Microbial life forms probably wouldn't shake anybody's faith.
-jws
Its funny you should say this because when I was a JW this was the one thing that I had ever thought about and that I knew would destroy my belief in the Bible. Intelligent life wasn't a big deal for me because "God could have just created them." However, if microbial life was ever found on mars I remember thinking that would show that evolution was true - and thus Adam and Eve were false. No Adam and Eve means no original sin. No original sin means no need for Jesus sacrafice.
Looking back I now know there are much better ways to disprove the claims in the Bible - but back then it was the one thing I knew would really shake my beliefs. I think for most JWs, finding microbial life in our solar system would make them serriously question their fath. The implications are too hard to ignore. And I'm sure the WTBTS would spend considerable resources trying to dispute such findings with their usual nonsense, "The microbes could have hitched a ride on the rover from earth, the microbes aren't really microbes they're an equiptment glitch, scientest can't be trusted, etc."