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- Title
Wave Velocities and Poisson Ratio in a Loose Sandy Martian Regolith Simulant Under Low Stresses: 1. Laboratory Investigation.
- Authors
Betancourt, J. P. Castillo; Delage, P.; Caicedo, B.; Lognonné, P.; Banerdt, B.
- 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/m3, 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 Vp and Vs, 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. Plain Language Summary: The InSight mission lander is a highly sensitive seismometer placed on Mars to detect seismic waves from Marsquakes and meteorite impacts. To help analyzing the seismic waves, a special laboratory device was developed to measure wave velocities on a loose sandy simulant mimicking the surface regolith, with wave emitting/receiving transducers placed on both sides of a cylindrical sand sample subjected to external stress. Due to reduced gravity, the stresses that control wave velocity (larger stress, faster waves) are very low at the Mars surface. To impose low stresses (down to 1.75 kPa), we used a horizontal sample with a careful stress calculation along the sample axis, along which waves propagate. We determined the changes in wave velocity with stress (governed by a power law) and observed greater variability under 5 kPa, showing the limit of our transducers at low stresses. Based on elasticity theory, derived from wave velocity parameters governing sample deformations under strain changes. We suspect a slight decrease in the Poisson ratio (that characterizes radial elastic strain of a cylindrical sample under axial loading) below 5 kPa. Data is useful to better constrain the response of Mars sandy surface materials at the InSight site and other areas for future Mars missions. Key Points: Bender Element measurements of Vp and Vs carried out on a novel device on loose Fontainebleau sand samples used as a Mars regolith simulantInvestigation carried out within a so far unexplored low stress range, below 10 kPa and as low as 1.75 kPaPoisson ratio observed to remain mostly constant at low stress, with a possible decrease in the zone below 5 kPa
- Subjects
FONTAINEBLEAU (France); POISSON'S ratio; ELASTICITY; STRAINS &; stresses (Mechanics); THEORY of wave motion; REGOLITH; STRESS waves; SEISMIC waves
- Publication
Journal of Geophysical Research. Planets, 2023, Vol 128, Issue 11, p1
- ISSN
2169-9097
- Publication type
Article
- DOI
10.1029/2023JE007988