The interaction between the SEIS seismometer of the InSight Martian mission and a regolith simulant

Abstract

A detailed investigation has been conducted of the interaction between a Martian regolith simulant and the foot of a seismometer (SEIS) recently deployed on the surface of Mars within the NASA InSight mission. A specific device used to investigate the SEIS/ground interaction was improved to provide accurate measurements of low forces and displacements, with a higher system stiffness and appropriate thermal insulation. A series of tests was carried out with a 60 mm dia. disc and the SEIS foot (disc with a spike at its centre). The maximum disc penetration in the loose sand used as simulant under the SEIS weight (10 N) was between 400 and 600 μm, with a tiny effect of the spike. Load cycles under various forces were performed to investigate the elastic interaction, with good reversibility and a linear change of the Young's modulus with respect to the average vertical stress. The tests provided comparable values, showing that the Young's modulus was around 20 MPa – which is compatible with that of loose terrestrial sands – and agreed well with the seismic wave velocities at the surface (from laboratory experiments and from measuring on the surface of Mars the travel times of waves received by the SEIS seismometer).