An investigation of Titan's aerosols using microwave analog measurements and radiative transfer modeling

A combination of laboratory experiments, theoretical modeling, and spacecraft observations is employed to characterize the aerosols in the atmosphere of Titan. The scattering properties of model aerosols were measured using the Microwave Analog Light Scattering Facility at the University of Florida and complemented with theoretical modeling of single scattering characteristics and radiative transfer in Titan's atmosphere. This study compares these modeling results with photopolarimetric observations made over a range of phase angles by the Pioneer 11 and Voyagers 1 and 2 spacecraft. Important results of this work include a survey of the scattering properties of different particle morphologies and compositions necessary to accurately interpret these observations without introducing non-physical assumptions about the particles or requiring additional free parameters to the radiative transfer models. Previous studies use calculation methods which, due to computing memory and processing time requirements, a priori exclude much of the parameter space that the microwave analog laboratory is ideal for exploring. The goal of the present work, to directly constrain aerosol physical characteristics, is addressed by studying in a consistent manner how a variety of particle morphologies and refractive indices affect the polarization and intensity reflected by Titan's atmosphere. Based on comparisons of model results to spacecraft observations, many model morphologies are excluded from further consideration. The most plausible physical particle models suggest that a combination of Rayleigh-like single particles and aggregates that are larger than those previously suggested and investigated [West, R.A., Smith, P.H., 1991. Evidence for aggregate particles in the atmospheres of Titan and Jupiter. Icarus 90, 330-333; Rannou, P., Cabane, M., Botet, R., Chassefière, E., 1997. A new interpretation of scattered light measurements at Titan's limb. J. Geophys. Res. 102, 10997-11013] provide the best fit to the existing data. Additional laboratory experiments and more refined modeling awaits the results of the new rich observational dataset from the Cassini/Huygens encounter with Titan.