The existence of microbial life on Mars remains highly controversial, but recent evidence of water, complex organic molecules, and methane in the Martian environment, combined with findings from the 1976 Viking mission, have led to the conclusion that existing life on Mars is a possibility that must be considered, as presented in an article in Astrobiology, a peer-reviewed journal from Mary Ann Liebert, Inc., publishers. The article is available free on the Astrobiology website until November 15, 2016.

In "The Case for Extant Life on Mars and Its Possible Detection by the Viking Labeled Release Experiment," coauthors Gilbert V. Levin, Arizona State University, Tempe, and Patricia Ann Straat, National Institutes of Health, Bethesda, MD (retired), clearly outline the evidence to support the "biological hypothesis", which argues that the results of the 1976 Viking Labeled Release experiment were positive for extant microbial life on the surface of Mars.

Further, Drs. Levin and Straat evaluate the "non-biological hypotheses" to explain the Viking results, which many scientists support, but the authors conclude that the experimental evidence supports a biological explanation and the likelihood that microorganisms were able to evolve and adapt to be able to survive in the harsh conditions of the Martian environment.

"Even if one is not convinced that the Viking LR results give strong evidence for life on Mars, this paper clearly shows that the possibility must be considered," says Chris McKay, PhD, Senior Editor of Astrobiology and an astrobiologist with NASA Ames Research Center, Moffett Field, CA.

"We cannot rule out the biological explanation. This has implications for plans for sample return from Mars and for future human missions."