A few years ago, when an academic publisher approached Dirk Schulze-Makuch about writing a book on the search for extraterrestrial life, the astrobiologist couldn't resist.

"You're not often getting asked to write a book about life in the universe," he recalls. "It was just too tempting."

Life in the Universe: Expectations and Constraints was published shortly before Schulze-Makuch joined Washington State University's Department of Geology in 2005. Coauthored by Louis Irwin of the University of Texas-El Paso, the book takes a close look at what's really necessary for life—not life as we know it here, but life as it might be on other worlds.

"We're looking at what is physically and chemically possible. It's just fun to think about, what is really needed?" says Schulze-Makuch. He has made a career of studying organisms that live in out-of-the-way places, first tracing the movement of microbes through groundwater and soils on Earth, and now speculating about how organisms might make their living on other planets.

According to Schulze-Makuch, so far all of our efforts to find life on other planets have suffered from a powerful bias: we keep looking for forms of life like those we know on Earth.

"What always kind of bothers me is that a lot of people are what I would call 'Earth-centric,'" he says. "I don't think there is anything magic in how we are put together. Life is always adapted to its environment, wherever it is."

He says our view of what's possible has expanded greatly since the mid-1970s, when researchers first found microbes living in hot springs in Yellowstone National Park. Since then, we've discovered organisms suited to life in vinegar, salt mines, and toxic waste. Researchers have even found microbes living within basalt rock along the Columbia River—two miles down, with no sunlight and no apparent means of support. Experiments have shown that the subterranean microbes metabolize—or "eat"—minerals in the rock.

If such life forms can survive on Earth, asks Schulze-Makuch, why do we continue to look for the most ordinary kinds of terrestrial life on other planets? Why not look for organisms that could use minerals or magnetism as a source of energy? Or those that use something other than water as an intracellular fluid?

The latter possibility arose last December, when he and Joop Houtkooper of Justus-Liebig University in Germany suggested we already have evidence pointing to the existence of such organisms on Mars. They analyzed results from experiments done by the Viking lander in the mid-1970s. Those results are often cited as proof that Mars is lifeless, but some of them have been so hard to explain, they were officially chalked up to instrument problems or unusual chemical reactions in the Martian soil.

Schulze-Makuch and Houtkooper hypothesize that Mars is home to microbes that use a mixture of water and hydrogen peroxide as their intracellular fluid. Such a mixture would provide three clear benefits in the cold, dry Martian environment. It can stay liquid down to -56.5°C; when it does freeze, it doesn't form cell-destroying ice crystals; and it's hygroscopic, which means it attracts water vapor from the atmosphere—a valuable trait on a planet where liquid water is rare or nonexistent.

Schulze-Makuch says that despite hydrogen peroxide's reputation as a disinfectant, when it's accompanied by stabilizing compounds, it performs useful functions in many Earth organisms. Some soil microbes tolerate high levels of hydrogen peroxide in their surroundings; one species even uses it in its metabolism.

"We may be wrong," says Schulze-Makuch. "But it would explain the Viking results. It's a little unusual for Earth organisms to adapt like this, given that there's plenty of water on Earth, but it's not impossible."

Besides, he says, whether Earth creatures use it or not is irrelevant to the situation on Mars.

"It's a different planet," he says with a laugh.

Sunset on Mars. Photo courtesy NASA.

Schulze-Makuch says the Viking tests were the same ones microbiologists use to detect microbes in soil here on Earth. Quarter-teaspoon-sized samples of dirt scooped from the Martian surface were mixed with water and other substances, then baked and examined for organic molecules that would show the samples had contained living organisms. The tests were the quickest, simplest way to detect microscopic life forms that use Earth-style metabolism.

Although the tests included nothing sinister, the fact that they would have killed any microbes in the samples grabbed headlines worldwide. After a solid initial report by the Associated Press, the straightforward science story morphed into lurid accounts of how the Viking mission had killed Mars's native fauna. The low point might have come when an Italian astronomy teacher e-mailed Schulze-Makuch, saying that newspapers in his country were running stories about NASA "murdering" Martians.

"You could get the impression [from press reports] that NASA just went there killing organisms, and that's not what happened," says Schulze-Makuch. "At some point it got very bizarre. Some of the people who interviewed me said, 'we didn‚'t kill ALL the life on Mars, did we?' No, of course not!

"Nobody makes a big deal of it when every day in a microbiology lab, there are zillions of microbes getting killed," he says. "Of course it would be nice if we don't have to do that, but ethically it's probably not a big problem."

Despite the goofy press reports and occasional sniping from colleagues with more pedestrian views, Schulze-Makuch and Houtkooper sparked the curiosity of NASA researchers. A lead scientist for the Phoenix mission, which launches August 2007 and will land on Mars May 2008, has enlisted their help in designing experiments to test for hydrogen-peroxide-containing microbes. This close to launch, the equipment for the mission has already been chosen, so the new experiments must be done with the materials already on board; but it's a great first step, says Schulze-Makuch.

Meanwhile, he will keep pressing the issue, as we send probes to worlds where life might thrive in the atmosphere (as on Venus), in lakes of liquid methane (as on Titan, a moon of Saturn), or other unearthly environments.

And for those who view his notions as wacky science fiction, he has a concise response.

"I think most of that is narrow-minded," he says. "I think it's a lack of imagination."

To read more about Schulze-Makuch's work, click here and here. The first article focuses on Schulze-Makuch's book.  The second is based on a paper Schulze-Makuch presented in early 2007 to the American Astronomical Society in Seattle.

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