Laboratory Chemical Dynamics in Hydrocarbon Rich Atmospheres of Outer Planets

Abstract

Investigating the formation of organic molecules in hydrocarbon rich atmospheres of planets and their moons is of paramount importance to understand the chemical composition, mixing ratios, and the future evolution of carbon bearing species in Solar System environments. The diacetylene molecule (C4H2) as detected in the stratosphere of Saturn and its moon Titan is the largest polyatomic organic species observed so far in the outer Solar System. In addition the methyl radical (CH3) together with methane (CH4), acetylene (C2H2), ethylene (C2H4), ethane (C2H6), methylacetylene (CH3CCH), propane (C3H8) and the CN bearing species hydrogen cyanide (HCN), dicyan (NCCN), methykyanide (CH3CN), cyanoacetylene (HCCCN), and presumably solid state dicyanoacetylene (NCC-CCN) were detected as well. Photochemical models of Jupiter, Saturn, Uranus, Neptune, Pluto and their satellites Titan and Triton predict further the presence of even more complex hydrocarbons up to triacetylene (C6H2) and tetraacetylene (C8H2). Previous experiments attempting to unravel the formation of these molecules in planetary environments employed and open reactors subjecting gas mixtures to high energy discharges or electron bombardments; often products were analyzed via gas chromatography coupled to a mass spectrometer (GS-MS). This approach can match the existence of major trace constituents. However, no information on radical intermediates could be supplied; further, possible open shell products and extremely labile products cannot be identified; hence involved reaction mechanism can only be guessed. However, the explicit identification of reaction intermediates, ALL reaction products, and the mechanism are a top priority to give a systematic picture on the chemistry in hydrocarbon rich atmospheres and to predict the formation of hitherto unobserved molecules.