One of the most interesting fields of speculation has to do with the Martian past. There seems to be ample evidence that Mars was totally different during the first years after accretion, and in many ways remarkably similar to Earth. Recent findings from the Mars Global Surveyor mission are giving even more proof for the existence of rivers. The presence of craters among the channels means that there was water on Mars at the period of heavy bombardment.
Liquid water is, in a way, the operational definition for life.
Mars is so dead now because of the absence of liquid water. One of the most asked questions today is whether there is life on Mars at the moment. The principal goal of the Viking landers was to settle this issue. Both twin landers performed identical experiments on soil that was sampled at their respective landing sites. The Viking team had different interpretations on the results, which could have been originated from purely inorganic chemical reactions triggered off by UV radiation or by some sort of living process.