Characterization of Mars Analog Soils with Microwave Radiation to Investigate Subsurface Water Extraction Utilizing Dielectric Heating

Abstract

Dielectric heating of planetary simulant soils with microwave radiation has been demonstrated by others to be a potentially sustainable method of extracting subsurface water in planetary soils. Investigating subsurface water extraction stems from the need to acquire water resources to support human and robotic missions to Mars. However, dielectric heating is fundamentally a function of temperature and frequency-dependent dielectric loss mechanisms and the thermal properties of indigenous soils. These loss mechanisms lead to energy dissipation (i.e., heat) from the coupling of microwave radiation to soils. It is well established that dielectric losses in the microwave region are optimized in materials with high liquid water content at ambient conditions (∼20◦C). However, due to the low temperature and pressure of the near-surface of Mars (∼ − 63◦C, 4.5 torr) stable liquid water does not exist; rather, subsurface ice is widespread across the planet. Considering these factors, an understanding of both the dielectric and thermal properties of simulant soils, which are strongly dependent on temperature and water content, will greatly benefit dielectric heating efforts on Mars. Therefore, this dissertation has analyzed dielectric loss mechanisms that contribute to the transfer of electromagnetic energy (2.6 – 18 GHz) to Mars simulant soils at environmental conditions comparable to those observed at the surface of Mars. In addition, dielectric heating of Mars simulants was performed using a low-power microwave transmission line system. Soil characterization studies demonstrated that dielectric loss mechanisms are significantly reduced at very low temperatures due to the absence of liquid water. However, unfrozen water films (indirectly observed in icy soils) produced higher losses than cold soils without ice. Moreover, heating icy, salty soils produced the highest heating rate among soil samples. Heating in lossy soils would be limited by the depth to which microwaves penetrate the subsurface (i.e., the penetration depth) combined with the thermal properties (e.g., thermal conductivity) of the soil. Given the insulative nature of soils observed on Mars; at low power, the subsurface could be heated just enough to liberate water vapor for extraction, without chemically altering the soil. Lastly, dielectric heating with microwave radiation could also have implications for the detection and sustainability of extant or extinct life in the subsurface of Mars.