While I am not a biologist myself, I did work 2 years at the NASA Astrobiology Institute in a supporting role, where I learned all about the science surrounding this stuff. I don't know any astrobiologist who seriously believes that whatever microbes might exist in the comfort of Europa's inner ocean might survive free-floating in the harsh Jupiter radiation belts, where it takes hours or days (not millions of years) to sterilize all but the most extreme life forms.