Mars after the viking missions: Is life still possible?

The results of the biology and organic chemistry experiments carried aboard the Viking landers seem to rule out the possibility of life on Mars. However, we present a possible model of Martian ecology which is based on the premise that Martian life could be endolithic, and thus missed by the Viking biology experiments. This model is based, in part, on the endolithic microbiota found in the dry valleys of Antarctica. In addition, we combine our model with a previously described model which suggests that Mars may go through ice ages and periods of relatively mild climate.