Seismic detection of a deep mantle discontinuity within Mars by InSight

Constraining the thermal and compositional state of the mantle is crucial for deciphering the formation and evolution of Mars. As predicted by mineral physics, Mars’ deep mantle is demarcated by a seismic discontinuity with a depth that is sensitive to both mantle temperature and composition, and is ascribed to the pressure-induced phase transformation of the mineral olivine to its higher-pressure polymorphs. Here, we report on the seismic detection of a mid-mantle discontinuity using the data collected by NASA’s InSight Mission to Mars that matches the expected depth and sharpness of this post-olivine transition. In five teleseismic events, we observed triplicated P- and S-waves, and constrained the depth of this discontinuity to be 1006±40 km by modeling the triplicated waveforms. From this depth range, we infer a mantle potential temperature of 1605±100 K, a result consistent with a crust that is 10-15 times more enriched in heat-producing elements than the underlying mantle. Our waveform fits to the data indicate a broad gradient across the boundary, implying that the Martian mantle is more enriched in iron compared to Earth. Through modeling of thermo-chemical evolution of Mars, we observe that only two out of the five proposed composition models are compatible with the observed boundary depth. These geodynamic simulations suggest that the Martian mantle was relatively cold 4.5 Gyr ago (1720-1860 K), and are consistent with a present-day surface heat flow of 21-24 mW/m2