They've thought about it. Such questions are kind of funny, because they take for granted that the scientists' standards are akin to the pop science stuff that

They've thought about it. Such questions are kind of funny, because they take for granted that the scientists' standards are akin to the pop science stuff that everybody else writes. They aren't, and that is what makes good straw man arguments. I submitted yesterday -with little success- a nice talk of Jonathan Lunine (Cornell) at the Harvard-Smithsonian Center for Astrophysics, here it is again: https://www.youtube.com/watch?v=m_mb59aQblY https://www.youtube.com/watch?v=m_mb59aQblY . This, albeit introductory, is what scientist do. If you find a planet with an atmosphere rich in free oxygen, you can bet it's life that has put it there, because we have a pretty good idea about the abundance of the elements and other scenarios are unlikely. At the beginning of the Archean, there was no O2 in our atmosphere. All there is now comes from water, CO2 and photosynthesis. As scientists are usually serious about their jobs, abiogenesis models take into account the more likely primeval atmosphere and conditions: https://en.wikipedia.org/wiki/Abiogenesis https://en.wikipedia.org/wiki/Abiogenesis What is exciting about the search of life in Europa, Enceladus, maybe Ganymede and who knows Triton (apparently there's no will to go there ever again), is that we have very good reasons to assume that at some depth under the ice crust there are hydrothermal vents like those on Earth. And maybe chemosynthesis is enough to get things started. Personally I find that there are more reasons to be excited about Enceladus than about... let's say Mars.