Geophysical evidence of progressive Noachian crustal thickening on Mars revealed by meteorite impacts

As a single-plate planet without plate tectonics, Mars lacks the horizontal motions of rigid plates. According to geochemical and geothermal analyses, the current thickness of the Martian crust from seismic observations is twice as thick as it was during the early Noachian period, indicating substantial crustal accretion over time. However, the mechanism behind this process remains unknown. Here, we employ the ground-truth-location nearby impact events to invert for the crustal layering and seismic velocities beneath the Elysium Planitia. We find a crustal thickness of 52 ± 9 km and propose that the lower boundary of the middle crust at ∼20 km corresponds to the early Noachian crustal base. Our models yield slower wave speeds in the lowermost crust and uppermost mantle compared to previous studies, based on additional sampling provided by six nearby impacts. Our inversion scheme allows the compressional-to-shear velocity ratio (Vp/Vs) to vary rather than assuming fixed values. The low Vp/Vs ratio (<1.7) in the mid-crust does not suggest the presence of liquid water in this region. The integration of refined velocity models with an elevated Vp/Vs ratio indicates the presence of partial melt, estimated to be less than 2 % in the lower crust and uppermost mantle. Altogether, these findings suggest that progressive magma underplating above a mantle plume led to a localized crustal thickening of the Noachian crust. A similar process might have taken place on the early Earth beneath cratons.