A detailed investigation 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 has been conducted. 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 were carried out with a 60 mm diameter disk and the SEIS foot (disk with a spike in the disk centre). The maximum disk 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 modulus with respect to the average vertical stress. The tests provided comparable values, showing that the Young modulus was around 20 MPa, compatible with that of loose terrestrial sands and agreeing well with the seismic wave velocities at surface (from laboratory experiments and from measuring on the surface of Mars the travel times of waves received by the SEIS seismometer).