The challenges of driving Charon's cryovolcanism from a freezing ocean

A combination of geological interpretations and thermal-orbital evolution models imply that Pluto's large moon, Charon, had a subsurface water (and possibly ammonia) ocean that eventually froze. Ocean freezing generates large tensile stresses in the upper part of the ice shell and pressurizes the ocean below, perhaps leading to the formation of Charon's large canyons and putative cryovolcanic flows. Here, we identify the conditions in which a freezing ocean could create fractures that fully penetrate its ice shell, linking Charon's surface with its ocean and facilitating ocean-sourced cryovolcanism. We find that current models of Charon's interior evolution predict ice shells that are far too thick to be fully cracked by the stresses associated with ocean freezing. Either Charon's ice shell was 100 km) or the surface was not in direct communication with the ocean as part of the eruptive process. If Charon's ice shell had been thin enough to be fully cracked, it would imply substantially more ocean freezing than is indicated by the canyons, Serenity and Mandjet Chasma. Due to the low radiogenic heating within Charon and the loss of tidal heating early in its history, a thin ice shell should have been short-lived, implying that ocean-sourced cryovolcanic flows would have ceased relatively early in Charon's history, consistent with interpretations of its surface geology. An additional (and perhaps implausibly large) heat source would be required to generate the substantially larger ocean implied by through-going fractures. We also find that ocean freezing can easily generate deep fractures that do not fully penetrate to the ocean, which may be the foundation of Charon's canyons. When ocean-bearing moons begin to cool down, their oceans can freeze. As new ice accretes to the bottom of the existing ice shell, the added volume of the ice can stress the shell. Pluto's largest moon, Charon, has canyons and cryovolcanic flows that may have formed in response to a freezing ocean. Here, we model the formation of fractures within Charon's ice shell as the ocean underneath it freezes to explore the evolution of Charon's interior and surface. We find that an ocean source for cryovolcanic flows is unlikely because the ice shell would have had to be much thinner than current thermal evolution models imply. However, freezing the ocean may have produced the stresses that formed canyons later in Charon's history. •Charon's