“Then you say, ‘A-ha, there must be compensation,’ ” Dr. Stevenson said. “Something more dense under the ice. The natural candidate is water.”
-It's sentences like that last one that give non-scientists pause. The article seems like a declaration of water definitively on Enceladus but then it appears to be speculative. Which is it? How certain are we?
Well, it's probably a little bit more complicated than that. If there's a cavity of known size, then one could predict with a high degree of precision how the gravitational pull should be affected. Then you see a different pull and you calculate that there must be a mass of size X causing the change. Then you say, ok, we have a mass X and a cavity of size Y; making a density calculation is trivial at that point. I'm guessing they did this calculation, and found that it is 1000kg/m^3 within error, and there probably aren't any other candidate liquids that fit that density. (For example, liquid methane, which has been observed on other moons, is about 450kg/m^3, or something like that. Not really even close.) Couple the density calculation with the fact that they know from observation that there are ice and ice geysers on the surface, and I think it's an if-it-looks-like-a-duck-and-quacks-like-a-duck type situation. These are just my speculations, however :)
I would consider that this is a journalist quoting a scientist. The fault might lie there. Whenever I have been a party to these things the result has always been cringe-worthy on our end. Every single time there is a quote that is either out of context, or lacks context. It amazes me, but the SOP for journalists when covering science seems to be to get the quotes, write the story, then send it to their editor. I don't think they typically ask the scientists for a proofread. At any rate, I would assume that their reasoning extends beyond speculation in this case. They probably have good reasons to exclude other possibilities.