If you wonder about the prospects for life on other worlds, take another look at what's going on at home. Microbes are surprising scientists by thriving where life as we thought we knew it shouldn't survive.
Last Friday, for example, Carnegie Institution microbiologist Anurag Sharma reported that his group had bacteria colonies living under 17,000 times Earth's sea level pressure. University of Southern California biologist Kenneth Nealson told the Associated Press the announcement "knocked my socks off." Yet it came less than a week after Dr. Nealson himself had described microorganisms' amazing versatility in a paper prepared for a meeting of the American Association for the Advancement of Science (AAAS) in Boston.
Such is the fast pace of discovery in the new science of geobiology. Its practitioners are tracing the relationships between microbes and inanimate matter that make Earth hospitable for so many other life forms, including ourselves.
According to geobiologist Edward Delong from the Monterey Bay Aquarium Research Institute in California, the "pervasiveness of microbial life" has become a basic theme for 21st-century biology.
Scientists, he says, now know microbes "have invaded almost every habitat we can imagine on Earth." They are found inside rocks on the surface and within rocks miles below it. The pressures under which Dr. Sharma's bacteria are living correspond to pressures many miles deep in Earth's crust.
Microbes thrive in hot volcanic springs and in superheated seawater venting from the deep sea floor. Fungi and bacteria live deep below glaciers and in Antarctic rocks and soils.
Life's temperature range probably runs roughly from 58 degrees below zero Fahrenheit to 302 degrees F above it, according to Norman Pace of the University of Colorado at Boulder.
All over our planet, microbes are "eating rocks, breathing rocks, making and dissolving minerals, and seemingly getting energy from the planet whenever it is there," says Nealson. If there's no sunshine for photosynthesis, it's no problem. Microbes can then use geothermal energy.
Nealson notes that such "processes invented by life are the very essence of life itself, and allow us to easily distinguish those environments where life abides." That's good news for astrobiologists. The more markers for life or for favorable environments scientists identify on Earth, the more clues they have to look for on Mars, in meteorites, or beneath the icy crust of Jupiter's moon Europa.
Desert varnish - a glossy coating on desert rocks - is one example. Although varnish formation is not fully understood, bacteria and microscopic fungi live on and within the coating. James Staley of the University of Washington at Seattle told the meeting in Boston that amino acids found in the varnish on Earth are possible signatures of biological activity, but not in themselves definitive proof of life. Similar coatings have been seen on Martian rocks. If biological signatures were found on Martian rocks, that would be some evidence for life there as well, Dr. Staley said. He added that, if we knew more about varnishes here, we could identify more of these signatures and know better what to look for on Mars.
The adaptability and toughness of Earth's microbial life bolsters astrobiologists' hopes for eventually finding signs of life elsewhere. Microbes might even survive in the molecular clouds drifting among the stars. However, microbes' tenacity has its downside for explorers.
Abigail Salyers of the University of Illinois at Urbana-Champaign explains that scientists didn't know about rugged microbes when they began sending spacecraft to the moon and Mars. They thought the vacuum and radiations of space would kill off any microbes that survived pre-launch sterilization.
Now they know that so-called extremeophiles survive, even thrive, in such conditions. Mr. Delong warns that this raises a crucial question in planning to search for life in our solar system: "Can we do that without overwriting the data with our own signature?" It's already too late to avoid writing that signature in parts of the moon and Mars, Dr. Salyers says.
Salyers also notes that, so far, scientists have identified only 1 to 2 percent of Earth's microbial life. There probably are more surprises ahead as more is learned about how adaptable that life is to what had been considered lethal circumstances. But, she explained, it already is clear that microbes prepared and now maintain the planetary environment that supports complex organic life. "We absolutely depend on them," she says.