Lorenz R. D., Smith P. H., Lemmon M. T.
The Search for Clouds on Titan Using HST Imaging
In 1994 we imaged Titan using the HST Planetary Camera in windows of low
atmospheric opacity in the near-IR. Our data cover almost an entire orbit
of Titan, allowing us to produce a map of Titan's surface (Smith et al.,
1995). Additionally, our observations included a close-spaced (6-8 hours
apart) sequence of 7 images to search for cloud activity and measure winds.
Although initial examination failed to yield any trackable cloud features,
after removal of surface features from the images a large cloud
is tentatively identified, moving W-E (prograde) at a latitude of about 20
degrees North. The feature may be seen in 3 frames in the 850LP filter,
and is (when closest to the center of the disk) about 4-sigma above the
background level, and is several HST pixels in size, although may not
The feature appears to move at about 25 m/s relative to the surface, a
windspeed expected at an altitude of 30km from models of the zonal wind
field (Flasar et al., 1994).
The feature is seen in one frame in the 673 and 619 filters (although at
poorer signal to noise). These frames were taken at about the same time as
the 850LP frame mentioned above. We compare the feature size and ocurrence
with the expected amount of cloud activity on Titan and the rate of cosmic
ray strikes on the CCD.
There is some evidence of cloud motion elsewhere in our image sequence. We
plan further monitoring for cloud activity using HST and groundbased
telescopes in October 1995.
References: Smith et al. 1995 `Titan's Surface, Revealed by HST Imaging'
Icarus, submitted. Flasar et al. 1994 `Titan's Zonal Wind' ESA SP-1177