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A. Coustenis (DESPA, Paris-Meudon Observatory, France), A. Salama (ISO Sc. Oper. Center, Madrid, Spain), E. Lellouch, Th. Encrenaz (DESPA), Th. de Graauw (S.R.O.N., Groningen, The Netherlands), G. L. Bjoraker, R. E. Samuelson (NASA/GSFC, Greenbelt, MD, USA), D. Gautier (DESPA), H. Feuchtgruber, M. F. Kessler (ISO Sc. Oper. C.), G. S. Orton (JPL, CA, USA)
Observations of Titan in the thermal infrared (2.36-45.2 \mum) were performed by ISO, in Jan. and Dec. 1997, with resolving powers between 1500 and 3000 in the Grating mode and up to 30000 in the Fabry-Perot mode. Two pure rotational water lines were observed using the ISO/SWS/Grating (R=2000) at 39.4 and 43.9 \mum, with fluxes of about 2 Jy over a continuum of 60 Jy [1], with S/N~8. The flux observed can be reproduced with a constant abundance of ~4 \times 10-10, or with a recent photochemical profile [2] multiplied by a factor of {\em 0.4}. This yields a H2O vapor mole fraction of about 10-8 at the 400 km altitude level (column density of 2.5 \times 1014 mol cm-2). The inferred water influx at Titan at 700 km of altitude is : (0.8-2.8) \times 106 mol cm-2s-1, compatible with the CO2 observed abundance and similar to that found at Saturn [3]. This suggests that infalling material from Saturn rings may not be the dominant source of Saturn's water. The analysis of the 233-1500 cm-1 spectrum of ISO/SWS has provided the thermal and compositional structure of Titan on a disk-average [4]. In particular, observations of the CH3D band at 1150 cm-1 significantly improved the determination of the D/H ratio in Titan's stratosphere. The new value of 7.5 \times 10-5, is four times lower than in comets and suggests that Titan's atmosphere is not of cometary origin, but rather formed by outgassing from the interior. We have also tested available vertical profiles and inferred upper limits for a few likely candidates in Titan's stratosphere (such as benzene and allene) [4].
{\bf References} [1] COUSTENIS, A., {\em et al.} 1998a. {\em Astron. Astrophys.} {\bf 336}, L85. [2] LARA, L. M., {\em et al.} 1996. {\em J. Geophys. Res.-Planets} {\bf 101}, 23261-23283. [3] FEUCHTGRUBER, H., {\em et al.} 1997. {\em Nature} {\bf 389}, 159-162. [4] COUSTENIS, A., {\em et al.} 1998b. Submitted for publication.
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