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The abundance of H_2 in Titan's troposphere from the Cassini CIRS investigation

机译：卡西尼号CIRS调查显示，泰坦对流层中H_2的丰度

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摘要

We derived the mole fraction of H_2 in the troposphere of Titan from an analysis of the H_2-N_2 dimer S_0(0) rotational transition seen near 355 cm~(-1) in spectra recorded by the Composite Infra-Red Spectrometer (CIRS) on the Cassini spacecraft between March 2006 and February 2007. We find a globally-averaged value of (9.6 ± 2.4) × 10~(-4), consistent with the earlier determination based on the Voyager IRIS dataset, and with the value obtained from the Gas Chromatograph Mass Spectrometer (GCMS) experiment on the Huygens probe, but a factor of ~4 lower than that measured by the Ion and Neutral Mass Spectrometer (INMS) in the 1025-1475 km altitude range. Interestingly, the latitudinal distribution of H_2 appears to be non-uniform, with a mole fraction above 50 N in excess of 30-70% ( + 40%) relatively to the globally-averaged value. Such a non-uniform distribution is unexpected since H_2 is thought to be chemically inert in the lower atmosphere of Titan. It is proposed that the observed enhancement in the north polar region is caused by downwelling occurring during the winter season in the polar vortex. This interpretation is dependent on the existence of a vertical gradient of the H_2 concentration in the stratosphere, as suggested by the differences between the INMS values and those measured by GCMS and CIRS, although such a gradient is not consistent with the present understanding of dynamics and chemistry in Titan's atmosphere.
机译：通过对复合红外光谱仪（CIRS）记录的光谱中355 cm〜（-1）附近的H_2-N_2二聚体S_0（0）旋转跃迁进行分析，得出了泰坦对流层中H_2的摩尔分数。 2006年3月至2007年2月之间的卡西尼号航天器。我们发现（9.6±2.4）×10〜（-4）的全球平均值与先前基于Voyager IRIS数据集确定的值一致，并且与从气相色谱质谱仪（GCMS）在惠更斯探针上进行的实验，但比在1025-1475 km高度范围内的离子和中性质谱仪（INMS）测量的因子低约4倍。有趣的是，H_2的纬度分布似乎不均匀，相对于全球平均值，摩尔分数高于50 N超过30-70％（+ 40％）。这种不均匀的分布是出乎意料的，因为人们认为H_2在Titan的低层大气中是化学惰性的。建议在北极地区观测到的增强是由于冬季在极地涡流中发生的下涌而引起的。这种解释取决于平流层中H_2浓度的垂直梯度的存在，这是由INMS值与GCMS和CIRS测量的值之间的差异所暗示的，尽管这种梯度与当前对动力学和动力学的理解不一致。泰坦大气中的化学反应。

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