The Viking Mars mission may have missed signs of life when it visited the red planet 30 years ago, a new study suggests.
If future missions are to set the record straight, the study's authors add, scientists may need to change the ways in which they search.
NASA's Viking Mission to Mars put two landers on the red planet in 1976. Their experiments uncovered mysterious chemical activity in the Martian soil but no clear evidence of life.
Now scientists suggest that telltale signs of life could have been there all along, but Viking's testing methods were not robust enough to recognize them.
"We simulated these [tests] that Viking did 30 years ago, this time in extreme regions of our own planet," said Rafael Navarro-Gonzalez, of the National Autonomous University of Mexico in Mexico City.
"We found low levels of organic compounds in those soils, but we cannot detect them by the same technologies used by the Viking mission."
Navarro-Gonzalez's report is published in the current issue of the Proceedings of the National Academy of Sciences.
Mars-Like Earth Environments
The Viking landers performed chemical analyses by heating Martian soil samples to vaporize their contents into a gas.
They examined the gas by spectrometer—a device that measures the wavelengths emitted by chemicals in the gas—which revealed no clear evidence of microorganisms in the soil.
But Navarro-Gonzalez's team duplicated the Viking tests in Mars-like Earth environments and found that they missed present signs of life in soil samples from Antarctic dry valleys, the Atacama Desert of Chile and Peru, and other locales.
(See a National Geographic magazine feature on the Atacama Desert: "The Driest Place on Earth".)
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