Modeling specular reflections from hydrocarbon lakes on Titan

During the 58th close flyby of Titan (T58), the Cassini Visual and Infrared Mapping Spectrometer (VIMS) observed a specular reflection of sunlight from Titan's Jingpo Lacus through the 5-μm methane window (Stephan, K. et al. [2010]. Geophys. Res. Lett. 37, L07104). The maximum intensity of this reflection is controlled by three basic factors: (1) the shape of the reflecting surface (its overall geometry and roughness), (2) the reflectance of the surface, as controlled by the real refractive index of the material (and that of the atmosphere), and (3) attenuation due to absorption and scattering by atmospheric gases and aerosols along the pathlength. Herein we model the expected intensity of a specular reflection off of a convex mirror-like surface on Titan. We assume the specular reflection is from a body of liquid hydrocarbons on Titan's surface with optical properties consistent with CH ₄ and C ₂H ₆ with smaller amounts of nitrogen and heavier hydrocarbons (e.g., C ₃H ₈) admixed. We assume the 5-μm opacity for the polar atmosphere is a factor of two higher than that of the tropical haze. For the geometry of the T58 observations, our model predicts a maximum I/F=1-to-5; for a Lambertian surface at normal illumination I/F=1. The maximum 5-μm intensity observed during T58 was I/F~2.6, from which we conclude that Jingpo Lacus is filled with a liquid that has a real index of refraction consistent with that of methane-ethane-nitrogen liquid and that the 5-μm atmospheric opacity was τ=0.5, consistent with the higher particle column expected in the winter polar atmosphere. Future VIMS observations will allow us to refine the refractive index of the liquid in the lakes and to place a quantitative constraint on the ratio of methane to ethane.