Dielectric and hardness measurements of planetary analog rocks in support of in-situ subsurface sampling

• Dielectric constant from radar sounding can measure subsurface hardness.
• On volcanic terrains dielectric constant is linearly proportional to rock hardness.
• Drilling penetration rate based on the rotary drill method is inversely correlated to the dielectric constant of volcanic rocks.
• Subsurface radar can optimize drilling experiments to assess targets of interest such as subsurface volatiles.

Accurate assessment of the subsurface mechanical characteristics and how they correlate with dielectric properties is crucial to optimize future drilling and sampling investigations on planetary bodies. For 12 different types of basaltic rocks with different hardnesses, we use capacitive cells to measure the real part of the dielectric constant over the frequency range 100–1000 MHz, and a Schmidt hammer hardness tester to measure the hardness using a scale of 10–100. Our measurements suggest that the real part of the dielectric constant and rock hardness are linearly correlated. Additionally, sample hardness was linearly correlated to density. For a density ranging from 0.82 to 3.05 g/cm3, the real part of the dielectric constant ε′ and rebound hardness values R ranged from ε′=1.8–7.6 and R=14.16–68 for the different basalt samples. Hence, high dielectric constants imply a high rock hardness value and vice versa. We concluded that for volcanic surfaces that are analogous to the Martian surface as well as other planetary surfaces, there is an inverse correlation between drilling penetration rate based on the rotary-percussive drill method and the dielectric constant. Dielectric inversion from planetary radar probing experiments proposed herein is a crucial method to locate regions with lowest hardness and hence highest drilling penetration rate in desiccated volcanic planetary subsurfaces. The use of these cross-correlation measurements can optimize future drilling experiments and ensure that they reach their targets of opportunities, minimize losses in drilling performance, or the unnecessary use of power that will be needed for the continuity of the investigation.