From Universe Today (yes I know it is linked elsewhere on BAUT, but I thought worthy of repeating here)
A new look at data from the Mars Viking landers concludes that the two landers may have found the building blocks of life on the Red Planet after all way back in 1976. The surprise discovery of perchlorates by the Phoenix mission on Mars 32 years later could mean the way the Viking experiment was set up actually would have destroyed any carbon-based chemical building blocks of life – what the experiment set about to try and find.
“This doesn't say anything about the question of whether or not life has existed on Mars, but it could make a big difference in how we look for evidence to answer that question," said Chris McKay of NASA's Ames Research Center. McKay coauthored a study published online by the Journal of Geophysical Research – Planets, reanalyzing results of Viking's tests for organic chemicals in Martian soil.
The Viking lander scooped up some soil, put it in a tiny oven and heated the sample. The only organic chemicals identified in the Martian soil from that experiment chloromethane and dichloromethane — chlorine compounds interpreted at the time as likely contaminants from cleaning fluids used on the spacecraft before it left Earth. But those chemicals are exactly what the new study found when a little perchlorate — the surprise finding from Phoenix — was added to desert soil from Chile containing organics and analyzed in the manner of the Viking tests.