37th DPS Meeting, 4-9 September 2005
Session 22 Outer Planets II
Oral, Tuesday, September 6, 2005, 11:00am-12:30pm, Law LG19

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[22.06] First Results of Middle-Infrared Spectroscopy of Uranus and Neptune from Spitzer

G. Orton (JPL / Caltech), M. Burgdorf (Astrophys. Res. Inst. / Liverpool J. M. U.), V. Meadows (IPAC / Caltech), J. Van Cleve (Ball Aerospace), D. Crisp (JPL / Caltech), J. Stansberry (U. Arizona), S. Atreya (U. Michigan)

We present disk-averaged middle-infrared spectra of Uranus and Neptune which were measured with the Infrared Spectrometer, IRS, aboard Spitzer. These high-SNR data span the spectral region between 5 and 37 \mum (270 and 1950 cm-1); one set of data has a resolving power R ~ 64--128 between 5.2 and 38 \mum which is useful for establishing the accurate photometry and another set has R ~ 600 between 10 and 37 \mum. For Uranus, besides verifying the presence of C2H2, these spectra show strong evidence for several new hydrocarbons in its stratosphere: C4H2, C3H4 and C2H6. A previous model (Orton et al. 1987. Icarus \bf 70, 1) of spectrally neutral high-altitude hazes in the stratosphere contributing substantially to the 7--14 \mum continuum is clearly incorrect; the continuum spectrum emulates the characteristics expected of the H2 continuum very well. The H2 S(2) quadrupole is evident in emission at 9.7 \mum. The spectrum of Neptune between 5.2 and 8.3 \mum, including a previously undetected spectral region between 5.2 and 7.3 \mum, is dominated by CH4 opacity, consistent with a 100-mbar volume mixing ratio in the range of 1--3 x 10-7. Besides verifying previous ground-based and ISO detections of CH4, C2H2, C2H4, C2H6, CH3D, CH3, H2, and CO2, we see a number of previously unidentified features, including two we have tentatively identified as C2H2 and C3H4. Comparisons between observations in May and November, 2004, show an apparent increase in C2H2 emission relative to C2H6 and CH4.

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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.