Observation of a Core‐Diffracted P‐Wave From a Farside Impact With Implications for the Lower‐Mantle Structure of Mars

Abstract

Since its deployment, the SEIS (Seismic Experiment for Interior Structure) instrument (Lognonné et al., 2019) as part of the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander has been passively recording marsquakes (Banerdt et al., 2020) for over 1300 Sols (a Sol is a Martian day and corresponds to ∼24 hr 40 min) since landing, including ∼90 marsquakes at teleseismic distances with moment magnitudes (M w ) in the range 2.5-4.2 (

Evidence for a Martian D"-like layer

“…As a consequence of the impedance contrasts at the top and bottom of the LSL, synthetic diffracted P-waves on both mantle-LSL (Pdiff ′ ) and LSLliquid core (Pdiff ′′ ) interfaces exist, in addition to a reverberation within the layer (Pdiff ′′′ ), all with similar move-out and waveform (Figure 3B and SM Figure S12). In line herewith, inspection of the observed waveform (Figure 3C) and scalogram (Figure 3D) shows, in addition to the main diffracted P-wave arrival (Pdiff ′′ ) [23], two arrivals with waveforms that match Pdiff ′′ (Figure 3E) and similar polarisation (Figure 3F). Furthermore, the polarisation of the observed Pdiff wavetrain is largely consistent with the imaged location of the impact (horizontal blue line in Figure 3F) [35].…”

“…Differential travel time and geophysical data őts are shown in SM Figure S11. In contrast to published geophysical models [6], we őnd that the core-diffracted P-wave arrival can be őt if a fraction of Mars's CMB region, previously believed to be the core [2, 6ś8], is allocated to a liquid silicate layer (hereinafter LSL) with Pdiff diffracting along its lower rather than its top boundary (Figure 2B) as considered earlier [23]. The radial seismic core proőles are compared to the models from [2] (Figure 2A) and indicate good agreement in terms of core P-wave velocity structure, but, as expected, differences in density since the shallow part of the core is found to belong to the mantle (see below).…”

“…This phase is, however, too weak to be identiőed in the observed data. Like [23], we were unable to identify diffracted S-waves.…”

“…To determine the seismic properties of Mars and thus the molten silicate layer, we rely on the crust, mantle, and core-converted, -reŕected, and -diffracted seismic phases (P, pP, PP, PPP, P-to-s, Pdiff, S, sS, SS, SSS, and ScS) described in [7,23] and the core-traversing phase (SKS) initially reported in [2] and updated here (SM section 3) from 15 low-frequency marsquakes and two meteorite impacts [35] with moment magnitudes M W in the ranges 3.0ś4.0 and 4.0ś4.1 [36], respectively, that cover an epicentral distance (∆) range from 23,35]. The updated differential travel time data set (all phases are relative to P or PP) presented herein (SM Table S3) comprise 82 seismic phase picks with bottoming depths to ∼2700 km that we simultaneously invert in combination with mean planet mass and moment of inertia for the radial seismic wave velocity structure and location (∆) for each event (SM Table S2).…”

“…In view of the recent observations of a P-wave diffracted along the coremantle-boundary [23] and P-waves traversing the core [2] from two far-side events [24], the causes of the low mean Martian core density can now be addressed quantitatively from P-wave velocities and densities. However, our ability to translate these measurements into composition and temperature is limited by the dearth of experimental measurements of the physical properties (P-wave velocity and density) of Fe-Ni-X (where X = S, C, O, and H) alloys at conditions relevant to Mars's core.…”