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A. Negrão, A. Coustenis, E. Lellouch (LESIA, Paris-Meudon Observatory), B. Schulz (Caltech, USA), A. Salama (ISO Science Operation Center, Villafranca, Madrid, Spain), E. Raynaud (LESIA, Paris-Meudon Observatory), P. Rannou (S. A., Univ. Paris 6 - Univ. de Versailles, France), H. Feuchtgruber (Max-Planck Institute, Garching, Germany)
The ISO PHT-S and SWS data, taken in January and December 1997, cover, among other, the 2.6-4.8 micron region offering for the first time a full view of the methane window on Titan at wavelengths close to 2.75 micron, and part of the 4.9 micron window.
The spectrum of Titan shows a weak CH4 absorption region near 3 micron, with a window region centered at 2.8 micron. Due to the CO2 telluric feature, only part of this region (2.9 -3.1 micron) has been observed from the ground [Griffith et al., 1998 ; 2003] who reported a geometric albedo of 0.02 on a normal night and 0.05 in the case of an abnormal increase in reflectivty in 1995, interpreted as a cloud in Titan's troposphere.
The ISO data give us for the first time the complete shape of this window, exhibiting two peaks centered around 2.7 and 2.78 micron. The maximum flux in the 3-micron window is 0.14 Jy at 2.7 and 2.78 micron, yielding a geometric albedo of 0.0405. To compare with the ground-based data, note that the ISO/PHT data for Titan near 2.9 micron give a geometric albedo of 0.024 (flux=0.07 Jy), in agreement with measurements by Noll et al. (1996) and with observations by Griffith et al. (1998) during a normal-observing night but in disagreement (factors of 3-10 times lower) with their 'cloud observations'.
We use a radiative transfer code to simulate Titan's geometric albedo from a microphysical model (with fractal particles by Rannou et al , 1997), by varying several atmosphere parameters in order to fit the data in the band. Then to fit in the window we must modify the surface's reflectivity.
Previous work suggests that Titan's surface albedo in the 2.8 micron window appears to be compatible with the strong water ice absorption band at 3 micron.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.