DPS Meeting, Madison, October 1998
Session 26. Solar System Astronomy with ISO and Prospects for SIRTF I
Invited Plenary Session, Wednesday, October 14, 1998, 9:10-10:25am, Madison Ballroom A and B

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[26.02] Detection of New Hydrocarbons on the Giant Planets

B. B\'ezard (Obs. Paris-Meudon)

The Short-Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO) recorded spectra of the four giant planets at a resolving power of ~1500. On Jupiter, Saturn, and Neptune, the 7-16 \mum range exhibits prominent emission bands from CH4 (and its deuterated isotope CH3D), C2H6, and C2H2. On Uranus, only C2H2 emission is observed in this spectral range.

On Saturn, methyl-acetylene (CH3C2H), diacetylene (C4H2), and benzene (C6H6) have been detected for the first time. The derived column densities are about 5\times1015, 3\times1014, and 7\times1013 molecules cm-2 respectively. ISO also detected benzene on Jupiter. On the other hand, a search for methyl-acetylene and diacetylene on Neptune yielded negative results.

In addition, the methyl radical (CH3), a direct product of methane photolysis, was detected on both Saturn and Neptune through emission in the \nu 2 band at 16.50 \mum. The CH3 column density is in the range 1.5-7.5\times1013 molecules cm-2 on Saturn and about 2\times1013 molecules cm-2 on Neptune. The Saturn measurement implies either a CH3 self recombination rate higher than assumed in current photochemical models or an eddy mixing coefficient near the methane homopause smaller than inferred from Voyager ultraviolet measurements.

Photochemical models are currently being developed with reference to these ISO measurements to better understand the quantitative details of hydrocarbon chemistry. Besides the existing observations, I will briefly discuss the information derived from these investigations.

The author(s) of this abstract have provided an email address for comments about the abstract: Bruno.Bezard@obspm.fr

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