Seismic Coupling of Short-Period Wind Noise Through Mars’ Regolith for NASA’s InSight Lander

Abstract

NASA’s InSight lander will deploy a tripod-mounted seismometer package onto the surface of Mars in late 2018. Mars is expected to have lower seismic activity than the Earth, so minimisation of environmental seismic noise will be critical for maximising observations of seismicity and scientific return from the mission. Therefore, the seismometers will be protected by a Wind and Thermal Shield (WTS), also mounted on a tripod. Nevertheless, wind impinging on the WTS will cause vibration noise, which will be transmitted to the seismometers through the regolith (soil). Here we use a 1:1-scale model of the seismometer and WTS, combined with field testing at two analogue sites in Iceland, to determine the transfer coefficient between the two tripods and quantify the proportion of WTS vibration noise transmitted through the regolith to the seismometers. The analogue sites had median grain sizes in the range 0.3–1.0 mm, surface densities of $1.3\mbox{--}1.8~\mbox{g}\,\mbox{cm}^{-3}$, and an effective regolith Young’s modulus of $2.5^{+1.9}_{-1.4}~\mbox{MPa}$. At a seismic frequency of 5 Hz the measured transfer coefficients had values of 0.02–0.04 for the vertical component and 0.01–0.02 for the horizontal component. These values are 3–6 times lower than predicted by elastic theory and imply that at short periods the regolith displays significant anelastic behaviour. This will result in reduced short-period wind noise and increased signal-to-noise. We predict the noise induced by turbulent aerodynamic lift on the WTS at 5 Hz to be $\sim2\times10^{-10}~\mbox{ms}^{-2}\,\mbox{Hz}^{-1/2}$ with a factor of 10 uncertainty. This is at least an order of magnitude lower than the InSight short-period seismometer noise floor of $10^{-8}~\mbox{ms}^{-2}\,\mbox{Hz}^{-1/2}$.