In the late 1970s, NASA landed two probes on the Martian surface. The Viking missions were designed to scoop up surface material and examine it for indications of life.
What was expected was for the missions to detect organic compounds: carbon-based molecules like amino acids that make up the building blocks of life. Instead, the results they got were disappointing. Instead of organics, they found compounds of chlorine like chloromethane and dichloromethane, which were interpreted as contamination from labs back on Earth (from cleaning fluids!).
However, in 2008, the Mars Phoenix lander did its own scooping, and found something unexpected: perchlorate. This molecule is made up of one chlorine atom and four oxygen atoms (ClO4) and has the interesting property of being very reactive with organic molecules. It’s found naturally on Earth, too.
What’s so very interesting about this is that recently, scientists took samples of soil in Chile, added perchlorate, and then analyzed those samples in the same way Viking did. Guess what they got?
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