is often rejected on Mars because it is contrary to "expectations." Expectations require a sterile surface. Thus, there can be no simple biological explanation for observations.

If there is (or was) life on the surface of Mars, I'm sure we have already seen plenty of evidence of it. But seeing and accepting are two entirely different things. Many would much rather erect incredibly complicated abiological hypotheses to explain observations that are much more simply explained by biology. And what is most delightful, there is then no test of these hypotheses on Mars, nor is there any further test for biology. Yet the abiological interpretations -- based on hypotheses that have never been tested on Mars (surely the exotic oxidants at least would have been tested for by now) -- are in line with expectations requiring a sterile surface, ergo, they are accepted, wheras a biological interpretation which is in line with the evidence is rejected.

It's just not time yet for another paradigm shift!

;-)