For several years now, NASA's Curiocity Rover has been probing around looking for signs of life on Mars. While it still has not found any evidence of past or present life on Mars (that we know of), the big announcement that came from NASA was that organic compounds have been found that could indicate that there was life on Mars, perhaps as much as billions of years ago. Essentially, these "organic" compounds are carbon-containing molecules that, on Earth, are produced by life forms. So, the idea is, that since these compounds have also been found on Mars, this might indicate that these compounds were once produced by life on Mars. But here's the catch: these "organic" compounds could have been produced by non-organic processes, as well.