Pluto's Ocean Under Sputnik Planitia: Insulation by Clathrate and the Survival of Liquid at 40 Kelvin

Pluto's surface sits near 40 kelvin, yet gravity and geology at Sputnik Planitia suggest a subsurface ocean whose top may even drive the basin's reorientation. Survival of liquid under so cold a surface is the puzzle addressed here. Pluto's radiogenic power is 39.87 gigawatts, a mean flux of 2.248 milliwatts per square metre. A pure-ice conductive shell in equilibrium with that flux would be 556.2 kilometres thick, thicker than Pluto's entire plausible water layer, so a pure-water ocean under pure ice does not survive in steady state. A basal methane-clathrate layer changes the verdict: 50 kilometres of clathrate at 0.5 watts per metre per kelvin absorbs 224.8 kelvin of the temperature drop, and the total shell, ice plus clathrate, falls to 103.9 kilometres. An unheated 100-kilometre ocean would freeze in 0.04232 billion years, so the ocean also requires the heating to be sustained, which radiogenic decay supplies. Insulation, not extra power, is what makes Pluto's ocean plausible. Computed from stated assumptions, not measured.