If water ran on Mars in the past, life may have originated there too. This became an important theme for the Viking mission, which had two spacecraft, each consisting of an orbiter and a lander and carrying identical sets of instruments. The orbiters measured global temperature and presence of water vapor and the landers photographed the surface, organically analyzed surface samples, and attempted to detect metabolic activity of soil microorganisms. The cameras sent back memorable landscapes of a desolate, rocky Mars with no traces of life. This was no surprise, since only microbial life seemed possible for Mars, and the Viking cameras could not have detected it.
Meteorites provide nonbiological sources of organic matter.
So it seems that Mars should have accumulated enough organic material over the ages to be detected by the sensitive Viking instrument. Of the three microbiological experiments Viking carried, two assumed that martian organisms, if any, lived in an aqueous environment. These experiments brought martian soil into contact with aqueous solutions of organic compounds and measured the metabolic response. All three experiments sent positive signals from Mars. Given the different assumptions of the experiments, it seemed that they could not all be detecting the same martian life.