The electrical properties of Titan’s surface at the Huygens landing site measured with the PWA–HASI Mutual Impedance Probe. New approach and new findings

Abstract

Ten years after the successful landing of the Huygens Probe on the surface of Titan, we reassess the derivation of ground complex permittivity using the PWA-MIP/HASI measurements (Permittivity, Waves and Altimetry-Mutual Impedance Probe/Huygens Atmospheric Structure Instrument) at the frequencies 45, 90 and 360 Hz. For this purpose, we have developed a numerical method, namely "the capacity-influence matrix method", able to account for new insights on the Huygens probe attitude at its final resting position. We find that the surface of Titan at the landing site has a dielectric constant of 2.5±0.3 and a conductivity of 1.2±0.6 nS/m, in agreement with previously published results but with much more reliable error estimates. These values speak in favour of a photochemical origin of the material in the first meter of the subsurface. We also propose, for the first time, a plausible explanation for the sudden change observed by PWA-MIP ~11 min after landing: this change corresponds to a drop in the ground conductivity, probably due to the removal of a superficial conductive layer in association with the release of volatile materials warmed by the Huygens Probe.

Titan's atmosphere and climate

“…Analyses of measurements taken by the Huygens Probe and assessment of its behavior upon landing indicate that the surface at the Huygens landing site was fine grained, relatively soft, and possibly covered in a 7 mm thick “fluffy” layer [ Zarnecki et al , ; Lorenz , ; Karkoschka et al , ; Atkinson et al , ; Schröder et al , ]. The surface also contained volatiles as indicated by the increase in abundance of CH 4 after the Huygens Probe landed, most likely resulting from the heating of liquid CH 4 that was present on the surface [ Niemann et al , , ], attenuation of acoustic signals after landing [ Lorenz et al , ], a change in the PWA‐MIP observations 11 min after landing [ Hamelin et al , ], and modeling of DISR spectra [ Rannou et al , ]. Located in a dry region of Titan, Williams et al [] interpret the moisture detected at the Huygens landing site as evidence of a recent local precipitation event or the result of recharging from a precipitation event located farther away because the surface should dry relatively rapidly.…”

Electrical Properties of Tholins and Derived Constraints on the Huygens Landing Site Composition at the Surface of Titan

“…More importantly, we highlight that the sounding depths of the Cassini Radar/radiometer and of PWA‐MIP/HASI are similar: both instruments are sensitive to the composition and structure of, roughly, the first meter below the surface (Hamelin et al, ; Janssen et al, ). More specifically, in the case of PWA‐MIP/HASI it is a weighted volume average of the first meter with a weight decreasing with depth (for a more in‐depth discussion see Lethuillier, ).…”

“…Lastly, the permittivity probe PWA‐MIP/HASI on board the Huygens probe measured both the real and imaginary parts of the complex permittivity of the HLS in the extremely and very low frequency range. At 45 Hz (frequency for which the PWA‐MIP calibration is best), Hamelin et al () found and in the interval 0.2–0.8 for the first 11 min after the landing and and in the interval 0–0.1 after. These values are consistent with the ones previously published (Grard et al, ) but with more reliable error bars that account for uncertainties related to the probe's resting attitude on the surface.…”

“…This work also offers the opportunity to investigate further the implications of our favorite scenario for the sudden drop of conductivity (or ) observed by the PWA‐MIP/HASI instrument about 11 min after the Huygens landing (Hamelin et al, ) (see also Table ).…”

“…The conductivity drop observed by PWA‐MIP/HASI occurred in a very short amount of time (2–4 s) and concerns an area, below the Huygens lander, as wide as the distance between the MIP electrodes, that is, 1–2 m (Hamelin et al, ). We reiterate that the concurrent observed decrease in is not strong enough (relative to measurement uncertainty) to be considered significant.…”