Other
Scientific paper
Oct 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995jgr...10011271k&link_type=abstract
Journal of Geophysical Research, vol. 100, p. 21,271,286
Other
9
Photochemical Reactions, Triton, Atmospheric Chemistry, Composition (Property), Extreme Ultraviolet Radiation, Interstellar Matter, Temperature Measurement, Turbulent Diffusion, Solar Radiation, Reaction Kinetics, Photolysis, Mixing Ratios, Lyman Alpha Radiation, Ions, Ice, Hydrocyanic Acid, Gas Density, F Region, E Region
Scientific paper
The photochemistry of 32 neutral and 21 ion species in Triton's atmosphere is considered. Parent species N2, CH4, and CO (with a mixing ratio of 3 x 10-4 in our basic model) sublime from the ice with rates of 40, 208, and 0.3 g/sq cm/b.y., respectively. Chemistry below 50 km is driven mostly by photolysis of methane by the solar and interstellar medium Lyman-alpha photons, producing hydrocarbons C2H4, C2H6, and C2H2 which form haze particles with precipitation rates of 135, 28, and 1.3 g/sq cm/b.y., respectively. Some processes are discussed which increase the production of HCN (by an order of magnitude to a value of 29 g/sq cm/b.y.) and involve indirect photolysis of N2 by neutrals. Reanalysis of the measured methane profiles gives an eddy diffusion coefficient K = 4 x 103sq cm/s above the tropopause and a more accurate methane number density near the surface, (3.1 +/- 0.8)x IO11/cu cm. Chemistry above 200 km is driven by the solar EUV radiation (lambda less than 1000 A) and by precipitation of magnetospheric electrons with a total energy input of 108 W (based on thermal balance calculations). The most abundant photochemical species are N, H2, H, 0, and C. They escape with the total rates of 7.7 x 1024/ s, 4.5 x 1025/s, 2.4 x 1025/s, 4.4 x 1022/s, and 1.1 x 1024, respectively. Atomic species are transported to a region of 50-200 km and drive the chemistry there. Ionospheric chemistry explains the formation of an E region at 150-240 km with HCO(+) as a major ion, and of an F region above 240 km with a peak at 320 km and C(+) as a major ion. The ionosphere above 500 km consists of almost equal densities of C(+) and N(+) ions. The model profiles agree with the measured atomic nitrogen and electron density profiles. A number of other models with varying rate coefficients of some reactions, differing properties of the haze particles (chemically passive or active), etc., were developed. These models show that there are four basic unknown values which have strong impacts on the composition and structure of the atmosphere and ionosphere. These values and their plausible ranges are the CO mixing ratio fco = 10-4 - 10-3, the magnetospheric electron energy input (1 +/- 0.5) x 108 W, the rate coefficient of charge-exchange reaction N2(+) + C(kappa) = 10-11 - 10-10cu cm/s, and the ion escape velocity upsiloni approx. equals 150 cm/s.
Cruikshank Dale P.
Krasnopolsky Vladimir A.
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