Volatile Transport Modeling on Triton with New Observational Constraints

Neptune’s moon Triton shares many similarities with Pluto, including volatile cycles of N2, CH4and CO, and represents a benchmark case for the study of surface-atmosphere interactions onvolatile-rich Kuiper Belt objects. The observations of Pluto by New Horizons acquired during the2015 flyby and their analysis with volatile transport models (VTMs) shed light on how volatilesublimation-condensation cycles control the climate and shape the surface of such objects. Withinthe context of New Horizons observations as well as recent Earth-based observations of Triton,we adapt a Plutonian VTM to Triton, and test its ability to simulate its volatile cycles, therebyaiding our understanding of its climate.Here we present numerical VTM simulations exploring the volatile cycles of N2, CH4 and CO onTriton over long-term and seasonal timescales (cap extent, surface temperatures, surfacepressure, sublimation rates) for varying model parameters (including the surface ice reservoir,albedo, thermal inertia, and the internal heat flux). We explore what scenarios and modelparameters allow for a best match of the available observations. In particular, our set of observational constraints include Voyager 2 observations (surface pressure and cap extent),ground-based near-infrared (0.8 to 2.4 μm) disk-integrated spectra (the relative surface area ofvolatile vs. non-volatile ice) and the evolution of surface pressure as retrieved from stellaroccultations.Our results show that Triton’s poles act as cold traps for volatile ices and favor the formation ofpolar caps extending to lower latitudes through glacial flow or through the formation of thinnerseasonal deposits. As previously evidenced by other VTMs, North-South asymmetries in surfaceproperties can favor the development of one cap over the other. Our best-case simulations areobtained for a bedrock surface albedo of 0.6-0.7, a global reservoir of N2 ice thicker than 200 m,and a bedrock thermal inertia larger than 500 SI (or smaller but with a large internal heat flux).The large N2 ice reservoir implies a permanent N2 southern cap (several 100 m thick) extendingto the equatorial regions with higher amounts of volatile ice at the south pole, which is notinconsistent with Voyager 2 images but does not fit well with observed full-disk near-infraredspectra. Our results also suggest that a small permanent polar cap exists in the northern (currentlywinter) hemisphere if the internal heat flux remains relatively low (e.g. radiogenic, < 3 mW m-2). Anon-permanent northern polar cap was only obtained in some of our simulations with high internalheat flux (30 mW m-2). The northern cap will possibly extend to 30°N in the next decade, thusbecoming visible by Earth-based telescopes. On the basis of our model results, we also discussthe composition of several surface units seen by Voyager 2 in 1989, including the bright equatorialfringe and dark surface patches.Finally, we provide predictions for the evolution of ice distribution, surface pressure and CO andCH4 atmospheric mixing ratios in the next decades. According to our model, the surface pressureshould slowly decrease but remain larger than 0.5 Pa by 2060. We also model the thermallightcurves of Triton for different climate scenarios in 2022, which serve as predictions for futureJames Webb Space Telescope observations.