Titan is tidally locked to Saturn and orbits the planet at an inclination of
0.33 degrees relative to Saturn's equatorial plane. Thus, due to the 26.73
tilt of Saturn's rotational axis, this satellite experiences seasonal variations
in insolation similar to those of the mother planet. When the Voyager
spacecraft arrived in 1980 and 1981, the sub-solar point was near zero degrees
and shifting northward and the brighter southern hemisphere was experiencing a
maximum accumulation of photochemical smog.
On August 26, 1990, a third of a Saturnian year after the Voyager 2
encounter, Caldwell et al. (1992) obtained images of Titan with WFPC-1 at
wavelengths 439, 547, and 889 nm. They reported that the north-south asymmetry
of the surface brightness at 439 and 547 nm was opposite to that seen in
Voyager images. Low signal-to-noise in the 889 nm images hampered analysis but
a positive south-north asymmetry was reported. On August 21, 1992, Smith and
Lemmon (1994) obtained 8 well-exposed images of Titan
with the PC mode of the WFPC-1. On Titan, at 1.33 billion kilometers, the
0.0439
pixelation corresponds to 284 km at the sub-earth point. Smith
and Lemmon confirmed the north-south asymmetry, reporting differences in
surface brightness of 15-20% at 439-589 nm and 45% at 889 nm. The image at
longest wavelength (850-1000 nm) showed small scale features that correlate
with University of Arizona ground-based 1-2 
m data. Smith and Lemmon
suggest that at these wavelengths the cloud is becoming optically thin and they
are detecting variations in surface albedo.