Titan's surface albedo variations over a Titan season from near-infrared CFHT/FTS spectra

Abstract

We have observed Titan in a series of campaigns from 1991 to 1996 with the Fourier Transform Spectrometer on the CFH telescope. The data acquired provide a lightcurve from the geometric albedos in the 0.9– 2.5 μ m spectral region. The 1991–1993 data were previously analyzed in Coustenis et al. [1995. Titan's surface: composition and variability from its near-infrared albedo. Icarus 118, 87–104] with a spherical particle code by McKay et al. [1989. The thermal structure of Titan's atmosphere. Icarus 80, 23–53]. We present here three new datasets from the 1994, 1995 and 1996 observations, with additional information from the 0.94- μ m methane window on Titan. All the data (1991–1996) are re-analyzed in this work using an updated radiative transfer code based on Rannou et al. [2003. A model of Titan's haze of fractal aerosols constrained by multiple observations. Planet. Space Sci. 51, 963–976] with fractal particles. A comparative study of methane absorption coefficients currently available from different sources is also performed demonstrating the great sensitivity of surface inferences to this model parameter. Based on our results, we recommend the methane absorption coefficients produced by Boudon et al. [2006. The vibrational levels of methane obtained from analyses of high-resolution spectra. J. Quant. Spectrosc. 98, 394–404.] and Irwin et al. [2006. Improved near-infrared methane band models and k-distribution parameters from 2000 to 9500 cm - 1 and implications for interpretation of outer planet spectra. Icarus 181, 309–319.] for future studies of Titan. The geometric albedos have been reproduced with our model. We have tested different types of haze profiles and inferred the impact of this parameter on the retrieval of surface albedos. The latter are finally proposed in the atmospheric methane windows centered at 0.94, 1.075, 1.28, 1.6 and 2 μ m for each of the methane coefficient sets. The surface albedos proposed here are not just points at the center of the windows but curves covering a large portion of these regions. This is useful for studies of the composition of the Titan surface. Indeed, we find our data to be compatible with mixtures of water ice and tholin but have strong indication for the presence of an additional, as yet unidentified, component (or components) for which we offer a spectral description. We also discuss the implications of the surface albedo retrievals (as a disk average) for the surface of Titan, recently investigated by the Cassini–Huygens mission.