The surface of Saturn's largest satellite--Titan--is largely obscured by an optically thick atmospheric haze, and so its nature has been the subject of considerable speculation and discussion. The Huygens probe entered Titan's atmosphere on 14 January 2005 and descended to the surface using a parachute system. Here we report measurements made just above and on the surface of Titan by the Huygens Surface Science Package. Acoustic sounding over the last 90 m above the surface reveals a relatively smooth, but not completely flat, surface surrounding the landing site. Penetrometry and accelerometry measurements during the probe impact event reveal that the surface was neither hard (like solid ice) nor very compressible (like a blanket of fluffy aerosol); rather, the Huygens probe landed on a relatively soft solid surface whose properties are analogous to wet clay, lightly packed snow and wet or dry sand. The probe settled gradually by a few millimetres after landing.

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