Passage: Callisto's battered surface lies on top of a cold, stiff, and icy lithosphere that is between 80 and 150 km thick. A salty ocean 150–200 km deep may lie beneath the crust, indicated by studies of the magnetic fields around Jupiter and its moons. It was found that Callisto responds to Jupiter's varying background magnetic field like a perfectly conducting sphere; that is, the field cannot penetrate inside Callisto, suggesting a layer of highly conductive fluid within it with a thickness of at least 10 km. The existence of an ocean is more likely if water contains a small amount of ammonia or other antifreeze, up to 5% by weight. In this case the water+ice layer can be as thick as 250–300 km. Failing an ocean, the icy lithosphere may be somewhat thicker, up to about 300 km.

Question: How thick can water and ice theoretically be on Callisto?
250–300 km