Elsevier

Icarus

Volume 82, Issue 1, November 1989, Pages 67-80
Icarus

Titan's atmosphere from Voyager infrared observations: II. The CH3D abundance and DH ratio from the 900–1200 cm−1 spectral region

https://doi.org/10.1016/0019-1035(89)90024-9Get rights and content

Abstract

We have studied the 900–1200 cm−1 range in three different selections of ∼30 Voyager 1 IRIS spectra recorded in Titan's equatorial region. In particular, we have reanalyzed the 8.6-μm emission feature attributed to both C3H8 and CH3D bands. Observations were compared to synthetic spectra generated from an atmospheric model which incorporates recent results on the thermal structure and gas composition (in particular on the abundances of methane and propane) at Titan's equator from A. Coustenis, B. Bézard, and D. Gautier (1989, Icarus, 80, 54–76.). The best fit approach and the error analysis yielded a CH3D mole fraction of 1.1+0.7−0.6 × 10−5 in the stratosphere and a CH3D/ CH4 ratio of 6+5.6−2.1 × 10−4. A value of D/H was derived, 1.5+1.4−0.5 × 10−4, which is in conflict with the results of the previous analysis by S.J. Kim and J. Caldwell (1982, Icarus 52, 473–482)—but consistent with their revised value (T. Owen, B. L. Lutz, and C. de Bergh 1986, Nature (London) 320, 244–246)—and in excellent agreement with the estimation deduced from observations in the 1.6-μm CH3D band by C. de Bergh, B. L. Lutz, T. Owen, and J. Chauville (1988, Astrophys. J. 329, 951–955). The associated error bars result mainly from instrumental noise and uncertainties in the thermal profile. Deuterium appears to be clearly enriched in Titan, but present uncertainties in the DH ratio, both in the satellite and in the protosolar nebula, preclude a firm interpretation of the origin of the observed enrichment.

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