YELLOWSTONE NATIONAL PARK - They are alive. The boiling, Caribbean-blue hellholes. The carpets of green, yellow, white, black and orange bathed in scalding runoff.

We are alive because untold trillions of microbes have lived. How the world's microbes - the planet's richest trove of life - survive and shape our world is the key to understanding the origins of life on Earth, scientists from the University of Colorado and elsewhere say. The $720 million Mars Reconnaissance Orbiter that launched Friday is NASA's latest bet that the same holds true elsewhere.

A series of missions to Mars has been dedicated to finding hints of past microbial life. In November 2006, from 190 miles above our neighbor planet's rusted deserts, the Mars Reconnaissance Orbiter is slated to begin an exhaustive, two-year search.

What the spacecraft really is looking for is a former Yellowstone.

Grand Prismatic Spring, at Yellowstone's Midway Geyser Basin, is among the national park's most popular attractions. The same combination of water and magma that so reliably flushes the contents of the nearby Old Faithful keeps the great pond's turquoise waters at a near-boil.

Bathed in the blistering trickle from Grand Prismatic's silicate lips are swaths of orange and brown that even casual visitors know to be microbial colonies.

Norman Pace, a thin man of 62, looked out over the spring's vaporous surface. He was among a minority on the boardwalk without a camera. But then, Pace was no tourist.

He once floated a bit of furnace wool in the spring. A week later, it came out pink - colonized with hydrogen-eating "extremeophile" microbes somehow at home in the 190-degree water. At such temperatures, human cells become a denatured soup.

The University of Colorado professor and renowned microbiologist has studied Yellowstone's pools for decades, using genetic mapping techniques he helped pioneer. But even Pace doesn't know exactly what's in the pools or the thick biological carpets - formed by billions of microbes per cubic inch - on display here and elsewhere in the park.

"What's going on out there?" Pace asks. "It's really wild cards."

But he and others have learned enough to believe what their genetic maps say: Places such as Yellowstone's hydrothermal springs probably were the font of life on Earth, which had been seeded with interstellar amino acids, carbohydrates, vitamins and other building blocks.

The strict grammar of physical chemistry says the same probably was true of life on Mars and, for that matter, anywhere else.

"The requirements for life and the way that life does it are going to be the same everywhere in the universe," Pace says.

Pace was at Yellowstone with several scientists and journalists for a workshop on life in extreme environments, sponsored by the Center for Astrobiology at the University of Colorado's Laboratory for Atmospheric and Space Physics. Astrobiology integrates space, life and geological sciences to explore the origins of and environments friendly to life on Earth and beyond.

A gram of soil - any soil - contains about a billion microscopic creatures, he says. Microbial mats form in mud puddles, and the smell of fresh rain is the aroma of microscopic life.

Others have found live microbes in ocean sediments hundreds of yards beneath the ocean floor. Steven D'Hondt, a University of Rhode Island professor of oceanography and lead scientist on his university's NASA Astrobiology Institute team, estimates that up to 5 percent of all life on Earth may be beneath the ocean floor. D'Hondt led the 2002 ocean-drilling expedition that first discovered such microbes.

Radiation and drought-resistant microbes also have turned up in Chile's Atacama desert, one of Earth's driest places. It is our planet's best imitation of modern Mars. Farther south, University of Colorado researcher Diane McKnight recently reported that, with the reintroduction of water, long stretches of microbial ecosystems in a dried-out stream of Antarctica's McMurdo Dry Valleys were revived after 20 years of desiccated dormancy. The implication, she said, is that long-dormant Martian microbes, should they exist, could be revived.

At Yellowstone, Pace can't help pointing out the greenish tinge of the water pouring off the Lower Falls. It's microbial algae, he says.

"We live in a microbial world," he says. "It's all around us."