Some breaking news, after this story was complete, was a news release that Callisto seems to have an underground ocean! I added a new sentence to the story to respond to this -- right now the evidence is shaky, and the story assumes that it will be refuted before these future events take place. The only evidence is that something is altering Callisto's magnetic field -- perhaps a liquid ocean? But then what is the heat source? I have trouble with the ocean idea, but what if something else is changing Callisto's magnetic field?