Are Titan aerosols really tholins? Constraining the aerosol extinction coefficient from VIMS in the 4 to 5 um range

Since the very first images of Titan made by Voyager I, it was evident that the planet was covered by a thick reddish haze that prevented the spacecraft from seeing the surface. In the last twenty years, various models have been developed in order to explain the production and aggregation of complex organic molecules on Titan to produce haze particles. In most models, the optical properties of the aerosol particles are usually assumed to be similar to those of the tholins first produced in the lab by Khare et al. (1984). However, the spectral properties of Titan aerosols inferred from CIRS and VIMS observations in the infrared are significantly different from those of the lab tholins both in the NIR, around 3 um, and in the FIR (Vinatier et al., 2012; Rannou et al., 2010). In this work, we analyze a collection of night-side VIMS limb spectra at stratospheric altitudes in the 4 to 5 um region, where so far there are no direct measurements of the aerosol extintion coefficient. Our simulated spec- tra include non-LTE emission due to CO and CH3D and thermal emission from the aerosols. We find that the optical properties of Titan's aerosols in this spectral region are not compatible with those of the tholins produced in the lab (Khare et al., 2002; Imanaka et al., 2004; Quirico et al., 2008) and infer the actual extinction coefficient in this spectral range from VIMS observations. In particular, the prominent 4.6 um peak in the tholins ab- sorbance spectrum, due to C-N stretching, appears to be shifted at smaller wavelengths, raising further questions about the actual composition of Titan aerosols.