Smith P. H., Lorenz R. D., Lemmon M. T.
New Information About Titan's Haze Contrast from HST Observations
Titan has been observed twice, in August 1992 and in October 1994, through
a set of filters selected to characterize the North-South albedo contrast
first observed by Voyager. The filters are centered on the following
wavelengths: 336, 439, 547, 588 and 673 nm for the continuum and 619 and
889 nm in methane bands. Another, poorly exposed, data set was taken in
August 1990 though only the 547nm filter is usable for analysis. The Voyager
filters overlap the 439, 547 and 588nm filters and the Pioneer spacecraft
took data in 1979 at both 450 and 650 nm. Analysis of these data shows a
strong wavelength dependence in both the contrast and the limb darkening.
The contrast has reversed since the spacecraft data were taken (1979-1981) so
that the northern hemisphere is now brighter than the southern by 15-20%;
However, outside the spacecraft filter region the contrast drops to less
than 5% at both 336 and 673 nm, then reverses again at 889 nm in the methane
band with a contrast of 40%. There is also a similar wavelength variation in
the limb darkening: the disk is nearly lambertian near 547 nm, but the
darkening decreases toward a uniform disk at both 336 and 673 nm. In the
methane band at 889 nm there is strong limb brightening.
In this work we present atmospheric models that seek to explain this data
with the aim of determining the altitude region where the north-south
discrepancy occurs. It seems that a layer near 80 km that is 0.5 optical
depths thicker in the south than the north could explain most of the effects
that are observed in the HST images. This layer would need to dissolve and
reform in the opposite hemisphere during the seasonal reversal that ocurred
between 1980 and the present.