EDP Sciences
Free access
Volume 413, Number 2, January II 2004
Page(s) L5 - L9
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20034637

A&A 413, L5-L9 (2004)
DOI: 10.1051/0004-6361:20034637


First detection of CO in Uranus

Th. Encrenaz1, E. Lellouch1, P. Drossart1, H. Feuchtgruber2, G. S. Orton3 and S. K. Atreya4

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

(Received 30 September 2003 / Accepted 24 November 2003)

The spectrum of Uranus has been recorded in Oct.-Nov. 2002, between 4.6 and 5.0  $\mu$m, 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 $3\times 10^{-8}$ at the tropopause and above. The tropospheric continuum of Uranus is also detected beween 4.7 and 5.0  $\mu$m. The observed continuum can be fitted assuming reflected sunlight above a cloud level at 3.1 bars, presumably attributed to H 2S. Upper limits of $2\times 10^{-8}$ and $1\times 10^{-6}$ 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.

Key words: planets and satellites: Uranus -- infrared: solar system

Offprint request: Th. Encrenaz, Therese.Encrenaz@obspm.fr

SIMBAD Objects in preparation

© ESO 2004