Sustainability of a subsurface ocean within Triton’s interior

Abstract

We present a study of coupled thermal and structural evolution of Neptune’s moon, Triton, driven by tidal dissipation and radiogenic heating. Triton’s orbital history likely involves capture from a binary system by Neptune, followed by a period of circularization. This work investigates Triton’s evolution past its circularization. We examine the rate of ice shell growth as a function of different orbital eccentricities, in the presence of radiogenic heating. Tidal dissipation in the ice shell, proportional to orbital eccentricity squared, concentrates heating near the base, reducing the basal heat flux. As the growth of the ice shell is proportional to the basal heat flux, increased tidal heating creates a blanketing effect, reducing the rate of ice shell growth. Radiogenic heating from Triton’s core is the other, more dominant, source of heat to the shell. Despite being several orders of magnitude higher than the tidal dissipation, radiogenic heating alone fails to sustain an ocean within Triton over 4.5Ga. For orbital eccentricities of 5×10−7 and 3×10−5 it takes approximately 2Ga and 3Ga, respectively, to completely freeze the ocean. For higher values of orbital eccentricities, an ocean can be sustained in Triton’s interior over 4.5Ga. If Triton’s history past circularization involves a slow decrease in orbital eccentricity to the current value, a thin, possibly NH3-rich ocean exists beneath Triton’s icy shell.