Organic chemistry on Titan

Abstract

Observations of nonequilibrium phenomena on the Saturn satellite Titan indicate the occurrence of organic chemical evolution. Features taken from various models of Titan's atmosphere are combined in a working composite model that provides environmental constraints within which different pathways for organic chemical synthesis are assessed. Experimental results and theoretical modeling studies suggest that the organic chemistry of the satellite is dominated by two atmospheric processes: photochemistry and energetic particle bombardment. Photochemical reactions of CH4 in the upper atmosphere can account for the presence of C2 hydrocarbons. Reactions initiated in various levels of the atmosphere by cosmic ray, Saturn ‘wind,’ and solar wind particle bombardment of a CH4‐N2 atmospheric mixture can account for the ultraviolet‐visible absorbing stratospheric haze, the reddish appearance of the satellite, and some of the C2 hydrocarbons. In the lower atmosphere, photochemical processes will be important if surface temperatures are sufficiently high for gaseous NH3 to exist. Hot H atom reactions initiated by photodissociation of NH3 can couple the chemical reactions of NH3 and CH4; if 0.1% of the incident ultraviolet light from 1600 to 2270 Å reaches the lower atmosphere, these reactions will be capable of producing organic matter at a rate comparable to or higher than that resulting from particle‐initiated reactions. Electric discharges are highly improbable on Titan; if they occurred at all, they would be restricted to the lower atmosphere and clouds. Their yield of organic matter might approach that of hot H atom reactions if the conversion of solar to electrical discharge energy on Titan was as efficient as that on earth. These assessments indicate that future missions to Titan should include organic chemical analyses of its atmosphere and surface among the prime science objectives.

Nitrogen Incorporation in CH4-N2 Photochemical Aerosol Produced by Far Ultraviolet Irradiation

“…The formation of HCN from the irradiation of CH 4 and N 2 mixtures at wavelengths > 125 nm was first reported by Dodonova (1966), with an approximate yield of 10 -5 HCN molecules per photon (Chang et al, 1979). This result was derived by using a very specific detection technique sensitive only to gas-phase HCN, with corroboration from measurement of trapped gases with a mass spectrometer.…”

“…Interestingly, when other workers in the 1970s and early 1980s attempted to repeat the results of Dodonova (1966) but failed to produce nitriles or N-containing organics in the UV (Ferris and Chen, 1975;Chang et al, 1979;Bossard et al, 1982;Raulin et al, 1982), Bossard et al (1982) concluded that ''UV light is not able to produce nitriles from CH 4 -N 2 mixtures'' and ''UV light is not an efficient source for producing unsaturated carbon chains.'' The results presented herein clearly contradict those conclusions.…”

Spectral Irradiance Measurements of Simulated Lightning in Planetary Atmospheres

“…Similar to the spectrum of terrestrial ability of convection energy (Flasar 1983, Lindal et al 1983 and condensable species in the lower atmosphere make it lightning (Orville andHenderson 1984, Wiedman et al 1989), the red portion of the spectrum is dominated by conceivable that some form of lightning discharge could be occurring (Chang et al 1979, Gupta et al 1981, Borucki atomic line radiation from the excited states of the neutral atoms, and the blue portion of the spectrum is dominated et al. 1984).…”

Physical properties of the organic aerosols and clouds on Titan

“…This basic picture of the organic nature of Titan's atmosphere (reviewed by Chang et al, 1979) was conÿrmed by the Voyager observations of Titan in 1980. Voyager images showed an optically thick haze in the stratosphere completely obscuring the surface at visible wavelengths (Smith et al, 1981.…”