Is, or was there ever, life on Mars? It's a question that's excited scientists since the dawn of the space age. And right now, there are orbiters, landers, roving vehicles all over the surface of the red planet, analyzing it. But it's still an open question: how habitable is our celestial neighbor and what, if anything, could have inhabited it? Here are four key moments in the search to answer that question: In 1976, NASA's 'Viking' lander, was the first mission to look for microbial life in soil samples it grabbed from Mar's surface. It didn't find evidence of even the simplest, organic molecule. Since then, some members of the Viking team have questioned whether they missed something in that initial analysis. But to this day, there's no accepted evidence of microbial life on Mars.

In July of 1996, NASA scientists announced the discovery of fossils of bacterial life in meteorites that they found in Antarctica—meteorites from Mars that had crashed into our planet12,000 years ago. After huge, initial excitement, NASA said that after two years of continuing to study that meteorites, those initial lines of evidence simply went away. But to this day, there are still scientists who maintain that initial analysis was valid. Both NASA's 'Curiosity' rover and the European Space Agency's orbiter have detected whiffs of the gas methane on Mars. Now that's tantalizing because on Earth, the vast majority of methane comes from microbial life so future martian missions have set their sights on detecting exactly where that gas is coming from. Most recently, and possibly most excitingly, researchers have found the first evidence of an existing body of liquid water on Mars. It was found using radar and this is a sub-glacial lake under the southern polar ice cap of the red planet.



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