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Issue A&A
Volume 403, Number 1, May III 2003
Page(s) L7 - L10
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20030414



A&A 403, L7-L10 (2003)
DOI: 10.1051/0004-6361:20030414

Letter

The infrared spectrum of Neptune at 3.5-4.1 microns: Search for H $\mathsf{_{3}^{+}}$ and evidence for recent meteorological variations

H. Feuchtgruber1 and Th. Encrenaz2

1  Max-Planck-Institut für extraterrestrische Physik, Postfach 1603, 85740 Garching, Germany
2  LESIA, Observatoire de Paris, 5 place Janssen, 92195 Meudon Cedex, France
    e-mail: Therese.Encrenaz@obspm.fr

(Received 24 February 2003 / Accepted 19 March 2003 )

Abstract
The infrared spectrum of Neptune at $3.5{-}3.75~\mu$m and $3.87{-}4.1~\mu$m has been measured at a spectral resolution of 1200. The observed flux is stronger by a factor ~3 compared to previous measurements, suggesting important meteorological variations between 1997 and 2002. The flux is detected mostly from a bright belt at mid-southern latitudes. Strong absorptions, identified as methane, are observed over the L band. The observed ${\rm CH_4}$ spectrum can be fitted by a multilayer model assuming that the solar light is partly reflected above several layers, including the ${\rm CH_4}$ haze and stratospheric photochemical hazes. An upper limit to a disk averaged column density of ${\rm H}_{3}^{+}$ of $2.9\left({{+7.1}\atop{-1.8}}\right)\times10^{10}~{\rm cm^{-2}}$ is reported, consistent with present ionospheric models.


Key words: line: formation -- radiative transfer -- planets and satellites: individual: Neptune -- infrared: solar system

Offprint request: H. Feuchtgruber, fgb@mpe.mpg.de

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© ESO 2003


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