Some independent evidence for winds on Titan, based on ground-based stellar occultation measurements, has been presented by Hubbard et al. (1993). Their analysis of the indicated latitudinal deformation of isopycnic surfaces in the stratosphere implies a zonal wind at the 0.25 mb level varying from some 80 m/s near the equator to more than 170 m/s at 60° latitude. Another promising new method for determining stratospheric wind motions are the infrared heterodyne observations of Titan's ethane emission at 841/cm, which originates primarily from the 1 mbar level (Kostiuk 1993, private communication). Initial measurements of the differential Doppler shift between east and west limb spectra indicate that the winds are prograde and have speeds of the order of 80 m/s.
Fifty near-infrared images of Titan were obtained with the refurbished planetary camera (WF/PC2) of the Hubble Space Telescope in the interval 4 - 18 October 1994 (Smith et al., 1994; 1995). A variety of continuum and methane filters were employed with the goal of obtaining cloud-tracked drift speeds associated with the differential motion of longitudinal structure of the type previously observed at 970 nm by Smith et al. (1992). A global surface map compiled from 14 processed images covering nearly the full Titan rotation period reveals a large bright feature in the leading hemisphere just to the south of the equator. Although some structure in the unaveraged images is suggestive of cloud features, attempts to derive cloud-tracked vectors have thus far proved unsuccessful. Further observations will be pursued at later opportunities.
The DWE will complement remote-sensing observations of temperatures and winds from the Cassini Orbiter. It will provide ground-truth corroboration of the thermal wind retrievals from Composite Infrared Spectrometer (CIRS) measurements, also providing a check on their assumed aerosol and cloud opacities. The high vertical resolution of the DWE retrieval of vertical shear near impact will provide an important characterization of the surface boundary layer unobtainable by thermal sounding. If the probe encounters sufficiently vigorous turbulence or vertical wave propagation, variations of the Doppler signal will provide information on the associated eddy momentum mixing and/or planetary waves.