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

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 3.3μm 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 1000km 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 500km. We found that the ν3→ground band dominates Titan's 3.3μmdaytime limb radiance above 750km whereas the ν3+ν4→ν4 band does below that altitude and down to 300km. Theν3+ν2→ν2, the 2ν3→ν3, and the 13CH4 ν3→ground bands each contribute from 5% to 8% at regions below 800km. The ν3+2ν4→2ν4and ν2+ν3+ν4→ν2+ν4 bands each contribute from 2% to 5% below 650km. Contributions from other CH4 bands are negligible. We have used the non-LTE model to retrieve the CH4 abundance from 500 to 1100km in the southern hemisphere from Cassini-VIMS daytime measurements near 3.3μm. 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 1000km. © 2011 Elsevier Inc.