Distribution of Haze in Titan's Atmosphere: Results from 1996 HST WFPC2 Imaging

E. F. Young (SETI), P. Rannou (NRC), C. P. McKay (NARC)

HST images were obtained in seven wavelengths (0.62, 0.89, 0.902, 0.916 0.928, 0.94, and 0.953 tex2html_wrap_inline14 m) in October of 1996. Titan's albedo in these wavelengths is a strong function of haze abundance, haze single scattering albedo, and methane abundance. Using a two-stream radiative transfer simulation, we fit for these quantities in two stages. First we solve for the relative optical depths due to haze and methane, assuming a uniform global column abundance for CH tex2html_wrap_inline16 and a very simple model for Titan's haze distribution. We then fit for haze abundances at higher spatial resolution (roughly 300 km per resolution element). Preliminary results from the 0.89 tex2html_wrap_inline14 m methane band image (which is mainly sensitive to Titan's upper atmosphere) indicate that the average optical depth due to aerosols for the layer between 200 and 350 km is 0.29, but it is much higher over the south pole ( tex2html_wrap_inline20 = 0.48) than over the north ( tex2html_wrap_inline20 = 0.10). The 0.62 tex2html_wrap_inline14 m observations can only be fit if we increase the haze single scattering albedo to tex2html_wrap_inline26 = 0.91. We find that the one-way optical depths due to haze between 0.89 to 0.95 tex2html_wrap_inline14 m is greater than 4, which means that fewer than 2% of photons in this wavelength range are unscattered as they travel from the surface up through the atmosphere, and fewer than 7% are singly-scattered. Given these haze optical depths, it will be extremely difficult for Cassini's ISS instrument to map Titan's surface at high resolution, even in the 0.94 tex2html_wrap_inline14 m methane window.