"These results might seem to bode well for life's spread; after all, it might take just one impact of a microbe-bearing rock to turn Europa or Enceladus from habitable to inhabited. But there are more factors to consider, and they tamp down the optimism."
There is a critical assumption that is the foundation of all searches for life on other worlds. The well attested law of abiogenesis, this law of science is as well attested as Kepler's planetary laws, Newton's laws of motion, or the law of gravity - thus the law of abiogenesis is operating throughout the universe and in our solar system billions of years ago. I was glad to read some caution in the report. I watched a recent SCI program (How The Universe Works) where molten creatures swim around in a hot, molten sea on some remote, hot jupiter type exoplanet - as a possibility for alien life too.