On The H2 Abundance And Ortho-to-para Ratio In Titan's Troposphere

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Titan

We have analyzed spectra recorded between 50 and 650 cm−1 by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft at low and high emission angles to determine simultaneously the H2 mole fraction and ortho-to-para ratio in Titan's troposphere.

We used constraints from limb spectra between 50 and 900 cm−1 and from in situ measurements by the Huygens probe to characterize the temperature, haze and gaseous absorber profiles. We confirm that the N2-CH4 collision-induced absorption (CIA) coefficients used up to now need to be increased by about 52% at temperatures of 70-85 K. We find that the N2-N2 CIA coefficients are also too low in the N2 band far wing, beyond 110 cm−1, in agreement with recent quantum mechanical calculations. We derived a H2 mole fraction equal to (0.88 ± 0.13) × 10−3, which pertains to the ∼1-34 km altitude range probed by the S0(0) and S0(1) lines. The H2 para fraction is close to equilibrium in the 20-km region. We have investigated different mechanisms that may operate in Titan's atmosphere to equilibrate the H2 o-p ratio and we have developed a one-dimensional model that solves the continuity equation in presence of such conversion mechanisms.

We conclude that exchange with H atoms in the gas phase or magnetic interaction of H2 in a physisorbed state on the surface of aerosols are too slow compared with atmospheric mixing to play a significant role. On the other hand, magnetic interaction of H2 with CH4, and to a lesser extent N2, can operate on a timescale similar to the vertical mixing time in the troposphere. This process is thus likely responsible for the o-p equilibration of H2 in the mid-troposphere implied by CIRS measurements.

Bruno Bézard, Sandrine Vinatier
(Submitted on 23 Sep 2019)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
DOI: 10.1016/j.icarus.2019.03.038
Cite as: arXiv:1909.10458 [astro-ph.EP] (or arXiv:1909.10458v1 [astro-ph.EP] for this version)
Submission history
From: Bruno Bézard
[v1] Mon, 23 Sep 2019 16:21:45 UTC (1,008 KB)
https://arxiv.org/abs/1909.10458
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