Wave Velocities and Poisson Ratio in a Loose Sandy Martian Regolith Simulant Under Low Stresses: 1. Laboratory Investigation

Abstract: Wave velocity measurements were performed on Fontainebleau sand samples used as a Martian regolith simulant to investigate the elastic properties of the surface material at the InSight landing site on Mars (Elysium Planitia). Loose samples (density 1.4 Mg/m 3 , density index 6%) were prepared by using the pluviation method to mimic the low regolith density at the InSight landing site. A novel device derived from triaxial testing was designed to measure wave velocities at low stresses along a horizontal cylindrical specimen. Four tests were made, in which the confining stress was applied by applying a vacuum between 1 and 80 kPa. Wave velocities were measured by using bender elements under stresses as low as 1.75 kPa, a very low value compared to the standard stress ranges generally considered in terrestrial geotechnics (>10 kPa). The changes in compression and shear wave velocities obey a standard power law, with two slightly different exponents for V p and V s , indicating a not perfectly elastic behavior. Data showed greater variability below 5 kPa, indicating some limitations of the bender element technique in this range. A slight decrease in Poisson ratio was detected below 5 kPa, which certainly deserves more investigation. This investigation is useful to better analyze the data of the InSight mission, both in terms of wave propagation at the surface and to interpret some in situ elastic tests carried out with the scoop. These data are interpreted in the light of a granular contact mechanics theory in a companion paper