Dione and Rhea as Negative Controls: Heat Budgets That Exclude Long-Lived Oceans

Not every icy moon should have an ocean, and a method that finds oceans everywhere is finding nothing. Dione and Rhea serve here as negative controls. Their radiogenic powers are only 2.71 and 4.672 gigawatts, giving equilibrium conductive shells of 1112 and 1255 kilometres, comparable to or exceeding any plausible water-layer thickness, and unheated freezing times of 0.01742 and 0.04238 billion years for the stated ocean depths. On present budgets both bodies are expected to be frozen through, consistent with the absence of confirmed induction or activity signatures. The same arithmetic that supports oceans elsewhere excludes them here. Computed from stated assumptions, not measured.