Life is tenacious. If you've ever battled a stubborn bathroom fungus, or been dosed with antibiotics until you fear you might die before your bacterial infection does, you know that life--and in particular single cell life--can really hang on when conditions are tough. Over the last few years, scientists have discovered life in some tough venues on our own
planet, fueling scientific interest in life on Mars or elsewhere in our solar system. As scientists keep searching, life's comfort zone continues to expand.

NASA's Opportunity and Spirit robots, now probing the geology of Mars, have a motto for their mission: "Follow the water." Biologists (and astrobiologists -- scientists who search for unearthly life) are sure that if life as we know it turns up, it will require liquid water.

Geological evidence from the Mars rovers and orbiter photos indicate that the planet was moist in the distant past. It looks very dry now, with any remaining water locked under the polar caps. But Europa, one of Jupiter's giant moons, seems almost certain to have liquid water, in a 60-mile-deep ocean sealed under a 10-mile-thick global ice sheet. By comparison, oceans here on earth are at most about six miles deep.

“Europa's oceans should have more than twice the volume of the Earth's,” says Torrence Johnson, the chief scientist of the Galileo mission to Jupiter and its moons. There's plenty of room for life, but can life flourish in a high pressure waterworld?

Deborah Kelley was looking at just such a world, at the bottom of the Atlantic ocean, in 2000. Her focus then was the geology of the Atlantis Massif, a sizeable mountain on the bottom of the ocean near the Mid-Atlantic Ridge. Late one evening, some other scientists on the mission were watching from a remote camera being "flown" over the sea-bottom valleys. Kelley remembers an excited Swiss oceanographer barging into her cabin to say: “I think we've seen something. I don't know what it is, but we haven't seen anything like it before." When they gathered around the monitors, the scientists saw white towers of rock, 200 feet tall, venting hot water, covered in bacterial slime. What the expedition had found at the Mid-Atlantic Ridge was a new kind of geological formation inhabited by previously unknown life forms.

The first geothermal vents, as these hot spots are called, were found in 1977 near the Galapagos Islands by scientists using Wood's Hole Oceanographic Institution's tiny submarine, ALVIN. At 8,200 feet down, these hot vents had been predicted. But Wood's Hole scientists photographed and brought back samples of life no one was expecting to find.

Before that time, it seemed clear that life was based on light: without plants to turn light energy and carbon dioxide into sugars, how could life exist? The ocean floors are almost totally dark, so under this logic, they should be barren. And yet the volcanic cloud-venting "black smoker" chimneys on the bottom of the seafloor were home to acid and heat-loving bacteria. A whole range of larger life -- clams, crabs, tubeworms--appeared to be living off those bacteria, much as we surface-dwellers live off plants. Scientists realized there was more than one way to get the energy needed for life.

Kelley led an expedition to the newly discovered vent, now called the Lost City hydrothermal field, in 2003. Unlike other deep sea vents, the energy at Lost City isn't generated by an underwater volcano, but from a chemical reaction between seawater and the underlying mineral, called peridotite. This reaction gives off heat, hydrogen, and hydrocarbons, leaving behind a common mineral called serpentine, a material you might find it in your own kitchen as a green "marble" counter-top.

Lost City is unique for now, but Kelley says that the geology at the Atlantis Massif is not rare, and that she expects other life islands based on peridotite will be found. "It really expands the places we find earthly life," she says.

The first discovery of living things growing where it was once thought impossible for them to grow was accidental. At Yellowstone National Park's hot springs in the 1960s, Thomas Brock of Indiana University found bacteria blooming at 176 degrees F, a species he named Thermus aquaticus. Decades later, in 2003, Derek Lovley and co-workers at the University of Massachusetts (Amherst) looked at a bacterium delivered from the Mothra vent field in the northeastern Pacific. They watched as it grew in their sterilizing equipment at 250 F, well above water's normal boiling point. (The crush of several atmospheres of pressure holds the water to its liquid form.) This yet-to-be identified organism, called Strain 121, is so far the champ for hot water survival, but no one is taking bets on where the limit for hot life might lie. Lovley says simply "We do not know why Strain 121 can grow at these temperatures."

Lovley's research focuses on evolution of very early life on Earth, a search for the still-living descendants of the first bacteria. Three billion years ago, this planet had no oxygen and no protective ozone layer, so to survive, life forms hid in hot, deep crevices in the earth, fed on chemicals scavenged from hydrothermal vents, and, as Strain 121 does, made use of iron in place of oxygen. These iron-rich environments may not be rare. Although Mars is cold, dry and volcanically inactive now, Lovely says there's plenty of iron there, indeed, the "red planet" is that color precisely because of the rusty iron minerals that Strain 121 "breathes."

Another sunless environment now beckons extreme life scientists, but this one is cold rather than hot. In 1974, radar surveys of Antarctica showed lakes of liquid fresh water that had been sealed over by a couple miles of ice for at least half a million years. In 1999, a team of French, US, and Russian scientists drilled down from the Russian base at Vostok, Antarctica, to within 400 feet of the liquid water. Ice cores, formed from frozen lake water, showed that there are indeed bacteria making a living in the hidden lake. Lake Vostok, about the size of Lake Ontario, averages about 27 F, yet remains unfrozen due to the pressure of the ice lying on it. (Pressure causes ice to melt and become more dense, that is, liquifies it.)

Now that the NASA rovers have yielded firm evidence that Mars once enjoyed persistent liquid water, Earth's geologists and microbiologists are planning a search for fossil remnants of the Martian life Martian. Submarine designers ponder a submersible probe for Europa's cold depths. And meanwhile, bookmakers and astrobiologists continue to revise the odds – in life's favor.