Lateral variations of the Martian crustal thickness from the InSight data set

By using arrival times of body waves recorded by the very broadband seismometer SEIS from the InSight mission, 1D average models of the crust, mantle, and core of Mars have been inferred [1, 2, 3]. However, possible causes of seismic wavefield disturbances such as 3D structure, undeniably complexify the interpretation as a 1D radial model. Because the InSight lander and the origin of marsquakes are located close to the crustal dichotomy between the Southern and Northern hemispheres, significant lateral variations of the relief along the crust-mantle interface and the surface topography can potentially affect the seismic wave arrival times, especially for the multiples (PP, PPP, SS, SSS).              

In contrast to investigations based on InSight seismic data, gravity and topography data allow one to address the question of lateral variations at global scale. However, the inversion of gravity and topography data is non-unique, in the sense that the latter generally assume constant values for the crust and mantle densities, and consider an average crustal thickness instead of lateral variations. We propose to use the crustal thickness range estimated below the InSight seismometer by [1] as an anchoring point.

To characterize the Martian crustal thickness, we use a Bayesian approach. The marsquake locations (back azimuth, epicentral distance, and depth) and the average radial seismic velocity model of Mars are randomly sampled. Vp, Vs, and density are parameterized in terms of quantities that govern the thermo-chemical evolution of Mars, which accounts for 4.5 Gyr of planetary evolution [5, 6]. In addition to improving the model self-consistency compared to varying independently seismological parameters along the inversion process (Moho depth, seismic velocities, etc.), this geodynamical approach can also constrain the value of physical quantities that are not accessible to direct measurements, such as the rheology of the Martian mantle, and provides the entire history of the planet associated. For each model, a 3D crust is generated, following the approach developed in [4], using the observed gravity field and topography as constraints, enabling to apply body wave arrival time correction for each seismic phase. The models that do not match the crustal thickness range estimated below the InSight lander [1] are rejected.

In addition to revealing the variability of the Martian global thickness maps, our approach shows that further insight into the value of the geodynamic quantities is gained by considering both seismic and gravity constraints on the present-day crustal thicknesses.

 

References:

[1] Knapmeyer-Endrun, B., et al., Science 373, 438-443 (2021)

[2] Khan, A., et al., Science, 373, 434-438 (2021)

[3] Stähler, S., et al., Science, 373, 443-448 (2021)

[4] Wieczorek, M., et al., JGR Planets, 124, 1410-1432 (2019)

[5] Drilleau, M., et al., JGR Planets, 226, 1615–1644 (2019)

[6] Samuel, H., et al., Nature, 569, 523–527 (2019)