On steep slopes in martian craters, recently melting snow may have created a system of gullies, says Philip Christensen, the principal investigator for the Mars Odyssey THEMIS camera system and a professor from Arizona State University. He says that the melted water collecting underneath these snow packs also could have created an ideal abode for life.
|This visible-light image, taken by the thermal emission imaging system on NASA's 2001 Mars Odyssey spacecraft, indicates that gullies on martian crater walls may be carved by liquid water melting from remnant snow packs. Credit: NASA/JPL/ASU
"I think we have discovered remnants of snow packs on Mars that in the recent past have melted," says Christensen. "I think if you were to land on one of those and stick a shovel in the ground, you'd be shoveling snow. And if life ever existed on Mars, I can't think of a more exciting place to possibly go and look."
Christensen examined martian images of gullies and what he termed "pasted-on materials." In his paper, published in the electronic February 13 issue of Nature, Christensen argues that that the pasted-on materials are remnants of a once very extensive layer of snow that covered the mid-latitudes of Mars.
"This snow draped the landscape, and as the climate warmed, the snows melted," says Christensen. The snow sheltered the water, which otherwise would have rapidly evaporated in the planet's thin atmosphere. "That melt water trickled through the snow and eroded the gullies underneath this overlying pack of snow."
Eroded gullies on crater walls and cliff sides were first observed in images taken by the Mars Global Surveyor in 2000. Most of the models to describe these formations had water coming out of the ground in aquifers and springs, or had ice-impregnated soils melting during warmer climatic periods and causing landslides.
In Christensen's model, however, the snow acts as a protective blanket that allows liquid water to collect a few centimeters or inches beneath the planet's surface. His model explains how gullies could form in unusual places such as the tops of isolated hills, on sand dunes, on the crests of ridges, and other places where it would be very difficult to have ground water creating the gullies.
"We've had, up until now, a number of different possible explanations for the gullies, and the fact is none of them have been very good," says Bruce Jakosky, a planetary scientist from the University of Colorado at Boulder. "I think (Christensen's theory) is the most consistent explanation with the observations so far. In the hierarchy of ideas that work and ideas that won't, this one is at the very top of the list."
|Polar regions show frost, often a mixture of dry ice (frozen carbon dioxide) Credit: NASA/JPL/Viking
In Christensen's model, the gullies formed about one hundred thousand years ago, when the tilt of the polar axis of Mars was higher than it is today. As the axis of Mars tilted, the poles became warmer while the mid-latitudes grew colder. The water ice in the warmer areas sublimed, or turned from a solid to a gas without going through a liquid stage. The wind carried this atmospheric moisture to the cold mid-latitudes, causing the moisture to condense into snow.
The axis of Mars slowly tilts over one hundred thousand-year cycles. As Mars tilts over, the poles warm and the mid-latitudes get colder, and then when the axis of Mars tilts back up again, the situation reverses itself, with the poles growing colder and the mid-latitudes growing warmer. During these cycles, water migrates between the polar regions and the mid-latitudes.
"To me, the exciting thing is it's an on-going process," says Christensen. "The snow will be back; these gullies will be rejuvenated and re-activated. What we see today are the fossil remnants of that snow, but it is by no means a dead, inactive process - it will occur again."
Today, the mid-latitude regions are too cold to allow ice to melt. But Jakosky says there still is a possibility for liquid water there, with partial melting forming thin films of water that might be adequate to support organisms.
"I think in terms of liquid water, Mars is a planet that's on the verge of being habitable," says Jakosky.
Jack Mustard, a planetary biologist from Brown University, notes that the only place we actually see water present on the surface of Mars is in the polar ice caps. While the Mars Odyssey found buried ice as far down towards the equator as 60 degrees, Mustard says we still have no direct evidence of ice or snow at the surface of the planet in those mid-latitude regions.
|This visible-light image, taken by NASA's Viking spacecraft, shows a close-up cliff Credit: NASA/JPL/Viking
Lynn Rothschild, a biologist with the NASA Ames Research Center, comments that biologists have been both excited and dismayed by the possibility of this buried mid-latitude water.
"We know that there are subsurface communities on the Earth, but the depressing thing is, we were talking about drilling maybe a kilometer or two below the surface," says Rothschild. "(Then the search for life on Mars) becomes very expensive and technologically difficult."
But Christensen's model suggests that liquid water could be within centimeters of the surface of Mars, negating the need for deep drilling.
Rothschild says that although Mars is very cold, there are organisms on Earth that can survive such temperatures.
"We have resting stages of organisms that can go down to the temperature of liquid nitrogen, say 200 degrees below freezing," says Rothschild. "We certainly know that warm-blooded organisms can live in very cold temperatures in the poles - penguins and polar bears and so on - but even things like microbes that don't regulate their own temperature can survive down to 20 degrees below freezing and still be active."
|Animation of Mars snow and water erosion remnants. Click image for larger picture. GIF Animation 91 kB. Also see image galleries Credit: NASA/JPL/ASU
Rothschild suggests that algae might be able to live in the patches of snow on Mars. Such algae blooms occur in places like the Arctic, the Sierra Nevada Mountains, and the Rocky Mountains. She says these red pigmented blooms become so dense in snow patches on Earth that they're sometimes called "watermelon snow."
"These organisms during much of the year are living in the soil in a resting stage, maybe like you'd see in Mars between the times that it snows, and they can survive long periods like that," says Rothschild.
"And then when the conditions are right, they're able to germinate. A mobile stage goes up towards the surface, and they are able to photosynthesize, complete their life cycle, and then go back into this spore form."
Even though the melting period on Mars is over, Christensen says that some patches of snow still remain in the mid-latitudes because of dirt and dust that settled on top of the patches. These dirt-covered snow patches occur in the colder hollows, on slopes facing towards the poles. If that hadn't occurred, says Christensen, then the melting would have continued and the snow would be completely gone.
Rothschild says these layers of dust and dirt on top of the snow would make it difficult to observe any life forms with orbiting cameras. To find life in such places, she says, we'll have to dig underneath the dirt layers.
"We've done a fair amount of work studying microbes that live under layers of sand or gravel, both in beaches and in Yellowstone National Park," says Rothschild. "In such places, you can't see any of the organisms underneath the layers of sand or gravel, but there's a very active photosynthetic community based on algae."
Mars Odyssey was launched from Cape Canaveral Air Force Station in April 2001 and arrived in Martian orbit in late October 2001. During the rest of the spacecraft's 917-day science mission, many of its priorities will highlight more of the current mysteries about Martian moisture.
In early 2004, two Mars Exploration Rovers will target what imagery indicates might have been ancient dry lake beds, as well as other geologically interesting sites. The European Space Agency, meanwhile, plans to launch a combined orbiter and lander mission in 2003. Its lander, named Beagle 2, will contain biological experiments designed to search for evidence of life on Mars.
The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Mars Odyssey mission for NASA's Office of Space Science in Washington, D.C. Investigators at Arizona State University in Tempe, the University of Arizona in Tucson and NASA's Johnson Space Center, Houston, operate the science instruments. Additional science partners are located at the Russian Aviation and Space Agency and at Los Alamos National Laboratories, New Mexico. Lockheed Martin Astronautics, Denver, the prime contractor for the project, developed and built the orbiter. Mission operations are conducted jointly from Lockheed Martin and from JPL.