We may have already discovered the essence of life on Mars 40 years ago, according to a former NASA scientist. Gilbert V. Levin, who was principal investigator on a NASA experiment that sent Viking landers to Mars in 1976, published an article in the ScientificAmerican journal last Thursday, arguing the experiment's positive results were proof of life on the red planet. The experiment, called Labeled Release, was designed to test Martian soil for organic matter. "It seemed we had answered that ultimate question," Levin wrote in the article. In the experiment, the Viking probes placed nutrients in Mars soil samples...