In a study published earlier this month in the journal Astrobiology, two researchers say the scientific community should take a closer look at a study of Mars' soil published in 1976. Because two NASA robots may have discovered signs of life on Mars almost four decades ago, say Gilbert Levin from Arizona State University and Patricia Ann Straat from the US National Institutes of Health. It all started when NASA sent two probes, named Viking 1 and Viking 2, to Mars in 1976 to test for signs of life on the Red Planet. As the first spacecraft from Earth to reach Mars, the Viking probes conducted three studies on the planet's biology. To conduct one of the studies, the labed release (LR) experiment, scientists took soil picked up by the Viking probes and mixed it with nutrient-rich water.