The 6-7 μm spectrum of Titan from ISO/SWS observations

We have analyzed spectroscopic observations of Titan performed in January 1997 by the Short Wavelength Spectrometer (SWS) on the Infrared Space Observatory in the wavelength range from 5.3 to 7.0 μm (1422-1890 cm−1). This spectral interval is of particular interest because it contains various molecular transitions, particularly the ν2 band of CH4 and the ν8 band of C2H6, as well as a spectral feature characteristic of the laboratory analogs of Titan's haze-or tholins-which have been studied for decades. In addition, this region is expected to show the transition between the thermal emission and reflected sunlight regimes of Titan's atmospheric radiation around 6 μm. All these features are indeed observed in the SWS spectrum, for the first time in the case of the ν2 band of CH4 at 6.5 μm. We present a radiative transfer model of these observations from which we derive the extinction cross section of the haze particles at 140 ± 20 km altitude. This result complements similar determinations recently obtained at shorter and longer wavelengths from Cassini VIMS and CIRS measurements, although large uncertainties still remain at 1700-1800 cm−1, as well as a gap at 1800-2000 cm−1. A comparison between the available extinction cross section determinations for the Titan haze and those derived for laboratory-synthesized tholins, from the near- to the far-infrared, shows some similarities but also striking discrepancies, thus reinforcing the idea that there is not a unique tholin analogue to the Titan haze material.