ISO observations of C2H2 on Uranus and CH3 on Saturn

Details ISO observations of C2H2 on Uranus and CH3 on Saturn ISO observations of C2H2 on Uranus and CH3 on Saturn

Mar 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999esasp.427..149a&link_type=abstract

The Universe as Seen by ISO. Eds. P. Cox & M. F. Kessler. ESA-SP 427., p. 149

Physics

4

Saturn, Uranus, Titan, Giant Planets, Photochemistry, Atmospheric Vertical Mixing

Scientific paper

In October 1996, ISO/SWS recorded the spectra of Uranus in the 7-16.5 μm. range and detected the spectral signatures of acetylene (C2H2). Interpretation of the C2H2 density profile with photochemical models leads to a value of (5-10)x103 cm2s-1 for the eddy diffusion coefficient (Kh) at the Uranus homopause, which for these limits of Kh corresponds to 354 km (0.037 mb) and 390 km (0.02 mb) above the 1-bar level. The eddy coefficient derived from these globally averaged observations is found to be in agreement with the values obtained by Voyager for the equatorial-low latitudes ten years earlier. The ISO-Voyager consistency in Kh indicates that whatever little manifestable internal energy Uranus possesses may still be adequate and responsible for controlling the behaviour of vertical mixing in its visible atmosphere, and that the dynamical behaviour of the upper atmosphere is fairly uniform over the planet. In December 1997, methyl radicals (CH3) were also detected by ISO/SWS in the atmosphere of Saturn. The derived stratospheric column abundance of CH3, (1.5-7.5)x1013 cm-2, is about a factor of 10 smaller than the predictions of methane photochemical models, however. The discrepancy could be explained by one of two means: either the value of the eddy diffusion coefficient used in the models -- the Voyager value -- is too high by at least a factor of 100, or the rate constant for the self-reaction loss of CH3 is too low by at least a factor of 10. It is argued that Kh could not possibly be reduced so drastically from the Voyager epoch. Instead, we believe the source of the discrepancy between the ISO derived CH3 and the model calculations lies in the poor knowledge of laboratory chemical kinetics data on certain CH3 loss reactions, which has serious implications for the interpretation of observations of atmospheres of the giant planets and Titan.

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