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T. Encrenaz (DESPA, Obs. Paris), G.S. Orton (JPL), P. Drossart, E. Lellouch (DESPA, Obs. Paris), S.K. Atreya (U. Michigan)
The 3-5 \mum spectrum of Uranus has been recorded at NASA's Infrared Telescope Facility (IRTF), using the SpeX imaging spectrometer, during two observing runs, in September 2000 and September 2001. The total integrating time was about 20 hours. We used a 0.8 x 15 arcsec slit which allowed us to record Uranus and the sky simultaneously along the slit and thus to subtract the sky emission. The spectral resolving power is about 1000.
The 3-5 \mum spectrum of Uranus is dominated by CH4 and CH3D absorptions which completely absorb the sunlight below 4.6 \mum. The absence of reflected sunlight allows the detection of numerous transitions in the \nu2 band of H3+, centered at 4 \mum, first detected in a few transitions by Trafton et al (Astrophys. J. 405, 761, 1993). Many H3+ emissions are detected in the IRTF spectrum and will be used to constrain the rotational temperature of H3+.
The 5-\mum window, previously detected by Orton and Kaminski (Icarus 77, 109, 1989) from photometric measurements, is also observed in our data with a maximum flux around 4.7 \mum. This window probes the lower troposphere of Uranus, presumably at pressure levels of a few bars. Calculations show that the shape of the window strongly depends upon the CO and PH3 tropospheric abundances of Uranus, which are presently unknown. Constraints upon these parameters will be discussed from the analysis of the IRTF data.