Triton's photochemical model: Update

Abstract

Titan, Triton, and Pluto are the Solar System objects with the nitrogen-ethane atmospheres. The progress in Titan's photochemistry, stimulated by the very successful Cassini-Huygens mission, facilitated photochemical interpretation of Pluto's atmosphere that was observed by the New Horizons mission and supporting ground-based measurements. Here is an update of Triton's atmospheric and ionospheric photochemistry that combines the previous photochemical model (Krasnopolsky and Cruikshank 1995, Photochemistry of Triton's atmosphere and ionosphere, J. Geophys. Res. 100, 21,271-21,286) with findings in Titan's and Pluto's photochemistry and progress in chemical kinetics.Our modeling of Triton's photochemistry is restricted to the conditions of the Voyager 2 flyby in 1989, when data on vertical profiles of temperature, haze, and densities of N2, CH4, N, and electrons were obtained using radio- and solar UVS occultations and imaging.Significant differences between the composition and photochemistry of Triton's atmosphere and those of Titan and Pluto are mostly caused by the low CH4 tropospheric abundance of 100 ppm, smaller than those on Titan and Pluto by two orders of magnitude. This methane is strongly depleted photochemically to a ppb level in the upper atmosphere of Triton, while it is enriched by diffusive separation on Titan and Pluto. Therefore atomic species and their chemistry are much more abundant on Triton than on Titan and Pluto. Atomic ions C+ and N+ with slow recombination rates dominate in Triton's ionosphere with electron densities, significantly exceeding those on Titan and Pluto.The proposed model agrees with all observational constraints and presumes thermal escape of H2 + H with a rate of 45 g cm−2 Byr−1, loss of nitrogen of 40 g cm−2 Byr−1 on thermal escape and precipitation of nitriles, and sublimation and loss of methane of 270 g cm−2 Byr−1.C2H6 and HCN are minor species in the photochemical precipitations on Triton, and their detection in Triton's ice may refer to either the primordial composition or ice photochemistry.Seven versions of the model are calculated to study its sensitivity to some basic input parameters. The model can be used for planning future studies of Triton's atmosphere. An analog of the Ion-Neutral Mass Spectrometer onboard Triton's orbiter looks promising.