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What is life?

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.

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