Vertical atmospheric flow on Titan as measured by the HASI instrument on board the Huygens probe

On January 14, 2005, the Huygens probe descended on the surface of Titan, the largest moon of Saturn and the only moon in the Solar system with a substantial atmosphere. After the deployment of the main parachute and the release of the heat shield at an altitude of about 150 km, the local pressure and temperature were measured by HASI (Huygens Atmospheric Structure Instrument) all the way down to the surface. These measurements have now been used to determine the vertical component of the atmospheric flow along the trajectory of the probe. The average flow direction is rising below about 40 km and sinking above that, with features that loosely match the strong shears in the horizontal wind profile obtained by radio telemetry. The prevalence of sinking motion in the stratosphere is an unexpected result for the given latitude, suggesting a layered atmosphere with a large thermally indirect cell. Wiggles superposed on the main upward flow are evidence of inertial instability characteristic of equatorial regions, pointing to a latitudinal shear of zonal wind throughout the lower atmosphere. Copyright 2006 by the American Geophysical Union.