What's the Biggest Question in Chapter Seven?

There are lots of fun moments and i'm not sure i can pull out any a particular one. I enjoyed taking a deeper dive into the state of knowledge on other planetary bodies. So questions about what controls the energy available for life, i think about that a lot for mars. It's a very similar set of questions for incelidus and europa. But you have to make very different assumptions as to what controls theenergy availability.