updated 1/5/2004 6:07:24 PM ET 2004-01-05T23:07:24

As NASA prepares to set twin robots loose on the Martian surface and makes plans to send another in 2007, the agency's long term goal is clear: Determine whether the red planet does or ever did harbor life.

But the current search for life is necessarily limited to life as we know it, organisms dependent on liquid water. A reader recently suggested that "we as humans are arrogant, simply believing that any other form of life will be just like us."

Researchers devoted to the search for ET have a similar view.

"Scientists’ approach to finding life is very Earth-centric," says Kenneth Nealson, a geobiologist at the University of Southern California.

The problem is, beyond carbon-based, water-dependent life, there isn't much intelligence to go on when designing a multi-million mission to Mars.

"Based on what we know about life on Earth, we set the limits for where we might look on other planets," Nealson said. Within that framework, however, there are extreme cases of life on Earth that suggest the range of places to look on frigid Mars.

Extreme beings
Nealson and his colleagues recently found the most extreme sort of organism known, in terms of the temperatures it can survive.

Corien Bakersman, a postdoctoral student in Nealson’s lab, discovered bacteria named psychrobacter cryopegella. It grows and reproduces in conditions as chilly as 14 degrees Fahrenheit (-10 degrees Celsius).

The discovery, in a salty liquid lake under the permafrost of Siberia, was reported in a recent issue of the journal Astrobiology.

The bacteria can continue to metabolize even at temperatures down to -4 degrees Fahrenheit (-20 Celsius), though they understandably stop reproducing at that extreme.

"This organism can exist at colder temperatures than any previously discovered," Nealson said.

Nealson's team figures that at certain times in the history of Mars, the tilting of its pole to a more oblique angle would have caused the north polar cap to warm to -4 Fahrenheit or higher as it received more sunlight.

"If the ice at the polar caps warmed to liquid water, organisms like cryopegella could have awakened and repaired any damage that might have occurred to their various cellular components," Nealson said. "Then, as the obliquity changed a few million years later and the planet got colder and colder, these organisms would have been the last survivors."

That does not mean there are necessarily dormant microbes within the ice caps of Mars. But it does suggest a broader range of potential cradles for life, places Nealson figures should be considered.

Other researchers agree, and a host of so-called "extremophile" discoveries on Earth in recent years indicate the polar regions of Mars might be prime hunting grounds. As on Earth, organisms there might be slathered in natural antifreeze or be able to go dormant for tens of thousands of years, waiting for a brief thaw, their moment in the Sun.

The two landers en route now will not explore the polar regions. NASA's Mars Phoenix lander, slated for launch in 2007, is intended to investigate the north polar area.

Life as we don't know it
Meanwhile, scientists recognize that there could indeed be life elsewhere in the universe that does not require water. And some astrobiologists are trying to explore the possibilities. But it is a tough problem to approach. In looking for "life as we don't know it," it's hard to even imagine what to expect.

Nealson is at the forefront of a nascent effort to at least contemplate what might be out there.

"It'll have shape and composition. It'll have structure," Nealson said at a gathering of astrobiologists last year. These characteristics are necessary to convert one type of energy to another, a process he considers fundamental to life.

There are other basics, he says: "Life replicates, and life evolves. And because it consumes energy, it produces waste products. And it has some particular activities that seem to be universal, one of which is movement."

Life might or might not exist on Mars. If there are critters there, they might or might not be like bacteria on Earth. In laboratory conditions, scientists in 2001 were able to get one-celled organisms to incorporate an amino acid -- a fundamental building block of life -- that no other known life uses. The discovery borders on the creation of artificial life, experts said. It also suggests that ET might operate by entirely different rules than those we're used to.

If life on Mars is fundamentally different from what scientists understand life to be, then current spacecraft and others in the works may well not recognize what's right under their mechanical noses.

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