Descent motions of the Huygens probe as measured by the Surface Science Package (SSP): Turbulent evidence for a cloud layer

The Huygens probe underwent vigorous short-period motions during its parachute descent through the atmosphere of Saturn's moon Titan in January 2005, at least some of which were excited by the Titan environment. Several sensors in the Huygens Surface Science Package (SSP) detect these motions, indicating the transition to the smaller stabilizer parachute, the changing probe spin rate, aerodynamic buffeting, and pendulum motions. Notably, in an altitude range of about 20-30 km where methane drops will freeze, the frequency content and statistical kurtosis of the tilt data indicate excitation by turbulent air motions like those observed in freezing clouds on Earth, supporting the suggestion of Tokano et al. [Tokano, T., McKay, C.P., Neubauer, F.M., Atreya, S.K., Ferri, F., Fulchignoni, M., Niemann, H.B. (2006a). Methane drizzle on Titan. Nature 442, 432-435] that the probe passed through such a cloud layer. Motions are weak below 20 km, suggesting a quiescent lower atmosphere with turbulent fluctuations of nominally <0.15 m/s (to within a factor of ∼2) but more violent motions in the upper troposphere may have been excited by turbulent winds with amplitudes of 1-2 m/s. Descent in part of the stratosphere (150-120 km) was smooth despite strong ambient wind (∼100 m/s), but known anomalies in the probe spin prevent investigation of turbulence in the known wind-shear layer from 60 to 100 km.