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 ^ { \circ } .
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 fluffy material identified by Atkinson et al .
( Icarus 210 , 843 ) .
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 .