When the Viking landers touched down on Mars in 1976 and scooped up soil samples, scientists were surprised that the two craft failed to unearth evidence that the Red Planet contained any organic compounds. The apparent lack of organic molecules – a basic requirement for carbon-based organisms – helped to cement the notion of Mars as an entity that would not easily support life.

But a new study, which relies on soil samples from Earth, now suggests that the Viking craft may have found organic compounds from Mars but failed to recognize them. The finding represents a sea change in the way many scientists think about Mars and suggests a specific strategy for searching for vestiges of life on the planet, says study co-author Rafael Navarro-González of the National Autonomous University of Mexico in Mexico City.

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The study was inspired by an analysis of soil samples conducted by the Mars Phoenix Lander, which arrived in the north polar region of Mars in May 2008 and operated for five months. Phoenix found that most of the chlorine at the landing site was in the form of perchlorate, rather than a chloride salt as had been assumed.

Perchlorate is an oxidizing agent that when heated, breaks down into highly reactive fragments that destroy organic compounds. These reactions take place at the same temperatures – 200° to 500° Celsius – to which Martian soil samples were heated by the Viking craft. The only organic compounds found by Viking, chloromethane and dichloromethane, were interpreted as contaminants from Earth, since they are common in cleaning fluid, solid rocket fuel, fireworks and other explosives.

But when Navarro-González and his colleagues added 1 percent by weight magnesium perchlorate to soil from the Atacama Desert in Chile, which is thought to closely resemble Martian soil and is known to contain organic compounds, they found an intriguing result. Heating the perchlorate-adulterated desert soil to temperatures comparable to those in the Viking experiments produced the same chlorinated organic compounds that were found by the landers in 1976 but dismissed as contaminants. Nearly all the organic compounds originally in the Chilean soil were destroyed during the heating.

Similarly, the team says, the soil at the two Viking sites likely contained plenty of organics that were destroyed upon heating and were turned into chlorinated methane compounds due to the presence of perchlorate.

“The bottom line of this work is that the Viking landers did detect organics on Mars, we just did not realize it,” McKay asserts. He and his colleagues estimate that the Martian soil contains a few parts per million of organics, comparable with the driest parts of the Atacama Desert.