Two U.S. spacecraft orbiting Mars have found signs that liquid water can survive on the Red Planet, despite its freezing climate and thin atmosphere. The clues pointing to this are recently discovered gullies apparently eroded by the water.
The U.S. Mars Global Surveyor satellite discovered the gullies three years ago high on Martian crater walls and cliff sides at the planet's middle latitudes. From the high altitude pictures, they look like twigs with several branches. They suggest that water carved them in recent times, but scientists have questioned how this could occur in Mars' frigid climate.
Some have suggested seeping groundwater made the channels. Others theorize they were made by mudflows caused by collapsing permafrost. Arizona State University researcher Philip Christensen says the latest photos from the Surveyor and the newer Mars Odyssey satellite indicate that snowmelt is the answer. "Mars seems to have quite a bit of snow," he says. "I think the young gullies of Mars were actually caused by melting of extensive snow packs."
Mr. Christensen's theory suggests the water flows beneath the snow, where it is sheltered from rapid evaporation in the thin Martian atmosphere.
It explains why gullies form in unusual places on Mars, at the crests of ridges and the tops of isolated hills where underground water is unlikely to reach. The high slopes face the Martian poles, the coldest areas on these formations. Mr. Christensen says snow accumulates on these slopes during times when the climate is coldest and melts during warmer ones, leaving exposed, dry gullies.
He points out that the potential for snowmelt has implications for the possibility of life on Mars. "Snow provides a wonderful abode for life. The snow itself acts as a miniature greenhouse. The sunlight can penetrate through the snow. That warms the snow and actually melts it," says Mr. Christensen. "Imagine now an environment where I have sunlight, I have temperatures above freezing, and I have liquid water. All of those make for a potential abode for life."
On Earth, there are forms of algae that survive in polar and mountain snow and turn it the color of pink watermelon. In fact, scientists have found hardy organisms surviving at far more inhospitable temperatures. At a Washington news conference, U.S. space agency biologist Lynn Rothschild told Dr. Christensen she hopes he is right. "Because what you have done is expanded the envelope of places on Mars that could conceivably be habitable, even today," says Ms. Rothschild. "Over the last 20 years or so, biologists have been expanding the envelope of what we know is possible for life on the Earth because we find it from boiling water and boiling acid baths all the way to under ice in the Antarctic."
Several geologists not involved with the research believe the snow melt explanation is plausible, although some are less convinced than others. Among the more skeptical is Brown University geologist John Mustard, who does not reject the theory, but seeks direct evidence. "For example, does it explain all of the observations that have been made of the gullies to date? And why do we see these only on steep slopes? Wouldn't shallow slopes also have been conducive to the flowing of water? Is it the final word? Well, I don't think so," says Mr. Mustard. "But I think it opens up intriguing new areas that we can take planetary science and the exploration of Mars."
The U.S. orbiters that provided the images for these findings are studying the Martian surface for signs of water and the distribution of minerals and chemicals. They are helping the U.S. space agency NASA locate the most interesting areas for future exploration by roving robot laboratories. Two such landing craft are to be launched to Mars in May and June.