Bouncing on Titan: Motion of the Huygens probe in the seconds after landing

Abstract

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 • . 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.2005; Karkoschka et al., 2007). The surface itself appears fine-grained, damp, and relatively soft (Zarnecki et al., 2005;Niemann et al., 2005;Lorenz et al., 2006;Karkoschka et al., 2007), and may be covered by a 7 mm thick fluffy layer (Atkinson et al., 2010). The motion of Huygens during atmospheric entry and descent has been studied well (Bird et al., 2005;Karkoschka et al., 2007;Lorenz et al., 2007), but its motion during and directly following landing has not. In the last phase of the descent a stabilizer parachute restricted the vertical speed to 4.5 m s −1 , ensuring a relatively soft landing. The horizontal speed was then only around 1 m s −1 . It is clear that, upon impact, the 200 kg probe could not immediately have come to rest. Indeed, evidence for continued motion exists. Schröder (2007) noticed that DISR measurements were variable for several seconds after impact, and suspected that motion and/or du...

Observation of 2nd Schumann eigenmode on Titan's surface

“…However, this is not consistent with the observations of other SSP instruments leading to claim that Huygens was definitely stabilized about 10 s after impact (Schröber et al, 2012). For instance, the value measured by the Y-tilt sensor seems to indicate still a permanent tilt of 2 • after offset correction.…”

“…Such coincidence, still under investigation, could perhaps be caused by some motion of at least one of the boom-antenna without any perceptible influence on the Y-tilt sensor. Alternatively, according to further studies in progress, a slow change in conductivity of the near surface, due to the presence of the massive gondola warmer than the dusty surface sediment (Schröber et al, 2012), might explain the simultaneous change of the MI measurements and of the pattern of the local SR wave field.…”

“…According to a recent work (Schröber et al, 2012), for about 3 s after touch down, Huygens gondola successively bounced back out of the hole impact, slid and wobbled back and forth five times, after which it commenced a 30-40 cm long slide on a flat surface for about 2 s. During an additional 5 s, the slide motion progressively slowed down until Huygens stayed definitely at rest. Because of the ELF-PWA electronic saturation visible on the first two spectra transmitted 2 and 4 s after impact (MT 0 = 8870 s), the first validated data are received at MT = 8878.375 s. Since there is a processingbuffering delay of about 2 to 4 s between the acquisition time of the electric field and the MT dating, we may only assume that the first available measurement after impact was performed during the first 10 s after touch down.…”

Silence on Shangri-La: Attenuation of Huygens acoustic signals suggests surface volatiles

“…The post-landing environment of the Huygens probe was very quiescent after the first $6 s when there was some bouncing and skidding (Bettanini et al, 2008;Schroeder et al, 2012). The orientation of the probe changed very slightly, with a tilt of $ 0.21 occurring over 72 min (Karkoschka et al, 2007).…”

“…Zarnecki et al, 2005). However, understanding of the probe post-impact motion has since improved (Schroeder et al, 2012), concluding that the probe bounced or skidded out of this impact depression. There is evidence that the probe was left sitting on the surface, rather than having penetrated into it (from the camera geometry, Karkoschka et al, 2007, and from the multipath interference pattern observed in the Huygens radio signal, Pérez-Ayúcar et al, 2006).…”