Even the most ardent fans of the Red Planet must occasionally wish for more than just hints of water popping up in ever-new places. So why not send a robot to hunt directly for little green men? One word: Viking. NASA's Viking landers did just that in 1976, laying out a tasty solution of nutrients to attract any microbes that might be living in a soil sample, like cookies left on a plate for Santa. The nutrients were laced with radioactive carbon, so if the solution was digested, a radiation monitor above the sample would detect the resulting gas. Intriguingly, radioactive carbon was detected, but then another experiment found no evidence of organic compounds in the soil - there were no alien bodies.
"They were hoping to find signs of life but the results came back basically negative - there is no life as we know it," says Ralph Milliken at the University of Notre Dame in Indiana. The US did not send another mission to Mars for 20 years. The $2.5 billion Curiosity rover will hunt for organic molecules and isotopic hints of life, but NASA is still shying away from the L word. "NASA cannot say to taxpayers that they put $2.5 to $3 billion to search for life, and then say, 'We have found no life - thank you, bye bye',' says Michel Cabane, leader of one of Curiosity's organics-sniffing instruments, who is based at the Pierre and Marie Curie University in Paris, France.
"If you project the message that you are hunting for life, even though it is very important to many of us, and you return with a null or ambiguous answer, people would be disappointed," says jack Mustard of Brown University in Providence, Rhode Island, who is a former chair of NASA's advisory panel on Mars. In any case, he and others say the problem may simply be too hard to solve. "If I posed the question 'prove that life existed in Earth's past' to you, it would be tough," Mustard says. "Geologists would say, we'll go find a fossil. But bodies are not always preserved on Earth."
He points out that Curiosity and other missions that touch down on the planet are only exploring a limited region for a limited time. Bethany Ehlmann at the California Institute of Technology in Pasadena agrees. "Think locally, not globally - that's a slight perversion of what the environmental movement thinks we should do here on Earth;' she says. Curiosity's landing site may once have been a lake, but other intriguing sites suggest life might have found a refuge in hydrothermal springs below the surface. "Environments during the first billion years of Mars history varied substantially," she says. "Now that we know there's this diversity out there, it becomes harder to say that the evidence says 'Mars did not have life'."
SOURCE : NEW SCIENTIST MAGAZINE NOVEMBER 2011