Detection of water vapour and atmospheric structure on Titan from ISO observations

A. Coustenis ¹ A. Salama ², E. Lellouch ¹, Th. Encrenaz ¹, Th. de Graauw ³, G. L. Bjoraker ⁴, R. E. Samuelson ⁴, D. Gautier ¹, H. Feuchtgruber ², M. F. Kessler ², & G. Orton ⁵

¹ DESPA, Paris-Meudon Observatory, France

² ISO Science Operation Center, Madrid, Spain

³ SRON, Groningen, The Netherlands

⁴ NASA/GSFC, Greenbelt, MD, USA

⁵ JPL, Pasadena, CA, USA

Observations of Titan in the thermal infrared were performed by ISO, using the Grating and the Fabry-Perot modes of SWS in the 2-45  region in January and December 1997, during Titan's Greatest Eastern Elongation. The resolving power of the SWS/Grating varies between 1500 and 3000 in the Grating mode and reaches 30000 in the Fabry-Pérot mode, where some short observations were dedicated to Titan in the 15-16  region. Two pure rotational water lines were observed using the ISO/SWS in the Grating mode (R=2000) at 39.4 and 43.9 m, with fluxes of about 2 Jy over a continuum of 60 Jy (Coustenis et al., 1998a). The associated signal-to-noise ratio is about 8. The observed flux can be reproduced with a constant abundance of , or with the photochemical profile of Lara et al. (1996) multiplied by a factor of 0.4. This yields a H₂O vapor mole fraction of about 10^-8 at the 400 km altitude level (column density of ). The inferred water influx at Titan at 700 km of altitude is : , compatible with the observed CO2 abundance and similar to that found at Saturn (Feuchtgruber et al., 1997). This suggests that the infalling material from Saturn rings is not the dominant source for Saturn's water. Future work on the input from the local or the interplanetary components is required before a better understanding of these processes is achieved. The 233-1500  spectrum of ISO/SWS affords 10 times higher spectral resolution than Voyager 1/IRIS. We have modeled the spectrum using, as a first step, the Voyager-derived atmospheric parameters (Coustenis et al., 1993; Coustenis and Bézard, 1995) and have improved the precision of the thermal and compositional structure of Titan inferred on a disk-average. >From the ISO data we have recovered a new temperature profile for Titan's disk average (from the analysis of the n4 CH₄ band at 7.7 ), as well as more precise mean molecular abundances for the minor species with emission bands in the thermal infrared (Coustenis et al. 1998b). In particular, we find a new determination of the CH3D abundance to yield a D/H ratio in Titan's stratosphere of . This value is four times less than in comets and suggests that Titan's atmosphere is not of cometary origin, but rather formed by outgassing from the interior. The enrichment in deuterium observed in Titan today with respect to the protosolar nebula can be explained by the enrichment found in CH₄ present in grains which formed Titan in the saturnian subnebula and by fractionation mechanisms. We have inferred upper limits for some likely candidates in Titan's stratosphere (such as benzene and allene) and tested available vertical profiles. Future work involves the analysis of ISO/LWS, ISO/SWS/Fabry-Pérot, ISO/PHT-S and other ISO observations of Titan.