The abundance of H2 in Titan's troposphere from the Cassini CIRS investigation

Abstract

We derived the mole fraction of H2 in the troposphere of Titan from an analysis of the H2–N2 dimer S0(0) rotational transition seen near 355cm−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 1025–1475km altitude range. Interestingly, the latitudinal distribution of H2 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 H2 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 H2 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.