Modeling Specular Reflections from Hydrocarbon Lakes on Titan's Surface

Kurzfassung

During the 58th flyby of Titan, the Cassini Visual and Infrared Mapping Spectrometer (VIMS) observed a specular reflection of sunlight from Titan's surface within the 5-µm methane window (Stephan, K., and 14 colleagues, 2010). Motivated by this observation, we model the expected magnitude of the specular reflection of solar radiation off a hydrocarbon lake on Titan's surface. The intensity of this reflection is controlled by a number of factors: the convex shape of Titan's surface, which reduces the magnitude of the reflection by the ratio of the solid angle of the field of view to the expanded solid angle projected off of the convex surface, attenuation and scattering by gases and aerosols in Titan's atmosphere, and the reflectance of the lakes surface in a specular geometry. We can describe the expected intensity of the specular reflection as the I/F VIMS would observed when directly viewing the Sun reduced by the geometric defocusing of the flux, the Fresnel reflection of the lake surface, and attenuation of the incoming and outgoing flux by the atmosphere. For the geometry described by the T58 flyby, our model predicts a maximum I/F 1-3. We thus conclude the surface of this body is in fact liquid. With future observations it will be possible to constrain the refractive index of the liquid as well as the total attenuation of the atmosphere at 5 µm. The Cassini Project, managed by the Jet Propulsion Laboratory, California Institute of Technology and under contract with NASA, supported this work.