Imagine it's 2030, and your ground crew has just strapped you into a spacecraft headed for Mars. Your assignment: Look for life on the Red Planet.
Astronomers think that Mars and Earth were very similar when they were young. Other scientists point to fossils on Earth that show life was thriving on our planet when it was "only" a billion years old (it's about 4.5 billion years old now). Maybe life emerged on Mars, too.
But where do you look for Martians? What do they look like? Are fossils all that remain on Mars - now a frigid planet with a wisp of an atmosphere? Or might life still exist beneath the planet's dry, rocky, windswept surface? If life never existed on Mars, are there other places in the solar system besides Earth where life exists?
Those questions are driving scientists to travel to some of the coldest, hottest, and otherwise most inhospitable spots on Earth to look for living organisms. And they're finding them by the billions: colonies of single-celled creatures living in Antarctic lakes; bacteria deep in the Earth's crust; tube worms that cling in silent darkness to the sides of volcanic vents on the ocean floor.
Call it X-life: life in extreme environments.
"Studies of life elsewhere in the solar system really aren't about what we used to call LGMs - little green men," says Matthew Kane, a biologist working with the National Science Foundation. Instead, the studies focus on microbes: tiny one-celled organisms with a remarkable ability to boldly thrive where no human could.
"When you think about life in extreme environments or elsewhere in the solar system, you think about microbes," he says, "because as organisms grow more complex, they have a more limited range" of habitats.
Scientists say microbes are the most widespread form of life on Earth. By some estimates, if you could weigh every living thing on Earth - from microbes and monkeys to giant Sequoias and blue whales - microbes would account for more than half the total weight. In soils, microbes help turn minerals into chemicals that plants can use. In the ocean, microbes are food for larger creatures. Inside mammals' stomachs, microbes help digest food.
Microbes also can change the chemical makeup of their surroundings. Vast mats (communities) of microbes that existed when the Earth was young probably gave our atmosphere its initial supply of oxygen, which allowed other life forms to emerge and thrive.
In fact, researchers say, early microbes "taught" plants about photosynthesis - the process by which green plants make food using carbon dioxide, water, and light.
An easy place to look for the hardiest of these microbes is in Yellowstone National Park. The park is on a geological lid atop a hotspot in the Earth's crust where hot magma has pushed its way near the surface. It bubbles with hot springs, foul-smelling fumaroles, and its famous geysers.
Many of the microbes found around hot springs rely on photosynthesis and form large slimy mats.
"There is a huge diversity of life there," says Anna-Louise Reysenbach, a scientist at Portland State University in Oregon. She studies relationships among microbes and between microbes and their surroundings. "You can find more diversity in one square centimeter" of these mats "than you'll find in a square mile of tropical rainforest," she says.
Comparing these slimy mats to forests is pretty accurate, says David Ward, a scientist from Montana State University in Bozeman. Visitors to Yellowstone see hot springs (the water is far too hot to touch) and "lots of green stuff" along the channels flowing from them. Under a microscope, the green stuff - not much thicker than a few sheets of notebook paper - resembles a forest. Smaller organisms that require less light (the understory) live beneath taller ones that require more light (the canopy). "It's equivalent in the microbial world to what you would see in a forest. The difference is only a matter of scale," Professor Ward says.
Dr. Reysenbach adds that Yellowstone is an amazing place to study X-life because it offers such a broad variety of microbial environments - from water that is almost pure acid to water that is extremely alkaline. The closer you get to either end of this range, the more corrosive the liquid is - except to the microbes thriving in it. Many of these microbes are thought to be descendants of the earliest forms of bacteria that inhabited the young, volcanically active Earth.
But do all microbes need light to live? Nope. Take undersea volcanic vents. They burble up from the sea floor at depths sunlight can't reach. In 1977, scientists visited one of these vents in the Pacific and were stunned to find it teeming with life - tube worms, blind shrimp, crabs, and other creatures. And - surprise, surprise, - microbes that have never seen the light of day sat at the bottom of that food chain. In fact, a tube worm couldn't live without microbes living inside it. The worm has no mouth, stomach, or other digestive organs. Instead, chemicals from the surrounding water seep into it and bacteria convert them to food the worm can use. The bacteria feed off the minerals dissolved in the water emerging from the vents, called "black smokers."
Paul Johnson, a marine geologist at the University of Washington at Seattle, had another question: If you can find microbial life around vents - which appear in places where magma is welling up from deep inside the Earth to form new crust - could microbes be found in old crust as well?
Yes. He has found microbes living more than 100 feet deep in crust that's about 3.5 billion years old. And while new crust quickly cools from 2,190 degrees F. to just above freezing, the temperature under old crust is about 140 degrees F. It's kept warm by a blanket of sediment that sits on top the crust, trapping heat rising from inside the Earth. The upper crust has lots of small spaces into which water can flow.
So undersea microbes appear to live just about anywhere you'd care to poke a hole and look deep in the undersea crust. And that undersea crust makes up 70 percent of the Earth's "surface."
Finally, let's chill a bit and look at life in the slow lane down in Antarctica's Dry Valleys. Except they're not exactly dry. They contain ice-covered lakes. In the summer, it gets warm enough for water to flow into them from nearby snowfields and glaciers.
Here, too, life thrives, says Peter Doran, a scientist at the University of Illinois at Chicago. He studies the bacterial mats that form on the lake bottoms.
Here, the variety of microbial critters is pretty small, he says, because the habitat appears to change very little. The water at the bottom is salty and stable, with little mixing between it and the layers above. But it's cozy. Despite the wide swings in air temperature from winter to summer (minus-50 to 32 degrees F.), the water in the lakes stays at room temperature year-round. It's warmed by the sun in summer and insulated by its icy cover in winter.
As they learn about how life survives in these relatively shallow lakes, scientists are setting their sights on the hunt for life in the continent's granddaddy, Lake Vostok. It's about the size of Lake Ontario, is some 3,280 feet deep, and it's trapped under a layer of ice 2-1/2 miles thick.
John Priscu, another Montana State University scientist, is helping to plan a search for life in Lake Vostok. He notes that columns of ice brought back from drilling expeditions there show ample evidence of microbial life. But the deepest sample penetrated only partway into a 650-foot layer of ice that marks a boundary between the ice above and the surface of the lake below. He hopes to lead a team to drill deep enough to bring up water samples. His dream: Send down a remotely piloted underwater vehicle.
Russian and French scientists' research suggests there may be hydrothermal activity at one end of the lake, just as there is on the sea floor and at Yellowstone.
If that's true, Dr. Priscu says, "then you have an energy source. You could have these unique organisms there. You could have an environment that supports multicellular organisms," as black smokers do.
"Wouldn't that be great to put down a submarine and see tube worms meters long under 2-1/2 miles of ice?" he wonders. "It gives me goosebumps...."
• Next Week: If life is in such odd places on Earth, what about life in outer space? Watch for our 'Design an Alien' contest!