system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can scientists know one of Saturn's moons has water?
gpt
There is a probe out among Saturn's moons we sent out about 17 years ago. While we already knew there was ice on the moon (it is all over the surface, kilometers thick) we didn't know that any was liquid. The moon was thought to be geologically inactive.

However the probe was precisely measuring the gravity around the southern pole and detected an anomaly; there was slightly more mass than could be accounted for by ice. This combined with the discovery of massive plumes of water being ejected from some cracks in the surface of the ice allowed the deduction that the unexplained density variation was due to liquid water (which is more dense than ice). By measuring the gravitational anomaly they could even determine the rough size of this subsurface ocean.