A pair of stubborn molecules, prized because they can survive deep time, just passed an important Mars test by failing a very different one on Earth. The molecules are pristane and phytane, hydrocarbons tied closely to living organisms. They have long attracted interest as possible biosignatures, chemical traces that might hint at ancient life on Mars. But when researchers examined those compounds in the Murchison meteorite, a famous space rock that fell in Australia in 1969, the result pointed in another direction. The molecules did not behave like fresh biological leftovers. Instead, they looked more like contamination from petroleum. That may sound like bad news for the search for life. In one way, it is the opposite. The work gives scientists a tougher and more realistic way to tell whether certain organic molecules are biological, non-biological, or simply picked up after a rock reaches Earth. It also gives a trial run to an instrument built for ESA’s Rosalind Franklin rover, part of the ExoMars mission, which is scheduled to begin searching for organic molecules on …