Titan surface mechanical properties from the SSP ACC-I record of the impact deceleration of the Huygens probe

Conference paper: the Huygens probe landed on Titan at 11:38:11 UT on 14 January, 2005. The The main impact event lasted approximately 20ms, with possible bounce and rocking for up to 2s thereafter. The peak deceleration was 15g. The deceleration was characterized by several accelerometers as well as a penetrometer : we focus on the Surface Science Package ACC-I whose sole function was to record surface impact. This sensor sampled deceleration along, but offset 0.325m from, the probe's axis at 500 samples per second. Simple mechanics suggests the probe decelerated over a stroke of 0.15 to 0.2m, although whether it subsequently bounced or slid out of that cavity is not clear. Comparison of the deceleration amplitude with models of various target mechanical properties suggests sand, soft clay or lightly packed snow are appropriate analogs. However, the fairly sharp onset of the deceleration peak argues against a target that increases in hardness sharply with depth such as dry sand - a more cohesive (perhaps damp) target seems more likely. Interestingly, the deceleration record implies a target strength several times weaker than that indicated by the penetrometer - one possible interpretation is that a number of cobbles on the surface, rather than the curved bottom of the probe, acted to define the contact area with the surface material during the impact event. The relatively soft impact meant that the deformation of the probe was not significant (as evidenced by the lack of any probe or instrument failures at impact) and thus to a first order, the event may be reproduced in physical or numerical models that assume the probe is rigid. We will review the latest simulations and interpretations of the impact event.