Bouncing on Titan: Motion of the Huygens probe in the seconds after landing

Abstract

While landing on Titan, several instruments onboard Huygens acquired measurements that indicate the probe did not immediately come to rest. Detailed knowledge of the probe's motion can provide insight into the nature of Titan's surface. Combining accelerometer data from the Huygens Atmospheric Structure Instrument (HASI) and the Surface Science Package (SSP) with photometry data from the Descent Imager/Spectral Radiometer (DISR), we develop a quantitative model to describe motion of the probe, and its interaction with the surface. The most likely scenario is the following. Upon impact, Huygens created a 12cm deep hole in the surface of Titan. It bounced back, out of the hole onto the flat surface, after which it commenced a 30–40cm long slide in the southward direction. The slide ended with the probe out of balance, tilted in the direction of DISR by around 10°. The probe then wobbled back and forth five times in the north–south direction, during which it probably encountered a 1–2cm sized pebble. The SSP provides evidence for movement up to 10s after impact. This scenario puts the following constraints on the physical properties of the surface. For the slide over the surface we determine a friction coefficient of 0.4. While this value is not necessarily representative for the surface itself due to the presence of protruding structures on the bottom of the probe, the dynamics appear to be consistent with a surface consistency of damp sand. Additionally, we find that spectral changes observed in the first four seconds after landing are consistent with a transient dust cloud, created by the impact of the turbulent wake behind the probe on the surface. The optical properties of the dust particles are consistent with those of Titan aerosols from Tomasko et al. (2008). We suggest that the surface at the landing site was covered by a dust layer, possibly the 7mm layer of fluffy material identified by Atkinson et al. (2010). The presence of a dust layer contrasts with the dampness measured just centimeters below the surface, and suggests a recent spell of dry weather at the landing site.