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 …