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 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 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 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 ( = 0.48) than over
the north ( = 0.10). The 0.62 m observations can only
be fit if we increase the haze single scattering albedo to
= 0.91. We find that the one-way optical depths due to
haze between 0.89 to 0.95 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 m methane
window.