Improving Constraints on Planetary Interiors With PPs Receiver Functions.

D Kim,V Lekić,F Karakostas,S Ceylan,R Joshi,Q Huang,B Knapmeyer‐Endrun,B Tauzin,R Maguire,A Khan,W B Banerdt,P Lognonné,M P Panning,J C E Irving,M A Wieczorek,D Giardini,N Schmerr

doi:10.1029/2021je006983

Abstract

Seismological constraints obtained from receiver function (RF) analysis provide important information about the crust and mantle structure. Here, we explore the utility of the free‐surface multiple of the P‐wave (PP) and the corresponding conversions in RF analysis. Using earthquake records, we demonstrate the efficacy of PPs‐RFs before illustrating how they become especially useful when limited data is available in typical planetary missions. Using a transdimensional hierarchical Bayesian deconvolution approach, we compute robust P‐to‐S (Ps)‐ and PPs‐RFs with InSight recordings of five marsquakes. Our Ps‐RF results verify the direct Ps converted phases reported by previous RF analyses with increased coherence and reveal other phases including the primary multiple reverberating within the uppermost layer of the Martian crust. Unlike the Ps‐RFs, our PPs‐RFs lack an arrival at 7.2 s lag time. Whereas Ps‐RFs on Mars could be equally well fit by a two‐ or three‐layer crust, synthetic modeling shows that the disappearance of the 7.2 s phase requires a three‐layer crust, and is highly sensitive to velocity and thickness of intra‐crustal layers. We show that a three‐layer crust is also preferred by S‐to‐P (Sp)‐RFs. While the deepest interface of the three‐layer crust represents the crust‐mantle interface beneath the InSight landing site, the other two interfaces at shallower depths could represent a sharp transition between either fractured and unfractured materials or thick basaltic flows and pre‐existing crustal materials. PPs‐RFs can provide complementary constraints and maximize the extraction of information about crustal structure in data‐constrained circumstances such as planetary missions.

Highlights

Planetary crusts preserve information about the thermal and magmatic history of a planet
We demonstrate the efficacy of PPs-receiver function (RF) before illustrating how they become especially useful when limited data is available in typical planetary missions
We show that using PP waves to compute receiver functions provides complementary information, to more commonly used direct P and S seismic arrivals and maximizes the amount of information extracted from limited data, which is helpful in the context of planetary missions

Summary

Introduction

Planetary crusts preserve information about the thermal and magmatic history of a planet. Lognonné et al (2003) illustrated the non-uniqueness of the RF travel times for determination of the lunar crustal thickness and the bias between low/high seismic velocities and thin/thick crust and suggested much thinner crust than that initially determined for the Moon by Toksöz et al (1974). These thin crust models were confirmed by the evidence for high crustal porosity in the GRAIL lunar gravity mission (Wieczorek et al, 2013). On Mars, preliminary RFs derived from two marsquakes (Lognonné et al, 2020) showed the first evidence of subsurface layering on Mars with low seismic velocities in the first upper 8–11 km; additional observations and inversions complemented by gravitational field modeling, have enabled average crustal thickness of Mars to be constrained between 24 and 72 km, with important geochemical and geodynamical implications (Knapmeyer-Endrun et al, 2021)

Results

Discussion

Conclusion