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Volume XXIX Number 2
Spring 2007

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mine entrance
Photo courtesy Lisa Pratt and Trustees of Indiana University

Unknown Life Way Down Under

Truth can be stranger than fiction, and alien life on Mars may be no stranger than life on Earth, or more precisely, life beneath the Earth’s surface.

An international team of scientists from nine institutions, including Lisa Pratt, a biogeochemist at Indiana University Bloomington, has discovered a self-sustaining community of bacteria two miles below the surface of a South African gold mine. The tube-shaped bacteria were found living in fracture water seeping from a tunnel wall. The researchers determined that the bacteria in the water, Firmicutes, were 3 to 25 million years old, and that the microorganisms have sustained themselves over the millennia without relying on the sun. Instead, the "extremophiles" get their energy by feeding on hydrogen and sulfur compounds produced by the decay of radioactive uranium ore.

The scientists descended dozens of times over 54 days to sample the fracture water. They found that the water contained hydrocarbons and hydrogen created from decomposition of water exposed to radiation from uranium-bearing rocks. As the research team observes in their paper published in Science, "deep crust biosphere . . . is capable of sustaining microbial communities indefinitely by geological processes."

"We know surprisingly little about the origin, evolution, and limits for life on Earth," says Pratt. "Scientists are just beginning to study the diverse organisms living in the deepest parts of the ocean, and the rocky crust on Earth is virtually unexplored at depths more than half a kilometer. The organisms we describe live in a completely different world than the one we know at the surface."

The unusual and extreme conditions of the mine’s microbial life have spurred the scientists’ speculation that life forms may exist below the surface of Mars. Tullis Onstott, a professor of geosciences at Princeton University and leader of the South African mine research team, says the fact that the bacteria have thrived in inhospitable conditions "raises the possibility that organisms could survive even on planets whose surfaces have long since become lifeless."

Pratt and Onstott are part of the Indiana-Princeton-Tennessee Astrobiology Institute (IPTAI), a long-term NASA-funded research collaboration studying what deep-Earth environments can tell us about extraterrestrial life. Pratt, who is IPTAI’s director, also has been working in the Lupin mine, a deep gold mine located near the Arctic Circle in Canada. Using the Lupin mine as an analogue (the Canadian Arctic and Mars both have permafrost), Pratt and her research colleagues are working on life-detection probes for deployment on future Mars missions, which just may turn up life where we least expect it.

—L.B.