PHOTOCHEMISTRY OF TRITON ATMOSPHERE AND IONOSPHERE

Citation
Va. Krasnopolsky et Dp. Cruikshank, PHOTOCHEMISTRY OF TRITON ATMOSPHERE AND IONOSPHERE, J GEO R-PLA, 100(E10), 1995, pp. 21271-21286
Citations number
83
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Geosciences, Interdisciplinary","Astronomy & Astrophysics
Journal title
JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS
ISSN journal
2169-9097 → ACNP
Volume
100
Issue
E10
Year of publication
1995
Pages
21271 - 21286
Database
ISI
SICI code
2169-9097(1995)100:E10<21271:POTAAI>2.0.ZU;2-2
Abstract
The photochemistry of 32 neutral and 21 ion species in Triton's atmosp here is considered. Parent species N-2,. CH4, and CO (with a mixing ra tio of 3x10(-4) in our basic model) sublime from the ice with rates of 40, 208, and 0.3 g/cm(2)/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 C2H 2 which form haze particles with precipitation rates of 135, 28, and 1 .3 g/cm(2)/b.y., respectively. Some processes are discussed which incr ease the production of HCN (by an order of magnitude to a value of 29 g/cm(2)/b.y.) and involve indirect photolysis of N-2 by neutrals. Rean alysis of the measured methane profiles gives an eddy diffusion coeffi cient K = 4 x 10(3) cm(2)/s above the tropopause and a more accurate m ethane number density near the surface, (3.1 +/- 0.8) x 10(11) cm(-3). Chemistry above 200 km is driven by the solar EUV radiation (lambda < 1000 Angstrom) and by precipitation of magnetospheric electrons with a total energy input of 10(8) W (based on thermal balance calculations ), The most abundant photochemical species are N, H-2, H, O, and C. Th ey escape with the total rates of 7.7 x 10(24) s(-1), 4.5 x 10(25) s(- 1), 2.4 x 10(25) s(-1) 4.4 x 10(22) s(-1), and 1.1 x 10(24) s(-1), res pectively. Atomic species are transported to a region of 50-200 km and drive the chemistry there. Atomic spheric chemistry explains the form ation 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. Th e ionosphere above 500 km consists of almost equal densities of C+ and N+ ions. The model profiles agree with the measured atomic nitrogen a nd electron density profiles, A number of other models with varying ra te coefficients of some reactions, differing properties of the haze pa rticles (chemically passive or active), etc., were developed. These mo dels show that there are four basic unknown values which have strong i mpacts on the composition and structure of the atmosphere and ionosphe re, These values and their plausible ranges are the CO mixing ratio f( infinity) = 10(-4)-10(-3), the magnetospheric electron energy input (1 +/- 0.5)x 10(8) W, the rate coefficient of charge-exchange reaction N -2(+) + C k = 10(-11)-10(-10) cm(3)/s, and the ion escape velocity nu( i) approximate to 150 cm/s,