[scoofy]

>So from a strictly physical point of view, I would argue that the odds of Earth "infecting" Europa with life in the past 3 billion years are pretty much 100%.

This is exactly why europa is so important. It's that the gravity well of jupiter is so great, that infection of europa is extremely unlikely, rather than being extremely likely. Europa would be a vastly better case for arguing separate origins of life than any other place in our solar system.

Since earth has only existed for a short time in the grand scheme of things, and supposing that life indeed sprung here on its own, the odds of a random rock from a random few impacts landed on europa, rather than jupiter, in such a way to get to survive is absurdly small.