Abstract
The spectrum of Uranus has been recorded in Oct.-Nov. 2002, between 4.6 and 5.0 μm, using the ISAAC imaging spectrometer at the VLT-UT1 (ANTU) 8-m telescope of ESO. The spectral resolving power was 1500. In addition to a few strong H 3 + emission lines, the spectrum of Uranus distinctly shows the emission lines of the CO(1-0) band from R7 to P8. The relative intensity distribution of the observed CO emission is not compatible with a thermal distribution, for any value of the rotational temperature. The most likely emission mechanism is fluorescence, and a good fit is obtained assuming a constant CO mixing ratio of 3 x 10 - 8 at the tropopause and above. The tropospheric continuum of Uranus is also detected beween 4.7 and 5.0 μm. The observed continuum can be fitted assuming reflected sunlight above a cloud level at 3.1 bars, presumably attributed to H 2 S. Upper limits of 2 x 10 - 8 and 1 x 10 - 6 are inferred for the CO and PH 3 tropospheric mixing ratios above this level. The low CO tropospheric upper limit might suggest that the CO vertical distribution is not uniform.
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