Smoking Craters: Home to Martian Life?
(Click for larger image.) Olympus Mons, the largest volcano in the solar system.
Mars may be smaller than Earth, but it’s still huge to a roving spacecraft that can cover only 100 meters a day. For that reason, Mars mission planners must go to great lengths to find landing sites that might still carry evidence that life once existed on Mars.
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.
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?
Like much of astrobiology, the study was a bit of a shot in the dark, Rathbun says. "A lot of efforts were incredibly theoretical, and ours was certainly one of those. But there hadn’t been any strict physical modeling of what water would do in the temperatures available in an impact crater, just qualitative work."
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.
Whether life could begin quickly enough to form in that transient lake, Rathbun admits, is "an open question." Although she says biologists have not provided "any hard and fast numbers" for how rapidly life could start, "a lot of biologists believe life would have emerged quickly" in the right circumstances.
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."
(Click for larger image.) Gusev crater, located at 14.6°S, 184.6°W, appears to be the site of an ancient lakebed, according to some scientists who believe that Mars was once warmer and had liquid surface water.
In addition, she says the very warmth that makes craters such alluring targets may backfire. "Large impact events have a tendency to sterilize the surrounding environment, leaving the area initially devoid of life. However, with time life may migrate to such areas through warm water being circulated through the extensive fracture systems generated by the impact."
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."
Similar speculation also indicates the type of life that may have lived in Martian hydrothermal systems. Rathbun and Squyres expect to see evidence of organisms akin to those found in deep-sea vents and geysers on Earth. These members of the kingdom Archaea live in anaerobic (oxygen-free), high-temperature conditions; some metabolize rocks for energy and can live without sunlight.
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.
This picture illustrates the high permeability at the rim of a buried crater on Mars.
Perhaps the most important limitation of any Mars modeling effort is the reliance on estimates rather than data for key parameters. "Normally on Earth you have far more information about what you’re trying to model," says Horton Newsom, a solar-system geologist from the University of New Mexico who has been speculating about hydrothermal systems on Mars for 20 years. "It’s a much more difficult job to try to model on Mars, where you have no geological constraints."
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.
Short of actual exploration, perhaps the best way to verify the results of computer modeling, Rathbun says, would be to investigate historic craters on Earth, and check whether their conditions match those that the computer model predicts.