Photochemistry of the atmosphere of Titan - Comparison between model and observations

Abstract

The photochemistry of simple molecules containing carbon, hydrogen, nitrogen, and oxygen atoms in the atmosphere of Titan has been investigated using updated chemical schemes and our own estimates of a number of key rate coefficients. Proper exospheric boundary conditions, vertical transport, and condensation processes at the tropopause have been incorporated into the model. It is argued that he composition, climatology, and evolution of Titan's atmosphere are controlled by five major processes: (a) photolysis and photosensitized dissociation of CH4; (b) conversion of H to H2 and escape of hydrogen; (c) synthesis of higher hydrocarbons; (d) coupling between nitrogen and hydrocarbons; (e) coupling between oxygen and hydrocarbons. Starting with N2, CH4, and H2O, and invoking interactions with ultraviolet sunlight, energetic electrons, and cosmic rays, the model satisfactorily accounts for the concentrations of minor species observed by the Voyager IRIS and UVS instruments. Photochemistry is responsible for converting the simpler atmospheric species into more complex organic compounds, which are subsequently condensed at the tropopause and deposited on the surface. Titan might have lost 5.6 x 10(4), 1.8 x 10(3), and 4.0 g cm-2, or the equivalent of 8, 0.25, and 5 x 10(-4) bars of CH4, N2, and CO, respectively, over geologic time. Implications of abiotic organic synthesis on Titan for the origin of life on Earth are briefly discussed.

On the volatile inventory of Titan from isotopic abundances in nitrogen and methane

“…Voyager 1 data, combined with photochemical models, yield a current photolysis rate that is well-determined to within a factor of two (Yung et al, 1984;Lara et al, 1994;Toublanc et al, 1995). Although the published models di er in the proportions of the various products of methane photolysis, there is close agreement in total destruction rate of methane: photolysis is limited by the solar ultraviolet ux with energies above the threshold for methane dissociation.…”

“…The homopause altitude has been constrained from Voyager ultraviolet spectrometer pro®les of methane, along with other data, to lie at 985 to 1125 km altitude, equivalent to radii of 3560 to 3700 km (Strobel et al, 1992;Vervack, 1997). Earlier authors computed homopause levels as low as 3200 km from Titan's center based on photochemical models (Yung et al, 1984). Hence we parameterize our results in terms of a homopause situated between 3200 and 3700 km, but prefer values above 3550 km.…”

Reaction of the C2H Radical with 1-Butyne (C4H6): Low-Temperature Kinetics and Isomer-Specific Product Detection

“…6,27,28 Low temperature rate constants and product branching ratios are used when available to describe accurately and reliably 29 the chemical schemes 27 that are proposed. In this respect, much progress 30 has been made experimentally to obtain kinetic data on key chemical reactions down to very low temperatures relevant to interstellar chemistry.…”

Analytic investigation of climate stability on Titan: sensitivity to volatile inventory

“…The methane in Titan's atmosphere is destroyed by photolysis on short timescales (H10 6 years, see Yung et al, 1984;Lara et al, 1994Lara et al, , 1996Toublanc et al, 1995)Ðmainly through conversion into ethaneÐits presence in the present epoch implies a resupply mechanism and/or bu ering by a surface or near-surface reservoir. Thus we consider that a reservoir of ethane, methane and nitrogen is in thermodynamic equilibrium with the atmosphere, although the reservoir may in part be subsurface (Eluskiewicz and Stevenson, 1990;Kossacki and Lorenz, 1995).…”