TEMPERATURE, N2, AND N DENSITY PROFILES OF TRITONS ATMOSPHERE - OBSERVATIONS AND MODEL

Improved analysis of the Voyager Ultraviolet Spectrometer (UVS) observ ations of the solar occultation by Triton yields the isothermal temper ature and N2 number densities in the altitude range 475-675 km: T = 10 2 +/- 3 K, [N2]= (4 +/- 0.4) x 10(8) cm-3 at 575 km. A distinct step-l ike absorption feature at 850 angstrom is due to the atomic nitrogen i onization continuum. It allows a measurement of N number densities in the range from 170 to 570 km. which correspond to diffusive equilibriu m above 300 km with [N] = (1 +/- 0.25) x 10(8) cm-3 at 400 km and T = 100 +/- 7 K. Deviations from diffusive equilibrium become important be low 300 km, and [N] = (5 +/- 2.5) x 10(8) cm-3 at 200 km. The exobase altitude is 870 km, and the total escape rate of atomic nitrogen is (1 +/- 0.3) x 10(25) s-1. The main condensible product of methane chemis try is ethylene, C2H4, with a peak number density of 6 x 10(6) cm-3 ne ar 25 km. The striking similarity of the thermospheric properties at b oth occultation sites despite substantial differences in latitudes, se asons, local time, and incoming flux of magnetospheric electrons impli es very effective winds and suggests that one-dimensional modeling is applicable. Temporal variations of the temperature profile should be r ather small in spite of strong variations of the electron flux. The pr esence of CO in the atmosphere as suggested by recent measurements of the CO absorption bands in the surface ice results in cooling by the r otational lines, deactivation and radiation of vibrational excitation of N2, and heating by quenching of N(2D). Heating efficiencies of elec tron precipitation and solar EUV radiation in the ranges of the N2 con tinuum and bands are calculated equal to 0.2-0.29), 0.24, and 0.19, re spectively. The column integrated cooling by the CO lines is weaker th an 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, H-2, and H densities are calculated. The calculated profile of atomic nitro gen and its escape rate are in excellent agreement with the measured v alues. Mixing ratios of H-2 and H at 400 km are equal to 240 and 23 pp m, respectively. The total escape rate of hydrogen atoms (H + 2H-2) is determined by photolysis of methane by H Lyman alpha radiation, does not depend on the ionospheric processes which transform H-2 to H, and is equal to 2.3 x 10(26) s-1. The ratio of H to N escape rates on Trit on, 20, does not correspond to the measured ratio H+/N+ is similar to 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 t he temperature profile calculated for a CO mixing ratio of 10(-3), a r atio of the average to the maximum electron fluxes of 0.162, and inclu des data on temperature, N2, N, CH4, H-2, and H number densities,