Properties of Titan's haze and surface.

Abstract

I have developed a model of aerosol microphysics and scattering in Titan's atmosphere that incorporates the concept of irregular aggregates. By comparing models with spacecraft observations of Titan's intensity and polarization phase laws and ground-based observations of Titan's solar reflection spectrum, I have constrained several properties of the aerosols. The aerosols are assumed to be irregular aggregates of small spheres made of Titan tholin--the spheres have a radius of approximately 0.06 μm, and through most of the atmosphere the aggregates comprise 10 to 100 spheres. The aggregates form at an altitude of 250 to 300 km, and are the primary source of scattering opacity down to the lower stratosphere, where there must be some removal process, such as rain-out, although there is likely to be some scattering opacity in the troposphere. This model fits the polarization data well, in addition to being highly forward scattering. It is used to predict the penetration of sunlight into the atmosphere, and it predicts that the surface becomes visible at wavelengths longer than about 0.9 μm. I then carried out a program of 0.9-2.3 μm spectroscopy of Titan in which Titan's albedo at wavelengths sensitive to the surface was monitored. Results of this program show that Titan's albedo does vary, with the leading hemisphere consistently brighter than the trailing hemisphere, and that the variations repeat with a period equal to Titan's orbital period. The source of the variation is very likely the surface, and there is no evidence of surface absorption features. The amplitude of the light-curve at 2 μm (32 ± 3%) suggests that the bright component is unlikely to have a strong 2 μm absorption feature, and, therefore, is not pure water ice. It is clear from the existence and repetition of the variation that the surface is not homogeneous, and specifically that a global ocean is not allowed.