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

• Published in: Journal of geophysical research. Planets Vol. 123; no. 4; pp. 807 - 822
• Main Authors: Lethuillier, A., Le Gall, A., Hamelin, M., Caujolle‐Bert, S., Schreiber, F., Carrasco, N., Cernogora, G., Szopa, C., Brouet, Y., Simões, F., Correia, J. J., Ruffié, G.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-04-2018; Wiley-Blackwell
• Subjects: Altimetry; Analogs; Astrophysics; Atmospheric structure; Cassini mission; Cassini‐Huygens; Complex permittivity; Computer simulation; Constraint modelling; Dust; Electrical conductivity; Electrical properties; Electrical resistivity; Huygens probe; Impedance probes; Landing; Lunar surface; Moon; Mutual impedance; Numerical simulations; Permittivity; Porosity; Radar; Saturn; Saturn satellites; Sciences of the Universe; Spacecraft; tholins; Titan; Water ice; Cassini-Huygens; Permittivity; Titan; Tholins
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1002/2017JE005416

In 2005, the complex permittivity of the surface of Saturn's moon Titan was measured by the PWA‐MIP/HASI (Permittivity Wave Altimetry‐Mutual Impedance Probe/Huygens Atmospheric Structure Instrument) experiment on board the Huygens probe. The analysis of these measurements was recently refined but could not be interpreted in terms of composition due to the lack of knowledge on the low‐frequency/low‐temperature electrical properties of Titan's organic material, a likely key ingredient of the surface composition. In order to fill that gap, we developed a dedicated measurement bench and investigated the complex permittivity of analogs of Titan's organic aerosols called “tholins.” These laboratory measurements, together with those performed in the microwave domain, are then used to derive constraints on the composition of Titan's first meter below the surface based on both the PWA‐MIP/HASI and the Cassini Radar observations. Assuming a ternary mixture of water ice, tholin‐like dust and pores (filled or not with liquid methane), we find that at least 10% of water ice and 15% of porosity are required to explain observations. On the other hand, there should be at most 50–60% of organic dust. PWA‐MIP/HASI measurements also suggest the presence of a thin conductive superficial layer at the Huygens landing site. Using accurate numerical simulations, we put constraints on the electrical conductivity of this layer as a function of its thickness (e.g., in the range 7–40 nS/m for a 7‐mm thick layer). Potential candidates for the composition of this layer are discussed.
Key Points
The complex permittivity of tholins was measured in laboratory at low frequencies and low temperatures
By comparison with measurements from Cassini/Huygens, constraints on the composition of the Huygens landing site are derived
There is evidence for the presence of a conductive superficial layer at the Huygens landing site