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A. Coustenis (DESPA, Paris-Meudon Observ., France), B. Schulz, A. Salama (ISO Science Oper. Center, Villafranca, Spain), E. Lellouch, Th. Encrenaz, E. Raynaud (DESPA, Paris-Meudon Observ., France), T. Owen (Inst. for Astronomy), P. Rannou (Serv. Aeronomie, Univ. Paris 6, France)
The PHT-S and SWS ISO data cover the 2.6-4.8 micron region offering, among other and for the first time, a full view of the methane window on Titan at wavelengths close to 2.8 micron (Coustenis et al., in preparation). Only part of this window (2.9-3.1 micron) is observable from the ground and Griffith et al. (1998; Nature 395, 575) reported in this region a geometric albedo of 0.02 on a normal-observing night and 0.05 in the case of a sudden increase, 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. We have used a radiative transfer code to simulate Titan’s spectrum in this region, using fractal particles, and find the shape of the surface albedo to be compatible with the presence of the strong water ice band at 3 micron.
The PHT-S data for Titan near 2.9 micron give a geometric albedo of 0.024 (flux=0.07 Jy), in agreement with ground-based spectroscopic measurements by Noll et al. (1996; Icarus 124, 625) and with observations by Griffith et al. (1998) during a « normal night », but much lower than their reported « cloud » fluxes. In other words, no tropospheric clouds on Titan were seen by ISO. The maximum flux in the methane window is ~ 0.14 Jy at 2.7 and 2.78 micron, yielding a geometric albedo of 0.04, compatible with water ice and tholin mixtures on the surface.
PHT-S gives a maximum flux of 0.02 Jy at 4.8 micron (0.006 in geometric albedo), in good agreement with Noll et al. (1996). The exact location of the center of this window, near 4.9 micron is not observed by PHT-S with sufficient signal-to-noise so as to give access to the center albedo value.
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The author(s) of this abstract have provided an email address for comments about the abstract: Athena.Coustenis@obspm.fr