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.
The Viking scooped up a tiny amount of Martian regolith (soil) in situ and exposed it to a rich nutrient soup. The soup had been injected with radioactive carbon atoms back on Earth with the idea being alien microbes in the soil would consume the soup then release a bit of radioactive carbon dioxide (expelling the injected radioactive carbon out the old shute, so to speak). The experiment was a success - well, sort of. Yes, the soil loved the soup and ate it up. It was evidence of a metabolic reaction - a clear sign of life, right? Not exactly.
A companion experiment aboard Viking scooped up martian regolith, too, and then searched for organic compounds of carbon. None could be found. This second experiment negated the first, labeled-release experiment. No carbon, no microbes. Apparently, chemicals in the Martian regolith "ate" the nutrient and produced radioactive CO2. Something sure looked like life signs but was, instead, a non-living reaction.
The infamous August 1996 NASA news conference claiming fossil life inside Mars meteorite ALH84001 gave a blackeye to the space agency. Agency researchers claimed polycyclic aromatic hydrocarbons (PAHs) found inside the space rock were the fingerprints of microbial life. NASA researchers also claimed the meteorite - found in Antarctica in 1984, hence the "84001" label - contained microscopic structures that looked like fossilized microbes. Here again, so-called life signs can be deceiving.
After considerable scrutiny by outside researchers, the PAL molecules were found to have been produced by non-living reactions - in fact, PAHs are an ingredient of terrestrial air pollution. And, the tiny "fossils" inside the meteorite? Well, these objects became suspect, too; the wormlike structures are an order of magnitude smaller than microbes on Earth - again, inorganic reactions can produce similar structures right here on Earth.
Figuring out how to finely distinguish between geochemical and biological signs is a daunting challenge in the search for life beyond Earth.
What's in the Sky: On Saturday, July 31, look for a clustering of planets in the western sky 45 minutes after sunset: Mercury, Venus, Mars and Saturn all appear in order in an ascending, imaginary curve rising to the left of the star Regulus.
Lou Varricchio, M.Sc., lives in Vermont. He was a former science writer at the NASA Ames Research Center in California and is a member of the NASA/JPL Solar System Ambassador. He is the recent recipient of the U.S. Civil Air Patrol's Brig. Gen. Chuck Yeager Aerospace Education Achievement Award.