科研之友
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摘要
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 mu m 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 N-2 is also of importance below that altitude. CH4-CH4 vibrational exchange of v(4)-quanta affects their population below 500 km. We found that the v(3) -> ground band dominates Titan's 3.3 mu m daytime limb radiance above 750 km whereas the v(3) + v(4) -> v(4) band does below that altitude and down to 300 km. The v(3) + v(2) -> v(2), the 2v(3) -> v(3), and the (CH4)-C-13 -> 53 ground bands each contribute from 5% to 8% at regions below 800 km. The v(3) + 2v(4) -> 2v(4)and v(2) + v(3) + v(4) -> v(2) + v(4) 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 mu m. 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.
- 出版日期2011-8
