What leads astronomers to believe that many of the larger moons associated with the giant planets have compositions that are roughly half water?

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Astronomers use the average density of a moon to estimate its composition. If the density is midway between that of water and rock, it suggests that the moon is about half water and half rock. Spectroscopic analysis can also provide insights into a moon's composition, but it alone may not definitively show that a moon is roughly half water. There is currently no evidence that space probes have drilled into the surfaces of many of these moons and detected water, and the observation of rocks and features forming in the presence of water is also not a definitive proof of the composition of the moon as a whole.

What Leads Astronomers to Believe That Many of the Larger