The Seismic Moment and Seismic Efficiency of Small Impacts on Mars

N Wójcicka,I Daubar,P Lognonné,G S Collins,K Miljković,A Rajšić,I D Bastow,N A Teanby

doi:10.1029/2020je006540

Abstract

AbstractSince landing in late 2018, the InSight lander has been recording seismic signals on the surface of Mars. Despite nominal prelanding estimates of one to three meteorite impacts detected per Earth year, none have yet been identified seismically. To inform revised detectability estimates, we simulated numerically a suite of small impacts onto Martian regolith and characterized their seismic source properties. For the impactor size and velocity range most relevant for InSight, crater diameters are 1–30 m. We found that in this range scalar seismic moment is 106–1010 Nm and increases almost linearly with impact momentum. The ratio of horizontal to vertical seismic moment tensor components is ∼1, implying an almost isotropic P wave source, for vertical impacts. Seismic efficiencies are ∼10−6, dependent on the target crushing strength and impact velocity. Our predictions of relatively low seismic efficiency and seismic moment suggest that meteorite impact detectability on Mars is lower than previously assumed. Detection chances are best for impacts forming craters of diameter >10 m.

Highlights

Since early 2019, the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander has been measuring seismic signals on Mars
Modifications to SALE include an elastoplastic constitutive model, fragmentation and strength models for dilatant geological materials (Collins, 2014; Collins et al, 2004; Ivanov et al, 1997; Melosh et al, 1992), various equations of state (EoS), and a porosity compaction model (Collins et al, 2011; Wünnemann et al, 2006). In this first investigation of seismic signals generated by impacts on Mars we considered a range of typical impact scenarios likely to be detected by InSight, including the size range of the newly occurring nearby crater
Our simulations show that each estimate of the scalar seismic moment is linearly proportional to impact momentum (Figure 6)

Summary

Introduction

Since early 2019, the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander has been measuring seismic signals on Mars. By September 2019, it had detected 174 marsquakes, providing the first measurements of Martian seismicity (Banerdt et al, 2020). No InSight signals have been unequivocally attributed to meteorite impacts (Banerdt et al, 2020; Giardini et al, 2020). Nominal prelanding predictions estimated that in one Martian year between a few and tens of small impacts (forming craters of diameter 0.5–20 m) would be detectable by InSight, the majority of which would be occurring on a regional scale (Daubar et al, 2018; Teanby, 2015; Teanby & Wookey, 2011). While the uncertainty on these detection estimates is large (ranging 0.1–30 per Earth year), the discrepancy between predictions and observations suggests that some of the assumptions relating to impact detectability require revision

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Conclusion