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What is life?

The search for extraterrestrial life is today's greatest scientific challenge. Setting aside the vast distances between worlds, the challenge to astrobiologists comes down to the age-old question: what is life? You may think you'd know life when you see it, but consider two recent controversies that illustrate the problem.

The best working definition of life was provided by biologist Gerald Joyce of the Scripps Research Institute. Joyce is a key member of NASA's exobiology team. His definition is now guiding the space agency's search for life elsewhere.

"Life is a self-sustained chemical system capable of undergoing Darwinian evolution," according to Joyce's definition.

More specifically, he identifies life as possessing three characteristics:

1. Life must be a chemical system.

2. Life must grow and sustain itself (metabolize energy from its environment).

3. Life must display variation.

But, even with Joyce's guidelines, it's not so easy.

Several respected paleontologists remain stymied as to how to recognize Earth's earliest fossil lifeforms.

Ancient chert rock found in Australia was once believed to hold Earth's earliest microscopic lifeforms.

In 1993, J. William Schopf of the University of California at Los Angeles stirred up a scientific firestorm when he claimed he had evidence of Earth's oldest bacteria - microscopic structures representing 11 fossil species.

Upon more detailed analysis, other researchers found Schopf's 3.5-billion-year-old microscopic structures weren't fossils; instead, the microstructures were the products of geochemical reactions in prehistoric hydrothermal vents. Despite the apparent setback, researchers dusted themselves off and continue the quest for paleontology's Holy Grail.

In the case of the search for life on Mars, NASA scientists have learned their lessons the hard way over the years. Frequently, things that appear to be related to life on the surface, turn out to be more easily explained via geochemistry.

In 1976, NASA's first landing on Mars by the twin Voyager spacecraft included a famous experiment, the labeled-release experiment.

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