Mimas is only about 246 miles wide.
When Lainey and his colleagues ran computer models, the only simulated moon that precessed like the real Mimas was one with a deep ocean layer of liquid water inside, hidden away beneath 12 to 18 miles of ice.
That's surprising because, from the outside, Mimas looks like a solid ball of ice with a big dent on one side: a crater called Herschel, which is the centerpiece of the moon's whole Death Star cosplay.
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