The Viking biological experiments on Mars

The essential findings of the three biological experiments aboard the two Viking Mars landers are reviewed and compared. All three of the experiments yielded significant data in repeated tests of Martian surface samples. Some of the results are consistent with a biological interpretation, although there are serious reservations in accepting this conclusion. Most of the findings, however, are inconsistent with a biological basis. The combined data suggest the presence of several classes of oxidants on Mars and these would account for the most of the observations. An explanation for the apparent small synthesis of organic matter in the pyrolytic release experiment remains obscure.