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

Abstract

We report on the observation of a diffracted P-wave (Pdiff) along the core of Mars from a distant impact that has been recorded by the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission. The identification of Pdiff allows us to sample the P-wave velocity structure of the lower mantle that hitherto could not be constrained because of lack of lower-mantle-traversing P-waves. In addition to Pdiff, we are able to pick PP-, PPP-, and SS-wave arrivals and locate the event to the farside of Mars in the vicinity of Tharsis, in agreement with the imaged location of the impact. This indicates that our joint single-station seismic event-location and structure-inversion scheme is both robust and accurate. Based on inversion of the body-wave arrival time picks made here, we find lower P-wave velocities in the deep mantle relative to predictions based on thermochemically homogeneous models.