2004A&A...413L...5E

The spectrum of Uranus has been recorded in Oct.-Nov. 2002, between 4.6 and 5.0µm, 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 H₃⁺ 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 3x10^–8 at the tropopause and above. The tropospheric continuum of Uranus is also detected beween 4.7 and 5.0µm. The observed continuum can be fitted assuming reflected sunlight above a cloud level at 3.1bars, presumably attributed to H₂S. Upper limits of 2x10^–8 and 1x10^–6 are inferred for the CO and PH₃ tropospheric mixing ratios above this level. The low CO tropospheric upper limit might suggest that the CO vertical distribution is not uniform.