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Cassini and Enceladus really allowed us to see the kind of things we could do with mass spectrometers and, more importantly, with material that's coming up straight out of the ocean, it's a way of viewing the ocean without drilling into it. We didn't necessarily have to land ; we could sit there and and sample to study quite a bit about these ocean worlds just from flying through the material that comes out of the interior. That's what the plumes are about on Europa as well. It's that connection to the interior ocean.