Analysis of Titan CH₄ 3.3 μm upper atmospheric emission as measured by Cassini/VIMS

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• 作者：Maya Garc&#xed ; a-Comas^a ; ^{maya@iaa.es" rel="nofollow} ; Manuel Ló ; pez-Puertas^a ; Bernd Funke^a ; Bianca M. Dinelli^b ; Maria Luisa Moriconi^c ; Alberto Adriani^d ; Antonio Molina^a ; ^e ; Angioletta Coradini^d
• 关键词：Titan ; atmosphere ; Infrared observations ; Atmospheres ; composition ; Abundances ; atmospheres ; Radiative transfer
• 刊名：Icarus
• 出版年：2011
• 出版时间：August 2011
• 年：2011
• 卷：214
• 期：2
• 页码：571-583
• 全文大小：1035 K
• ISSN：S0019103511001114

文摘

After molecular nitrogen, methane is the most abundant species in Titan’s atmosphere and plays a major role in its energy budget and its chemistry. Methane has strong bands at 3.3 μm emitting mainly at daytime after absorption of solar radiation. This emission is strongly affected by non-local thermodynamic equilibrium (non-LTE) in Titan’s upper atmosphere and, hence, an accurate modeling of the non-LTE populations of the emitting vibrational levels is necessary for its analysis. We present a sophisticated and extensive non-LTE model which considers 22 CH₄ levels and takes into account all known excitation mechanisms in which they take part. Solar absorption is the major excitation process controlling the population of the v₃-quanta levels above 1000 km whereas the distribution of the vibrational energy within levels of similar energy through collisions with N₂ is also of importance below that altitude. CH₄–CH₄ vibrational exchange of v₄-quanta affects their population below 500 km. We found that the ν₃ → ground band dominates Titan’s 3.3 μm daytime limb radiance above 750 km whereas the ν₃ + ν₄ → ν₄ band does below that altitude and down to 300 km. The ν₃ + ν₂ → ν₂, the 2ν₃ → ν₃, and the ¹³CH₄ ν₃ → ground bands each contribute from 5% to 8% at regions below 800 km. The ν₃ + 2ν₄ → 2ν₄and ν₂ + ν₃ + ν₄ → ν₂ + ν₄ bands each contribute from 2% to 5% below 650 km. Contributions from other CH₄ bands are negligible. We have used the non-LTE model to retrieve the CH₄ abundance from 500 to 1100 km in the southern hemisphere from Cassini–VIMS daytime measurements near 3.3 μm. Our retrievals show good agreement with previous measurements and model results, supporting a weak deviation from well mixed values from the lower atmosphere up to 1000 km.