Stratospheric global winds on Titan at the time of Huygens descent

Abstract

Measurements of stratospheric zonal winds on Titan were made in preparation for and during the time of the descent of the Huygens Probe into Titan's atmosphere on 14 January 2005. Fully resolved emission lines from ethane near 11.7 μm were measured on the east, center, and west positions on Titan using the NASA/GSFC Heterodyne Instrument for Planetary Wind And Composition, HIPWAC, mounted on the National Astronomical Observatory of Japan 8.2 m Subaru Telescope on Mauna Kea, Hawaii. Analysis of the Doppler shifts of the emission line shapes yielded mean prograde gas velocity ∼60 ± 65 m/s at altitudes below ∼120 km (∼5 mbar). This result is consistent with retrievals from the Huygens Doppler Wind Experiment and from other observations near this altitude range. Current spectral line shapes, however, differed significantly from those obtained in similar measurements on Subaru in 2004 and on the NASA IRTF in 1993–1996, which retrieved prograde zonal winds 190 ± 90 m/s at 230 km (∼0.4 mbar). The cores of the emission lines, which probe the high‐altitude region, could not be fitted as before to retrieve wind directly using the accepted atmospheric model for Titan. They imply an approximately tenfold increase in ethane mole fraction (1.2 × 10−4) with strong wind shear above the stratopause, providing a potential probe of the lower mesosphere and possible evidence of temporal and spatial variability. Results contribute to coordinated measurements of winds by various techniques providing information on the altitude distribution of wind velocity in Titan's atmosphere from near the surface to the lower mesosphere.