Letter to the Editor

First detection of CO in Uranus

¹ LESIA, Observatoire de Paris, 92195 Meudon, France
² MPI, Postfach 1603, 85740 Garching, Germany
³ JPL, Pasadena, CA 91109, USA
⁴ The University of Michigan, Ann Arbor, MI 48109-1243, USA

Corresponding author: Th. Encrenaz, Therese.Encrenaz@obspm.fr

Received: 30 September 2003
Accepted: 24 November 2003

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 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 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 H₂S. Upper limits of and 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.