  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 12 cm
deep hole in the surface of Titan. It bounced back, out of the hole onto the
flat surface, after which it commenced a 30-40 cm long slide in the southward
direction. The slide ended with the probe out of balance, tilted in the
direction of DISR by around 10 degrees. The probe then wobbled back and forth
five times in the north-south direction, during which it probably encountered a
1-2 cm sized pebble. The SSP provides evidence for movement up to 10 s 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. (P&SS 56, 669). We suggest that the surface at the
landing site was covered by a dust layer, possibly the 7 mm layer of...
