The results from tests performed by the Viking space probes in the 70s indicate to some scientists that we may have found life on Mars, then immediately killed it, eliminating forever the Martian threat.
The Viking landers of the disco era were searching for microbes on the evil red planet and mixed water with local rocks to search for life. At the time, it was thought that life required water to exist, but since then, we've learned that life could probably exist with other liquids as solvent. Mysterious test results from the experiments could indicate the first interplanatary murder.
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