The 1976 Viking missions to Mars have been our only attempts to date to search directly for life on another planet. The pair of identical Viking landers carried a trio of life detection experiments that were a marvel of early 1970s-era miniaturized technology. In addition, a gas chromatograph-mass spectrometer (GCMS) was carried to analyze the vapors released during heating of a sample to detect any organic compounds in the Martian soil. Unfortunately the scientific results of the Viking life detection experiments were ambiguous at best. While there was activity observed in all the life detection experiments with activity in one in particular (i.e. the LR or Labeled Release experiment) giving a positive result based on protocols established before the Viking flight, the lack of any detectable organic compounds in the Martian soil cast doubt on this interpretation.
The consensus of the planetary science community was that Viking did not detect life on Mars and that the activity observed in the life detection experiments was due to simple, abiotic chemistry. Scientists hypothesized that some sort of powerful oxidant generated by the high UV flux and hyperarid conditions on the Martian surface was responsible for the observed activity instead. But after almost a third of a century of observations, laboratory experiments and debate, it was NASA’s Phoenix mission to Mars which seemed to finally identify the long sought oxidizing agent in 2008: a highly reactive family of compounds called perchlorates.
Unfortunately, the identification of perchlorates in the Martian soil only complicated matters further. The rejection of a biological explanation for the apparent positive results from Viking’s Labeled Release experiment and the unusual activity in the other experiments back in 1976 was based in part on the lack of any detectable organic compounds in the soil. Unfortunately even if organic compounds were present, they would have been destroyed when the Viking GCMS heated its soil samples in the presence of the perchlorates to produce carbon dioxide, water vapor and traces of other gases including chlorinated hydrocarbons which had been detected by Viking but were dismissed at the time as being just traces of cleaning agents used on the instrument before launch. While Phoenix and NASA’s Curiosity rover, currently exploring Mars, have been equipped with more sophisticated instruments to detect organic compounds on Mars, their efforts have been complicated by the presence of these perchlorates. And the more complex task of detecting life on Mars has not been directly addressed by any Mars mission since Viking.
Just a few days ago, I received my June 2014 issue of Scientific American and discovered a great article: “How to Search for Life on Mars” by Christopher P. McKay of NASA Ames Research Center and Victor Parro Garcia of the Center for Astrobiology in Spain. The authors, in addition to addressing the complications of the presence of perchlorates in the Martian soil, contend that the Viking life detection experiments were flawed from the start. These experiments, which had been developed back in the 1960s, used culture-based methods which are no longer considered definitive in detecting life even in terrestrial soil samples since only a small fraction of microbes can be grown in a culture. In this article, McKay and Garcia outline modern approaches to life detection experiments that rely on determining the presence of key biomarkers in the soil. Although there still remains the question of which of the multitude of potential biomarkers should be sought, these techniques are now advanced enough that instruments employing these new methods could be built and included in future Mars lander or rover missions. And while the authors were focused on the search for life on Mars in this article, these methods could also be applied to search for life or its traces in other biocompatible environments elsewhere in the solar system.
This article is well worth reading and I highly recommend it.
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Here is a 1978 NASA documentary on the Viking science results (narrated by Viking Science Team member, the late Harold Masursky, if I am not mistaken) called “19 Minutes to Earth”. The video was originally produced for use in high school science classes and is definitely a product of the late 1970s but it is still a good review of the Viking science results that is worth viewing.
“Automated Biological Laboratory”, Drew Ex Machina, August 7, 2014 [Post]
“Viking and The Question of Life on Mars, Part 1”, SETIQuest, Volume 3, Number 3, pp. 1-6, Third Quarter 1997 [Article]
“Viking and The Question of Life on Mars, Part 2”, SETIQuest, Volume 3, Number 4, pp. 1-7, Fourth Quarter 1997 [Article]
Christopher P. McKay and Victor Parro Garcia, “How to Search for Life on Mars”, Scientific American, Vol. 310, No. 6, pp. 44-49, June 2014 [Article Access]