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R\'egis Courtin (CNRS-DESPA), Franck Hersant (Universit\'e Paris-7), Emmanuel Lellouch (CNRS-DESPA)
In January 1997, ISO observed Titan between 1430 and 1900~cm-1 (\lambda=5.3--7 micron) using one of the grating modes of the SWS spectrometer. Although, the nominal spectral resolution was twice as good, the calibrated data were binned in 3~cm-1 intervals in order to achieve sufficient signal-to-noise. Globally, the flux density decreases rapidly from a maximum of about 2.5~Jy at 1430--1480~cm-1 to 0.1~Jy near 1800~cm-1. In addition, there is a narrow and prominent emission feature near 1540~cm-1. This part of Titan's spectrum has already been explored by the IRIS instrument on Voyager~1 with a comparable resolution but only up to 1580~cm-1. The two data sets compare very nicely. We interpret this spectrum with a radiative transfer model including the following components: a) a thermal continuum due to the photochemical haze, b) stratospheric emission due to the \nu4 and \nu2 bands of CH4, and to the \nu8 band of C2H6, c) sunlight backscattered by the haze and the surface. Thermal emission dominates up to 1700~cm-1. Reflected sunlight picks up at 1650~cm-1 and becomes dominant above 1800~cm-1. A good match is obtained with the following model parameters: a stratospheric CH4 mole fraction of about 2%; a C2H6 mole fraction of 7\times 10-6 around 100~km (and moderately increasing with altitude); an imaginary index of refraction for the haze 1--3 times larger than that of N2-CH4 tholins; and a surface albedo of about 0.10.
The author(s) of this abstract have provided an email address for comments about the abstract: Regis.Courtin@obspm.fr