Observations of hydrocarbons in the giant planets

Details Observations of hydrocarbons in the giant planets Observations of hydrocarbons in the giant planets

Mar 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999esasp.427..153b&link_type=abstract

The Universe as Seen by ISO. Eds. P. Cox & M. F. Kessler. ESA-SP 427., p. 153

Computer Science

6

Solar System, Planets And Satellites: Jupiter, Saturn, Uranus, Neptune

Scientific paper

Observations of the giant planets with the Short Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO) are presented. Emission bands from various hydrocarbons are detected in the 7-16.5 μm range. These hydrocarbons are produced from the photochemistry of methane in the upper atmosphere. Analysis of the C2H6 and C2H2 emission bands yields constraints on the abundance profiles of these compounds, and indirectly on the eddy mixing rate of the atmosphere in the 0.1-3 mbar region, a poorly-known parameter. On Saturn, methyl-acetylene (CH3C2H), diacetylene (C4H2), and benzene (C6H6) have been detected for the first time. ISO also found evidence for benzene on the Jovian disk, while the Voyager spacecraft could observe it only in the northern auroral region. Photochemical models are currently being developed with reference to these ISO measurements to better understand the quantitative details of hydrocarbon photochemistry. On Saturn, the overall agreement between model and observations is excellent except for C2H4 and C3H8 which were not seen by ISO and are overpredicted by the model. An eddy mixing coefficient of the order of 3x104 cm2 s-1 is derived around the 1-mbar level, much lower than at the methane homopause. On Uranus, analysis of the C2H2 emission combined with the lack of emission in the 7.7-μm CH4 band leads to a value of 5-10x104 cm2 s-1 for the eddy mixing coefficient at the homopause, confirming the sluggishness of the atmospheric mixing. Methyl radical (CH3), a direct product of methane photolysis, was detected on both Saturn and Neptune through emission in the ν 2 band at 16.5 μm. Column densities of 3.5+4-2x1013 and 1.6+1.2-0.9x1013 molecules cm-2 were respectively derived. The CH3 abundance is mostly sensitive to the poorly-known methyl recombination rates and to the eddy mixing profile. These measurements imply either a CH3 self recombination rate higher than assumed in current models or an eddy mixing coefficient smaller than inferred from Voyager ultraviolet measurements.

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