Retrieval of near-surface properties of martian regolith using under-sampled repeating hammer source during the 2018 insight mission

Abstract

The InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission will be 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 Probe (HP3) experiment will hammer a probe 5 m into the Martian subsurface to measure the heat coming from Mars' interior and reveal the planet's thermal history (Figure 1). The probe, which uses a self-hammering mechanism, will generate thousands of seismic signals that can be used to analyze the shallow (several tens of meters) subsurface and shed new light on the mechanical properties of Martian regolith. The descent will progress in ~0.5 m hammering intervals, each interval taking between 0.5-4 hours, and each interval being separated by several days of thermal measurements. Each hammering interval consists of several hundred to several thousand strokes ~3 s apart, depending on the regolith properties. Although not included in the mission’s level 1 science objectives, which 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 study of the shallow geological structure at the landing site. Understanding the seismic properties of Martian regolith and determining its thickness will certainly reduce InSight’s seismic measurement errors. However, an added benefit is the opportunity to conduct the first ever seismic geotechnical study of the Martian soil and provide new essential knowledge for future robotic and human exploration missions. Given that geotechnical analysis of the landing site was not part of the mission threshold objectives InSight’s Seismic Experiment for Interior Structure (SEIS) experiment was not designed to accommodate the high sampling rates and strict source-sensor timing synchronization requirements that are part and parcel of similar terrestrial surveys. Nevertheless, the seismometer’s sensitivity and the multitude of hammering impulses make near-surface seismic exploration of Mars feasible. In this paper we outline the methodologies that will be incorporated in the analysis of HP3’s seismic signals to overcome some of the technical challenges that SEIS operation presents. Through a combination of laboratory measurements and numerical simulations we demonstrate that we will be able to determine the seismic pressure and shear (P and S) wave velocity, the thickness of the regolith layer and the corresponding mechanical properties of the regolith Presentation Date: Wednesday, October 19, 2016 Start Time: 11:35:00 AM Location: 147/154 Presentation Type: ORAL