Temperature, N2, and N density profiles of Triton's atmosphere: Observations and model

Abstract

Improved analysis of the Voyager Ultraviolet Spectrometer (UVS) observations of the solar occultation by Triton yields the isothermal temperature and N2 number densities in the altitude range 475–675 km: T = 102 ± 3 K, [N2]= (4 ± 0.4) × 108 cm−3 at 575 km. A distinct steplike absorption feature at 850 Å is due to the atomic nitrogen ionization continuum. It allows a measurement of N number densities in the range from 170 to 570 km, which correspond to diffusive equilibrium above 300 km with [N] = (1±0.25) ×108 cm−3 at 400 km and T = 100±7 K. Deviations from diffusive equilibrium become important below 300 km, and [N] = (5 ± 2.5) × 108 cm−3 at 200 km. The exobase altitude is 870 km, and the total escape rate of atomic nitrogen is (1±0.3) × 1025 s−1. The main condensible product of methane chemistry is ethylene, C2H4, with a peak number density of 6 × 106 cm−3 near 25 km. The striking similarity of the thermospheric properties at both occultation sites despite substantial differences in latitudes, seasons, local time, and incoming flux of magnetospheric electrons implies very effective winds and suggests that one‐dimensional modeling is applicable. Temporal variations of the temperature profile should be rather small in spite of strong variations of the electron flux. The presence of CO in the atmosphere as suggested by recent measurements of the CO absorption bands in the surface ice results in cooling by the rotational lines, deactivation and radiation of vibrational excitation of N2, and heating by quenching of N(2D). Heating efficiencies of electron precipitation and solar EUV radiation in the ranges of the N2 continuum and bands are calculated equal to 0.2–0.29, 0.24, and 0.19, respectively. The column integrated cooling by the CO lines is weaker than the thermospheric heating and cannot form a mesosphere. A CO mixing ratio of up to 10−2 is consistent with thermal balance calculations. Sixteen optimized versions of the model are considered with profiles of T and [N2] which agree with the measurements. Profiles of N, H2, and H densities are calculated. The calculated profile of atomic nitrogen and its escape rate are in excellent agreement with the measured values. Mixing ratios of H2 and H at 400 km are equal to 240 and 23 ppm, respectively. The total escape rate of hydrogen atoms (H + 2H2) is determined by photolysis of methane by H Lyman α radiation, does not depend on the ionospheric processes which transform H2 to H, and is equal to 2.3 × 1026 s−1. The ratio of H to N escape rates on Triton, 20, does not correspond to the measured ratio H+/N+∼ 2–3 in Neptune's magnetosphere and should be taken into account in its modeling. The recommended model for Triton's atmosphere is based on the temperature profile calculated for a CO mixing ratio of 10−3, a ratio of the average to the maximum electron fluxes of 0.162, and includes data on temperature, N2, N, CH4, H2, and H number densities.