Titan's meridional wind profile and Huygens' orientation and swing inferred from the geometry of DISR imaging

Abstract

The altitude and zonal motion of the Huygens probe descending through Titan's atmosphere was determined early under the assumption of no meridional motion (Bird et al. [2005]. Nature 438, 800-802). By comparing images taken during the descent, Karkoschka et al. (Karkoschka et al. [2007]. Planet. Space Sci. 55, 1895-1935) determined the meridional motion of Huygens, which was generally much smaller than its zonal motion. Here, we present a comprehensive geometrical analysis of all images taken during the descent that is four times more accurate than the previous study. The result is a meridional wind profile across Titan's troposphere with northward winds by up to 0.4 m/s with an average of 0.1 m/s above 1 km altitude, and southward winds below, peaking at 0.9 m/s near 0.4 km altitude. The imaging data extend down to 0.22 km altitude, although additional information came from the horizontal impact speed near 0.8 m/s southward (Schröder et al. [2012]. Planet. Space Sci. 73, 327-340). There is a region between 5 and 8 km altitude with no significant meridional wind. In the stratosphere, the average meridional wind was 1.2 ± 1.5 m/s northward, and zero meridional motion is possible down to 15 km altitude. We present the difference between the zonal speeds of Huygens and the wind that was ignored in previous publications and amounts to up to 7 m/s. We determined the three rotational angles of Huygens for the times of each exposure that showed surface features. During 26 exposures, the swing speed of Huygens was fast enough to smear images. Inferred swing speeds were up to 20°/s during the calm phase of the descent, consistent with up to 40°/s swings reported before during the rough phase. The improved geometric calibration of images allowed identification of many features also seen in Cassini radar images. This comparison yields the location of the Huygens LandingSite as 192.34 ± 02° West and 10.47 ± 0.02° South.