Unearthly Life On Mars
From the media standpoint -- and therefore that of most people -- the Viking Martian biological experiments were uncompromisingly negative. However, R. Lewis points out that this is simple not so. The labelled-release experiments on both landers produced positive results every time a nutrient was added to fresh Martian soil. (The nutrient was tagged with carbon-14, and radioactive carbon dioxide always evolved, suggesting biological metabolism.) Further, the soil samples, when sterilized by heat, gave uniformly negative results. On earth. such repeatable experiments would be considered strong evidence that life existed in the samples. The reason the Viking experiments were described as "negative" is that the other two life detection experiments produced negative or equivocal results. The gas chromatograph, for example, detected no organic molecules in the Martian soil; and it is difficult to conceive of life without organic molecules. At first, most scientists preferred to explain the ambiguous life-detection-experiment results in terms of strange extraterrestrial chemistry. Nevertheless, strange extraterrestrial life would explain the data equally well. Everyone should be aware that the Viking biology team still considers life on Mars as a real possibility.
(Lewis, Richard; "Yes. There Is Life on Mars," New Scientist, 80:106, 1978.)
Comment. Most research into the possibility of extraterrestrial life assume "life-as-we-know-it." For more information on the Viking experiments, read entry AME14 in our Catalog: The Moon and the Planets. Ordering information here.