DPS 2001 meeting, November 2001
Session 38. Titan Posters
Displayed, 9:00am Tuesday - 3:00pm Saturday, Highlighted, Friday, November 30, 2001, 9:00-10:30am, French Market Exhibit Hall

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[38.01] The D/H ratio on Titan from ISO and IRSHELL data

A. Coustenis, B. Bezard, Th. Encrenaz, D. Gautier, E. Lellouch (DESPA, Paris-Meudon Observatory, France), A. Salama (ISOC, ESA/Vilspa, Spain), J. Lacy (Univ. of Texas, USA), G. Orton (JPL, Pasadena, USA)

The ISO/SWS 1997 observations of Titan in the 600-1400 cm-1 range, where emission bands of CH3D and of methane exist, were analyzed using a radiative transfer code and an atmospheric model as described in Coustenis et al. (2001). From the best fit of the 7.7 \mum methane band we obtained a nominal temperature profile which we used - assuming values of the CH4 stratospheric abundance in the 1-3.5% range (the nominal value is 1.9%) - to find: CH3D = 6.7+2.9- 1.9 10-6 and hence a D/H ratio of 8.8 +3.2-1.9 10-5, with most of the uncertainty due to the ignorance of the exact methane abundance value.

The ISO CH3D abundance and the D/H ratio are in good agreement with results from ground-based observations by Orton (1992), who found a D/H ratio of 7.75 ±2.25 10-5, and by Coustenis et al. (1993) who reported similar values from data obtained in 1992 with IRSHELL/IRTF and calibrated against the Voyager observations. Recently, by re-calibrating this data with ISO, we find a CH3D / CH4 measure of 3.2 10-4, from which we find a preliminary best fit value of D/H to be about 7.9 10-5. This work is in progress and aims to improve the D/H ratio determination significantly by combining all the data.

Indeed, it appears that we may be reaching a consensus on the D/H value. The Voyager 1 value (Coustenis et al., 1989), of 1.5 +1.4-0.5 10-4, is higher, but still compatible with the upper limits of the ISO and IRSHELL data. Based on these results new constraints may be set on cosmogonical scenarios such as the one proposed by Mousis et al. (2001) which suggests among other that the atmospheric CH4 is continuously replenished by outgassing from the interior of Titan.


- Coustenis, A., et al., 1993. XVIII EGS Assembly, Wiesbaden, Germany, 3-7 May, Proceedings in Ann. Geoph., Springer Intern. Eds, Vol. 11, p. C460.

- Coustenis, A., et al., 1989. Icarus 82, 67.

- Coustenis, A., et al., 2001. Submitted for publication.

- Mousis, O., et al., 2001. Submitted for publication in Icarus. See also paper in this DPS meeting.

- Orton, G. 1992. Proc. of the Titan Symp., Toulouse, France, 9-12 Sept. 1991. ESA-SP 338, 81.

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