Characterizing the Dielectric Properties of Geologic and Asteroid Regolith Analogue Material for Improved Planetary Radar Interpretation
Hickson, Dylan Christian
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Planetary radar has provided an increasingly growing number of datasets on the inner terrestrial planets and near-Earth and main-belt asteroid populations in our solar system. Physical interpretation of radar data for inference of surface properties requires constraints on the constitutive parameters of the material making up a given surface. For many planetary surfaces, the response to electromagnetic radiation is described by the complex permittivity. In this thesis, the dielectric response of several geologic materials as a function of frequency and porosity was characterized to supplement radar data interpretation. Using the coaxial transmission line method, the complex permittivity of seven powdered mineral samples was measured. The samples were characterized for their composition and structure using a variety of laboratory techniques. A detailed review of the theory and use of electromagnetic mixing equations was presented to introduce the range of models available to describe the experimental permittivity measurements. A thorough analysis of the experiments was performed which showed that the Looyenga-Landau-Lifshitz and Bruggeman (Symmetric) mixing models described the experimental results with the highest accuracy. Measurement bias in the coaxial transmission line method highlighted in previous research due to inhomogeneities at the sample/conductor interface was modelled using these mixing theories, providing a way to correct for these effects post-measurement. The variation in the permittivity of the solid mineral grains between different minerals was characterized based on the grain density of the minerals, as well as the chemical composition. The experimentally verified mixing models were incorporated into an existing asteroid radar model and were used to calculate the porosity in the near-surface of seven asteroids visited by robotic spacecraft. Comparing with bulk porosity estimates, the asteroid radar model indicated the presence of a porous regolith covering on each asteroid that is similar in porosity to the upper 30 cm of the Moon. The results from this research are important for future radar studies, and the model predictions for asteroid surface properties will be tested with results from upcoming space missions visiting asteroids, such as NASAs OSIRIS-REx and JAXAs Hayabusa2 missions.