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

The InSight mission is a NASA geophysical mission aimed at better understanding the structure of Mars and of the other rocky plan-ets 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 geophys-ical data (which have definitely enriched and completed existing knowledge on the structure of Mars), the InSight instruments, togeth-er with orbiter observations and tests carried out on the soil with the IDA, have significantly increased the knowledge of the geologi-cal 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 self-penetrating 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

Seismic investigations of the Martian near-surface at the InSight landing site

The InSight ultra-sensitive broadband seismometer package (SEIS) was installed on the Martian surface with the goal to study the seismicity on Mars and the deep interior of the Planet. A second surface-based instrument, the heat flow and physical properties package HP3, was placed on the Martian ground about 1.1 m away from SEIS. HP3 includes a self-hammering probe called the ‘mole’ to measure the heat coming from Mars' interior at shallow depth to reveal the planet's thermal history. While SEIS was designed to study the deep structure of Mars, seismic signals such as the hammering ‘noise’ as well as ambient and other instrument-generated vibrations allow us to investigate the shallow subsurface. The resultant near-surface elastic property models provide additional information to interpret the SEIS data and allow extracting unique geotechnical information on the Martian regolith

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

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

Characterization of the InSight Landing Site near Surface Properties using the Heat Flow and Physical Properties Probe (HP3) mole as a Seismic Source

The InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission is the first Mars lander to place an ultra-sensitive broadband seismometer on the planet’s surface. About a meter away from the seismometer, a Heat Flow and Physical Properties Package (HP3) experiment hammered a probe into the Martian subsurface to measure the heat coming from Mars' interior and reveal the planet's thermal history. The probe, which uses a self-hammering mechanism, generated thousands of seismic signals that can be used to study the shallow subsurface and shed new light on the mechanical properties of Martian regolith. While the mission’s science objectives focus on planetary-scale seismic and tectonic processes and their implications to rocky planet formation, the proximity of a repeating hammer source to a sensitive seismometer presents a unique opportunity to carry out the first geotechnical study of the shallow Martian subsurface. The HP3 mole hammering mechanism produced distinct seismic signals, but using these signals for a geotechnical seismic profiling presents several challenges: The InSight Seismic Experiment for Interior Structure (SEIS) requires 100 samples-per-second data that results in under-sampling the HP3 Although each HP3 penetration so far produced over nine thousand hammer strokes, the ~4 s interval between them varies slightly depending on the regolith properties and on the temperature of the mole. A second stroke, ~0.06s following the initial stroke, also varies in time, likely obscures a reflection from an anticipated basalt layer several meters below the surface at the InSight landing site. To overcome these difficulties the analysis took advantage of the variation in the interval between strokes, and the repeatability of the signal, which varies extremely slowly between strokes, to reconstruct the signal and recover information above the nominal Nyquist frequency. Combined with careful synchronization of the seismometer and the heat-probe, and use of the probe’s internal tiltmeter timing information to determine source timing, we were able to determine travel-times and apparent P-wave velocities in the top meter of the regolith layer. Regolith layer thickness was inferred from auxiliary measurements and analysis of the oscillations excited by the hammer in the regolith layer, which overlays a faster brecciated basalt layer. We will present a comprehensive description of the experiment, including terrestrial analogue preparations, data, analysis methodologies, and interpretation