Titan Ground Complex Permittivity at the HUYGENS Landing Site; the PWA-HASI and Other Instruments Data Revisited

Abstract:

Ten years after the successful landing of the HUYGENS probe on the surface of Titan, we reassess the complex permittivity measurements of the surface materials performed by the PWA-HASI experiment (Permittivity, Waves and Altimetry - Huygens Atmospheric Structure Instrument).

The complex permittivity is inferred from the mutual impedance of a classical quadrupolar probe, ie. the ratio of the voltage measured by a receiving dipole over the current emitted by another dipole. Using a simple model of the quadrupole configuration, the dielectric constant of the material at the landing site was first estimated to be of the order of 1.8. A more realistic numerical model that took into account the influence of the HUYGENS gondola yielded a dielectric constant in the range 2-3 and a conductivity in the range 0.4 - 0.8 nS/m. due to uncertainties about the system geometry ( Grard et al., 2006). However, a puzzling experimental fact remains to be explained, namely a sudden variation of the amplitude and phase of the received voltage 11 mn after landing that cannot be associated with any lander mechanical disturbance. Permittivity estimations were based on the first 11 mn sequence.

The present analysis takes advantage of a recent analysis of the landing process that provided more realistic final position and attitude for the HUYGENS lander (Schroder et al., 2012). The new results lie within former estimated ranges and attention is paid to their sensitivity to geometry and to the reference measurements collected immediately before landing. This point is particularly critical for the estimation of the conductivity.

The complete data set has been analysed, including the sequence collected after the first 11 mn. We consider various scenarios that may explain the observed phase and amplitude discontinuity. We tested two layers ground models in order to investigate the possibility that the upper layer may have experienced a fast physical change due to deliquescence or outgasing. Unfortunately a rigid quadrupolar array measure the average electric properties of the ground and cannot detect any inhomogeneity.

We present in addition the measurements made last May in the Dachstein ice cave in Austria, with a mockup of HUYGENS-PWA and a replica of the PP-SESAME instrument onboard the PHILAE lander of ROSETTA