Craddock said. "I
would argue that the evidence for Bigfoot or UFOs is better than for life on Mars."
He nonetheless supports planned missions that would scour the red planet for signs of biology, in part because even a failed
search, via a bit of twisted logic, could along the way answer the huge question of how life began on Earth - another gaping hole in scientific knowledge.
"If we don't find life, we might find the prebiotic
chemistry that went into life on Earth," Craddock explained. "That's more important, because we know what life is, but we don't know about the precursor chemistry that went it."