Exobiology: E.T., You May Be Home Already

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There are aliens living in the hot springs of Yellowstone National Park. Technically, the otherworldly creatures aren't all alive; many are just bacterial microfossils. They're not exactly extraterrestrials either, having spent their brief lives entirely on Earth. But if the tiny microbes prove anything, it's that living organisms can make their homes in the most unlikely places. For scientists like Sherry Cady, 43, a geologist at Portland State University in Oregon, this is big news indeed.

Cady is one of the leading lights in a brightening field known as astro- or exobiology--the study of how life could form elsewhere in the universe. In 1994 she made her first trip to Yellowstone to study the springs. Cady knew she might find heat-loving microbes in the scalding water, but she had no reason to think she would find remains of their deceased kin. When she looked at rock samples under a microscope, however, she discovered that they bore the fossilized imprints of bacterial sheets that gave silica a place to cling as the rocks were forming. "I was flabbergasted," she says.

With good reason. If microbes could live and fossilize in so punishing a place, they might do the same in other hot spots, like now vanished springs on Mars. Similar remains might be left behind where temperatures are low, like frozen lakes on the Martian surface before the planet dried out.

Other scientists are content to speculate. Not Cady. She's collecting samples not only in Yellowstone but also in the Taupo Volcanic Zone in New Zealand and the frigid depths of a lake in British Columbia. When astronauts or robot craft finally start hauling home rocks from other planets, those specimens can be compared with Cady's for the tiny thumbprint of life both may contain. The trick, she says, "is knowing what clues to look for."

--By Jeffrey Kluger