I have no difficulty imagining a scenario in which some forms of terrestrial microbes survive and proliferate extremely rapidly in nutrient-rich regions of warm oceans under Europa's ice. Assuming they have a correspondingly high mutation rate, certain aquatic life as may exist there might soon become their hosts, with potentially disastrous results. I would like to be assured that it couldn't happen, and I'd like to know why it couldn't.

"Time is what prevents everything from happening at once" - John Wheeler