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[20.03] Altitude dependent Titanfs atmospheric chemistry: UV irradiation experiments at low temperature combined with cold plasma irradiation

H. Imanaka, B.N. Khare, E.L.O. Bakes (SETI Inst./NASA ARC), Y. Sekine, C.P. McKay, D.P. Cruikshank (NASA ARC), M. McGuigan, J.H. Waite, R. Sacks (Univ. Michigan)

Active organic chemistry in Titanfs atmosphere forming organic haze and various gas species is induced by solar UV radiation and charged particle irradiations. The main available energy sources depend on altitude, so that the physical and chemical properties of the haze and the condensates of hydrocarbons and nitriles may depend on altitude. To understand Titanfs atmospheric chemistry as a function of altitudes, we have conducted laboratory simulations using UV irradiation and/or charged particle irradiation as energy sources. We utilize GCMS for analyzing the chemical components of gas products, condensed ice mixtures, and pylorizates from tholins.

First, the gas products and tholins were formed from methane-nitrogen gas mixture through cold plasma irradiation. This simulates the upper atmosphere of Titan where methane and nitrogen are dissociated by short UV and charge particle irradiations. The gas products were fractionally condensed by cold traps at several temperatures. More than 100 gas species were detected by GCMS, including saturated and unsaturated aliphatic hydrocarbons, substituted aromatics, nitriles, nitrogen heteroaromatics such as pyrrole, pyridine, pyrazine, pyrazole. Nitrogen containing compounds larger than (C+N)4 were mainly detected from ice mixture condensed at 196 K. Py-GCMS analysis of tholin revealed various alkanes, nitriles, and substituted aromatics as well as substituted pyrroles.

In the lower stratosphere of Titan, various gas species diffused from the upper atmosphere may undergo further photolysis by long UV. To simulate the lower stratosphere, the gas products from cold plasma were further irradiated by long UV lights (> about 150 nm) at about 196 K and 1 mbar. The unsaturated hydrocarbons were reduced and formed the tholin materials, although the production rate was very small.

Our results can be directly applied to the Huygens mission in 2005, which will measure gas compositions (GCMS) and aerosol particles (ACP-GCMS) at several altitudes in Titanfs lower atmosphere.

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.