Relative permittivity measurement of Martian regolith simulant at UHF-SHF band under the Martian surface environment

Abstract:

Analysis of radar data obtained by the radar sounder and ground-penetrating radar depends on the evaluation of relative permittivity values which determine the velocity of electromagnetic waves and the reflection coefficient between mediums. While the measurement of relative permittivity values is widely performed on the Earth, these values cannot be applied directly to the analysis of radar data obtained on other celestial bodies. This is because most measurements are not performed under a controlled environment in terms of such as the water content and temperature, which can significantly affect the values. We develop a new measurement system to evaluate the relative permittivity of materials under low temperature (up to -60℃) and dry condition, which is similar to the condition on the Martian surface, using the coaxial probe method. Our system is capable of measuring the value of the laboratory-based simulated materials (simulants) at UHF-SHF band, applied to a couple of GPRs for the Martian explorations.

We report results of our measurements, especially of the permittivity of Martian regolith simulants developed in our laboratory, and show that the value decreases almost linearly as the temperature decreases. For example, the relative permittivity value of basaltic simulant at 3 GHz is about 3.55, 3.25, and 2.95 at room temperature (20 °C), -20 °C, and -60°C, respectively. Also, the values of other regolith simulants measure about 7-20 % lower at -60 °C compared with those at room temperature.

By using theoretically estimated values of the porosity and the thermal gradient of the Martian subsurface, we calculate the relative permittivity on the Martian subsurface. We find that the real part of the relative permittivity of basaltic regolith on the Martian surface is about 10 % higher than that of terrestrial rocks.

This indicates that the estimated depth by using permittivity values measured under the terrestrial surface condition might be overestimated because the velocity of electromagnetic waves becomes slower in the medium of the Martian subsurface having the higher real part of the relative permittivity.