The evolution of icy satellite interiors and surfaces

The satellites of the outer solar system planets are thought to be mixtures of ices and rocky material, in which decay of radioactive nuclides can lead to internal melting and solid-state convection. Time-dependent models indicate that melting will reach its maximum extent approximately 2.0 GYr after formation; bodies of radius <500 km will never melt, and those <750 km in radius will be totally refrozen by present. Surface water flows are not expected for bodies of <1500-km radius. However, even small (100 km) bodies may be unstable against solid-state convection, and their surfaces may show signs of tectonism. Other processes altering the surfaces include sublimation and photolysis of ices. Sublimation likely explains the absence of CH₄ ices on any Saturnian satellite except Titan; photolysis explains the absence of NH₃ ices on these bodies, and possibly the absence of water ice on the surface of Callisto. The photolysis rate of CH₄ also implies a crustal reservoir of CH₄ on Titan.