Evidence for water vapor in Titan's atmosphere from ISO/SWS data

The infrared spectrum of Titan around 40 mu m was recorded in the grating mode of the Short Wavelength Spectrometer (SWS) of ISO, with a resolving power of about 1900. Two emission features appear at 43.9 and 39.4 mu m, where pure rotational water lines are expected. Line strengths are about 8 times the 1sigma statistical noise level. The H_2O vertical profile for water suggested by the photochemical model of Lara et al. (1996), rescaled by a factor of about 0.4(+0.3}_{-0.2) , is compatible with the data. The associated water mole fraction is about 8(+6}_{-4) x 10(-9) at an altitude of 400 km (column density of 2.6(+1.9}_{-1.6) x 10(14) mol cm(-2) above the surface). The inferred water influx at 700 km in Titan's atmosphere is in the range (0.8-2.8) x 10(6) mol cm(-2) s(-1) . Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of NASA and ISAS. The SWS instrument (P.I. Th. de Graauw) is a joint project of the SRON and the MPE. RES would like to thank D. Hamilton for illuminating discussions regarding dust transport in the Saturn System.