Can Higher Mode Surface-Wave Dispersion Discriminate Between Different Mars Mantle Models?

The dispersion of surface-wave phase velocity is commonly used on Earth to constrain the depth dependence of shear-wave velocity inside the upper mantle, which in turn can help constrain the thermal state of our planet. With InSight's broadband seismometer, there is the potential to detect surface waves on Mars and thus to constrain properties of the Martian mantle. Because it is sensitive to deeper structure than the fundamental modes, higher mode Rayleigh wave dispersion in the 10-100s period range has the potential to constrain Mars mantle down to about 1000km depth, just above the likely location of where the olivine to wadsleyite phase transition.

In this work, we compare different published Mars interior models and determine whether fundamental and higher mode surface waves can distinguish between them. We test the two end-member models of Sohl and Spohn (1997) which were based on the bulk chondritic composition of SNC meteorites and on the contemporaneous estimate of the polar moment of inertia. We also test the results of Zheng et al (2015) who proposed the presence of an upper mantle low velocity layer beneath a thin high velocity ''seismic lid''. This predicts shadow zones that can affect surface wave detectability. Similarly, the Martian core formation modeling by Brennan et al. (2019) produced various mantle and core compositional models with different crustal thicknesses, mantle temperatures, and core radii, and these can affect which higher mode surface waves can be detected and their dispersion curves. Those models are also tested.

We use normal mode code MINEOS to predict dispersion curves and calculate synthetic seismograms for these different models and assess the detectability of fundamental and higher mode surface waves, as well as their dispersion curves, in the 10s to 100s period range. We test the effects of data noise, event location and source parameters, including magnitude and depth, to determine the threshold of detectability under various conditions. We also change the thickness of the regolith layer since it can affect the shorter period fundamental mode surface waves. In the future, we will be able to compare these results with future recordings of long period seismic events on Mars.