Seismic Velocity Variations in a 3D Martian Mantle: Implications for the InSight Measurements

Abstract

We use a large data set of 3D thermal evolution models to predict the distribution of present-day seismic velocities in the Martian interior. Our models show a difference between maximum and minimum S-wave velocity of up to 10% either below the crust, where thermal variations are largest, or at the depth of the olivine to wadsleyite phase transition, located at around 1000 – 1200 km depth. Models with thick lithospheres on average have weak low-velocity zones that extend deeper than 400 km, and seismic velocity variations in the uppermost 400 – 600 km that closely follow the crustal thickness pattern. For these cases the crust contains more than half of the total amount of heat producing elements. Models with limited crustal heat production have thinner lithospheres and shallower but prominent low-velocity zones that are incompatible with InSight observations. Seismic events suggested to originate in Cerberus Fossae indicate the absence of S-wave shadow zones in 25° - 30° epicentral distance. This result is compatible with previous best-fit models that require a large average lithospheric thickness and a crust containing more than half of the bulk amount of heat producing elements to be compatible with geological and geophysical constraints. Ongoing and future InSight measurements that will determine the existence of a weak low-velocity zone will directly bear on the crustal heat production.