In a groundbreaking article in Scientific American, the case is laid out that we did detect life on the red planet in the 1970s and the results were suppressed. In fact, out of all of the probes, landers and rovers we have sent to Mars, only the Viking missions in 1976 actually tested for life and both of the Viking spacecraft which landed 4,000 miles apart.
On July 30, 1976, two Viking probes landed on Mars and conducted a Labeled Release (LR). A Labeled Release is used to check for microbial respiration. All four tests conducted on both spacecraft resulted in positive tests for life and the tests were very similar to tests conducted on Earth, indicating that the tests were not faulty.
The Viking Molecular Analysis Experiment did not detect organic matter though, resulting in the conclusion by NASA that there was not life but there was something that “mimicked life” which is probably even more troubling than just saying there is life.
The article outlines an impressive case for life on the red planet, including the detection of surface water in every Mars mission, the lack of correlation between the results and any known geological process, the fact that there were many tests on samples which yielded no false positives or negatives, etc. The famous ghost lights, which have been filmed on Mars, seem to be the same as will-O’-the-wisps on Earth which are formed by the release of methane.
It may be a while before we have an answer, other than the one we already have. NASA has announced that they will not conduct a life detection test in 2020 when the new Mars lander touches down.
This has led to many conspiracy theories and it is comforting to know that it also has scientists left scratching their heads as to why on earth we have not at least tried to answer the question since 1976.