Penetrometry of granular and moist planetary surface materials: Application to the Huygens landing site on Titan

Abstract

46The Huygens probe landed on the then unknown surface of Titan in January 2005. A 47 small, protruding penetrometer, part of the Surface Science Package (SSP), was pushed into the 48 surface material measuring the mechanical resistance of the ground as the probe impacted the 49 landing site. We present laboratory penetrometry into room temperature surface analogue 50 materials using a replica penetrometer to investigate further the nature of Titan's surface and 51 examine the sensor's capabilities. The results are then compared to the flight instrument's 52 signature and suggest the Titan surface substrate material consists of sand-sized particles with a 53 mean grain size ~2 mm. A possible thin 7 mm coating with mechanical properties similar to 54 terrestrial snow may overlie this substrate, although due to the limited data we are unable to 55 detect any further layering or grading within the near-surface material. The unusual weakening 56 with depth of the signature returned from Titan has, to date, only been reproduced using a damp 57 sand target that becomes progressively wetter with depth, and supports the suggestion that the 58 surface may consist of a damp and cohesive material with interstitial liquid contained between its 59 grains. Comparison with terrestrial analogues highlights the unusual nature of the landing site 60 material. 61 62

Detection of structure in asteroid analogue materials and Titan’s regolith by a landing spacecraft

“…A notable feature in the penetrometry data, over the first five centimetres of penetration, is a negative force-depth gradient that Atkinson et al (2010) reproduced in the laboratory with penetrometer impacts into coarse wet sand. Atkinson et al (2010) also investigated the particle size at the Huygens site and found that their data was consistent with a particle diameter of 2 mm but noted that larger particles may exist at the site as seen in the Huygens imager data (Keller et al, 2008).…”

“…The prominent features of the data, relevant to this study, are labelled. The feature labelled A is a low strength material that has been associated with a loose covering of deposited material (Atkinson et al, 2010) most likely airfall dust (Schröder et al, 2012). The feature labelled B is a peak, possibly caused by the impact with one of the ice pebbles observed on the surface.…”

“…The feature labelled C is a negative force-depth profile, which is the main focus of this study. It has been previously investigated by Atkinson et al (2010) who found that wet coarse sand had similar mechanical properties to the surface of Titan. The feature labelled D is the point where the main body impacts the surface and changes the surface's mechanical response to the penetrometer in an unknown way.…”

“…The penetrometry data was found to be consistent with a moderately firm granular material, possibly wet. The measurement was considered repre-sentative of the subsurface down to a depth of 5 cm after which the impact of the spacecraft main body modified the penetration mechanics in an unknown manner (Zarnecki et al, 2005;Atkinson et al, 2010). A notable feature in the penetrometry data, over the first five centimetres of penetration, is a negative force-depth gradient that Atkinson et al (2010) reproduced in the laboratory with penetrometer impacts into coarse wet sand.…”

“…The measurement was considered repre-sentative of the subsurface down to a depth of 5 cm after which the impact of the spacecraft main body modified the penetration mechanics in an unknown manner (Zarnecki et al, 2005;Atkinson et al, 2010). A notable feature in the penetrometry data, over the first five centimetres of penetration, is a negative force-depth gradient that Atkinson et al (2010) reproduced in the laboratory with penetrometer impacts into coarse wet sand. Atkinson et al (2010) also investigated the particle size at the Huygens site and found that their data was consistent with a particle diameter of 2 mm but noted that larger particles may exist at the site as seen in the Huygens imager data (Keller et al, 2008).…”