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B. Bézard (Obs. Paris-Meudon), J.I. Moses (Lunar and Planetary Inst.), J. Lacy, T. Greathouse, M. Richter (U.\ Texas), C. Griffith (Northern Arizona Univ.)
On November 30 and December 2--4 2000 UT, we observed Jupiter and Saturn with the new high--resolution, mid--infrared, grating spectrograph Texes mounted at the NASA/Infrared Telescope Facility (IRTF). In particular, we recorded spectra in the 948-- to 953--cm-1 range at a resolution of 0.017 cm-1 to search for ethylene (C2H4), a still undetected compound outside of the tempera\-ture--enhanced auroral regions. The integration time was about 15 min.\ on Jupiter and 30 min.\ on Saturn with the ~8--arcsec slit oriented North--South and centered on the disk. Another 30 min.\ was spent on Saturn with the slit 7" east of the central meridian to enhance stratospheric emissions. Spectra in the 1230--1234 cm-1 region were also recorded to constrain the stratospheric temperature profile. About ten C2H4 lines are clearly detected on Jupiter and Saturn. The stratospheric column density inferred from radiative transfer calculations is about 2--3\times1015 molecules cm-2 on both planets. Recent photochemical models (Moses {\em et al.\/} 2000, {\em Icarus\/} 143, 244--298 for Saturn; Moses {\em et al.\/} 2001, {\em Jupiter: Planets, Satellites & Magnetosphere\/} conference proceedings, p.\ 78, June 25-30, 2001, Boulder, CO for Jupiter) overestimate the actual C2H4 abundance on both planets by a factor 3--5. Implications of these results on the hydrocarbon photochemistry will be discussed. One interesting puzzle is the similarity of the C2H4 abundances on both planets. In contrast, the C2H2 abundance on Jupiter is observed to be a factor of ~2--4 times less that on Saturn. Because the photochemistries of C2H2 and C2H4 are highly interconnected, these new C2H4 observations may provide hints as to the different photochemical or transport processes occurring on the two planets.