Physical state of volatiles on the surface of Triton

Abstract

The most recent analyses of infrared spectrophotometric studies of Neptune's satellite Triton concluded that both condensed methane and nitrogen are present^1,2. It was also concluded² that the most likely surface configuration is a liquid nitrogen (N₂) ‘ocean’ with dry areas of solid methane (CH₄), and perhaps some exposed fine-grained water frost (H₂O). However, this model runs into some difficulties, especially when requirements of phase equilibrium between the solid and liquid components are imposed. Because an understanding of the distribution and state of volatiles is crucial in interpreting secular changes in Triton's appearance due to seasonal effects³, and in planning observing strategies for the Voyager–Neptune/Triton encounter, we assess here several possible configurations for these volatiles on Triton. We conclude that the simplest volatile configuration which best satisfies the constraints with the least number of ad hoc assumptions is N₂ and CH₄ both in solid forms, perhaps partly as a microscopic mixture, but more probably as a disequilibrium assemblage, non-uniformly distributed. Thermodynamic equilibrium is then limited by seasonal transport and the finite diffusion time of CH₄ in crystalline N₂. Although a nitrogen ocean cannot be excluded, it requires very restrictive assumptions.