[Previous] | [Session 41] | [Next]
R. D. Lorenz, M. T. Lemmon, P. H. Smith (LPL, U. Arizona)
HST images of Titan obtained 1992-1997 indicate several features we interpret to be clouds in the lower atmosphere, one of which was described previously (Lorenz et al., DPS 1995). The spectroscopic identification of tropospheric clouds by Griffith et al. (1998) prompted a re-examination of our data, and we find several distinct cloud features that are entirely consistent with their observations.
The features occupy about 10 per cent of Titan's disk, and exhibit several properties not shared by the large bright surface feature identified in Smith et al. (1996). First, while the center-to-limb brightness variation of surface features is consistent, the cloud contrast does not correlate with zenith angle. Secondly, although dimmer than the surface feature at 954nm the cloud features are brighter at 619 and 673nm - this wavelength dependence suggests a tropospheric cloud. Finally, the features move - each is seen only in one year's dataset.
The features could not be reliably tracked, although the 1994 feature appeared to have a slow (10-20 m/s) prograde motion. All the features were seen at northern midlatitudes. These observations suggest the Cassini ISS will see lots of clouds.
The author(s) of this abstract have provided an email address for comments about the abstract: rlorenz@lpl.arizona.edu