Coupling of Internal Volatile Transport and Heatflow on Triton

Recently Brown eta/. (Science 250, 1991) showed that Triton's internal heat source could amount to 5-20% of the absorbed insolation on Triton, thus significantly affecting volatile transport and atmospheric pressure. Subsequently, Kirk and Brown (Proc. 22cd LPSC, 1991) used simple analytical models of the effect of internal heat on the distribution of volatiles on Triton's surface, confirming the speculation of Brown et a/. that Triton's internal heatflow could strongly couple to the surface volatile distribution. To further explore this idea, we present numerical models of the permanent distribution of nitrogen ice on Triton that include the effects of sunlight, the two-dimensional distribution of internal heatflow, the coupling of internal heatflow to the surface distribution of nitrogen ice, and the finite viscosity of nitrogen ice. From these models we conclude that: (1) The strong vertical thermal gradient induced in Triton's polar caps by internal heatflow facilitates viscous spreading to lower latitudes, thus opposing the poleward transport of volatiles by sunlight, and, for plausible viscosities and nitrogen inventories, producing ~permanent caps having considerable latitudinal extent; (2) It is probable that there is a strong coupling between the surface distribution of nitrogen ice on Triton and internal heatflow; (3) Asymmetries in the spatial distribution of Triton's heatflow, possibly driven by large-scale, volcanic activity or convection in Triton's interior, can result in permanent polar caps of unequal latitudinal extent, including the case of only one permanent polar cap; (4) In contrast to the solid-state greenhouse mechanism proposed by Brown et al. (Science 251, 1990), melting at the base of a permanent polar cap on Triton caused by internal heatflow can significantly enhance viscous spreading, as well as providing the necessary energy,' fluids, and/or gases to drive Triton's geyser-like plumes; (5) The atmospheric collapse predicted to occur on Triton in the next 20 years (Spencer, Geophys. 3 I?es. Lett., 1991) may be plausibly avoided because of the large latitudinal extent expected for permanent polar caps on Triton.