The Site Tilt and Lander Transfer Function from the Short-Period Seismometer of InSight on Mars

Abstract

The National Aeronautics and Space Administration InSight mission has deployed the seismic experiment, SEIS, on the surface of Mars, and has recorded a variety of signals including marsquakes and dust devils. This work presents results on the tilt and local noise sources, which provide context to aid interpretation of the observed signals and allow an examination of the near-surface properties. Our analysis uses data recorded by the short-period sensors on the deck, throughout deployment and in the final configuration. We use thermal decorrelation to provide an estimate of the sol-to-sol tilt. This tilt is examined across deployment and over a Martian year. After each modification to the site, the tilt is seen to stabilize over 3–20 sols depending on the action, and the total change in tilt is <0.035°. Long-term tilt over a Martian year is limited to <0.007°. We also investigate the attenuation of lander-induced vibrations between the lander and SEIS. Robotic arm motions provide a known lander source in the 5–9 Hz bandwidth, yielding an amplitude attenuation of lander signals between 100 and 1000 times. The attenuation of wind sensitivity from the deck to ground presents a similar value in the 1.5–9 Hz range, thus favoring a noise dominated by lander vibrations induced by the wind. Wind sensitivities outside this bandwidth exhibit different sensitivity changes, indicating a change in the coupling. The results are interpreted through a finite-element analysis of the regolith with a depth-dependent Young’s modulus. We argue that discrepancies between this model and the observations are due to local compaction beneath the lander legs and/or anelasticity. An estimate for the effective Young’s modulus is obtained as 62–81 MPa, corroborating previous estimates for the top layer duricrust.

In Situ Regolith Seismic Velocity Measurement at the InSight Landing Site on Mars

“…Stott et al. ( 2021 ) derived an estimate of Young's modulus from the forcing of the lander in the frequency range of 0.1–0.9 Hz (assuming a density of 1,300 kg/m 3 and a Poisson's ratio of 0.25). Converting the values from lander‐overload to surface‐pressure conditions provides a Young's modulus range of 30–40 MPa.…”

“…Converting the values from lander-overload to surface-pressure conditions provides a Young's modulus range of 30-40 MPa. The larger moduli found by Stott et al (2021) may be due to the assumption of different density and Poisson's ratio values but could also be an effect of the lower frequency contents of the analyzed seismic data in the leveling system and lander resonance studies and, hence, the larger volume related to the effective moduli observations. However, all estimates come with a significant uncertainty and any differences should be discussed with care.…”

“…Converting this value to the mean depth between SEIS at the surface and the mole tip at depth of 16 cm results in a Young's modulus of around 18 MPa. Stott et al (2021) derived an estimate of Young's modulus from the forcing of the lander in the frequency range of 0.1-0.9 Hz (assuming a density of 1,300 kg/m 3 and a Poisson's ratio of 0.25). Converting the values from lander-overload to surface-pressure conditions provides a Young's modulus range of 30-40 MPa.…”

MarsQuakeNet: A More Complete Marsquake Catalog Obtained by Deep Learning Techniques

“…The VBB instrument records extremely small ground motions (Lognonné et al., 2020) and is consequently also sensitive to the local environment. The main source of background noise in the bandwidths of interest for seismic events (0–10 Hz) is the wind that predominantly couples to the ground through lander induced vibrations (Murdoch et al., 2018; Stott et al., 2021), as modeled pre‐mission (Mimoun et al., 2017) and observed on mission data (Charalambous et al., 2021). This, wind driven background noise varies throughout the day by several orders of magnitudes.…”

“…An outstanding feature of the data set is the excitation of resonances which correspond to (a) wind‐excited eigenmodes of the lander (Dahmen, Zenhäusern, et al., 2021; Murdoch et al., 2018) or seismometer system (Hurst et al., 2021), some of which are associated with different positions of InSight's robotic arm and change during the mission (Dahmen, Zenhäusern, et al., 2021; Stott et al., 2021); (b) a measurement artifact that leads to thin spectral peaks at 1 Hz with overtones (Zweifel et al., 2021); and (c) a broad resonance at 2.4 Hz that has been associated with a subsurface structure (Compaire et al., 2021; Dahmen, Zenhäusern, et al., 2021; Hobiger et al., 2021). In contrast to the lander modes, the 2.4 Hz resonance is only seen in quiet periods and is excited by seismic events.…”