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.

Vote on this Story by clicking on the Icon


Use the comment form below to begin a discussion about this content.

Sign in to comment