Smoking Craters: Home to Martian Life?
A key zone of speculation exists just beneath Mars' cold, dry, dusty and inhospitable surface - where two prerequisites for life, water and heat, may be found. Such heat may come from volcanism, and indeed Olympus Mons is the largest volcano in the solar system.
Asteroid impacts (most likely in the first half-billion years of the solar system but conceivably even today) are a second possibility. When a big piece of rock crashes into Mars at about 5 kilometers per second, could that liberate enough heat to melt underground ice, drive the circulation of liquid water, and perhaps allow the formation or survival of life?
Julie Rathbun, who now teaches astronomy and physics at the University of Redlands (Redlands, California), and Steven Squyres, a planetary scientist at Cornell University, decided to answer the question by modeling hydrothermal circulation - the flow of liquid water through geologic structures. "We were looking to see if a hydrothermal system would set up, and if so, what kind temperature it would establish, and for what period," says Rathbun.
The model indicated that lakes might have lingered for thousands of years after an impact, conceivably long enough for life to form. The lakes were much warmer than the planet as a whole. And they may have been deep enough to connect to aquifers - underground water bodies -- where microbial life may already have been living.
In the journal Icarus (June 2002), Rathbun and Squyres described two theoretical impact craters on Mars. In both cases, a lake formed from melted ice in the Martian permafrost and was soon covered by ice.
The smaller crater was 7 kilometers (4.3 miles) across, and the lake probably froze rather quickly. (Under current Martian conditions, any water at the surface will rapidly boil and freeze, then eventually sublimate into the atmosphere.) The larger crater, with more astrobiological interest, was 180 kilometers (112 miles) in diameter. Water in parts of that lake ranged from 50 degrees C (122 F) to 100 degrees C (212 F). Depending on assumptions used for geologic conditions, the lake may have persisted for 15,000 years.
Virginia Gulick, an astrobiologist with the SETI (Search for Extraterrestrial Intelligence) Institute, agrees that cmay be hospitable to life. "From what we know about life, life requires water, an energy source, and time. Hydrothermal systems can provide such an environment."
However, she notes that hydrothermal systems can also be powered by rising magma, volcanism and tectonic shifting. "It's not clear whether craters would be a better place to look, especially considering that hydrothermal systems powered by the intrusion of magma may last far longer - millions or hundreds of millions of years."
While the search for past life on Mars may seem a long shot, Gulick thinks recent biological discoveries indicate otherwise. "We know on Earth that microbial life inhabits environments formerly thought to be inhospitable, such as in the deep subsurface, in the extremely cold and dry Antarctic soils, rocks and ice-covered lakes, in deep ocean basins at mid-oceanic rift hydrothermal systems, and also in high altitude (20,000 foot) icy volcano lakes. Given that life is found in these extreme environments on Earth, it isn't such a far stretch to think that similar microbial life may have existed deep in the subsurface of Mars."
Rathbun agrees that the accuracy of estimates of conditions on Mars is only as good as the assumptions of Martian conditions on which they rest. All bets are off, for example, if any water is absent from the near-surface environment of Mars.
The lifetime of a lake, Newsom notes, "depends on the amount of heat, and permeability. But in geological material, permeability can vary over many orders of magnitude, and this has a major influence" on when the hydrothermal system will freeze up.
Nonetheless, even transient hydrothermal systems might be a smart place to look, Newsom says. "In a 150-kilometer crater, you could have hot rock and hot water around for thousands of years. Even if life did not originate there, the lake will draw in groundwater, so you have essentially a giant Petri dish that can culture and grow microorganisms that may have grown elsewhere."
Thus he, like Rathbun and Squyres, agree that large impact craters are promising sites for evidence of life. Indeed, Newsom says, two impact craters (Gusev and an unnamed, buried, 150-km crater rim) are top candidates for the Mars Exploration Rover, scheduled for launch in 2003.