The mechanical properties of the Martian soil at the InSight landing site

Abstract

The InSight mission is a NASA geophysical mission aimed at better understanding the structure of Mars and of the other rocky planets of the solar system. To do so, various instruments are used, including a very sensitive seismometer (SEIS) and a dynamic self-penetrating heat probe (HP3) that have been placed on the Mars surface by the Instrument Deployment Arm (IDA). Besides geophysical data (which have definitely enriched and completed existing knowledge on the structure of Mars), the InSight instruments, together with orbiter observations and tests carried out on the soil with the IDA, have significantly increased the knowledge of the geological and geotechnical characteristics of the surface material at the InSight site, which is made up of a basaltic sand. In-situ data were also successfully compared with terrestrial previous estimates from terrestrial lab tests, carried out on various soil simulants. Small strain (elastic) parameters at small strains were derived from wave velocity measurements between the selfpenetrating probe and the seismometer. Strength data were derived from both IDA operations and penetration data. The soil includes some pebbles within a somewhat cohesive sandy matrix, limiting the heat probe penetration to only 40 cm length. Thermal data were also obtained, allowing for some thermo-elastic modelling of the effect of the Phobos (one of the “Moons” of Mars) eclipses. Elastic data were also derived from the effects of wind on the ground, detected by SEIS.