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