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 be resolved.

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 (in press)

Titan paper