Seismic detection of a deep mantle discontinuity within Mars by InSight

Abstract

Constraining the thermal and compositional state of the mantle is crucial for deciphering the formation and evolution of Mars. Mineral physics predicts that Mars’ deep mantle is demarcated by a seismic discontinuity arising from the pressure-induced phase transformation of the mineral olivine to its higher-pressure polymorphs, making the depth of this boundary sensitive to both mantle temperature and composition. Here, we report on the seismic detection of a midmantle discontinuity using the data collected by NASA’s InSight Mission to Mars that matches the expected depth and sharpness of the postolivine transition. In five teleseismic events, we observed triplicated P and S waves and constrained the depth of this discontinuity to be 1,006 ± 40 km by modeling the triplicated waveforms. From this depth range, we infer a mantle potential temperature of 1,605 ± 100 K, a result consistent with a crust that is 10 to 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 thermochemical evolution of Mars, we observe that only two out of the five proposed composition models are compatible with the observed boundary depth. Our geodynamic simulations suggest that the Martian mantle was relatively cold 4.5 Gyr ago (1,720 to 1,860 K) and are consistent with a present-day surface heat flow of 21 to 24 mW/m 2 .

Structure Along the Martian Dichotomy Constrained by Rayleigh and Love Waves and their Overtones

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Using seismic recordings of event S1222a, we measure dispersion curves of Rayleigh and Love waves, including their first overtones, and invert these for shear velocity (Vs) and radial anisotropic structure of the martian crust. The crustal structure along the topographic dichotomy is characterized by a fairly uniform vertically-polarized shear velocity (Vsv) of 3.17 km/s between ˜5-30 km depth, compatible with the previous study by Kim et al (2022). Radial anisotropy as large as 12 % (Vsh > Vsv) is required in the crust between 5-40 km depth.

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“…Using the available frequency content from the surface waves, we invert the group velocities to obtain profiles of S-wave velocity and radial anisotropy down to ~90 and 50 km depth, respectively. We compare our results with previously published models derived from group velocity dispersion of fundamental-mode Rayleigh waves based on S1000a and S1094b (Kim et al, 2022) and discuss implications for lateral variations in crustal structure across the topographic dichotomy on Mars. surface wave arrivals with dispersion.…”

“…Since then, analysis of waveforms of body wave phases from marsquakes (Giardini et al, 2020) and impacts (Garcia et al, 2022;Posiolova et al, 2022) have resulted in important discoveries about its interior structure. They include the characterization of crustal structure (Lognonne et al, 2020;Knapmeyer-Endrun et al, 2021;Kim et al, 2021a) and mantle velocity structure (Khan et al, 2021;Drilleau et al 2022;Duran et al 2022), as well as the detection of the martian transition zone (Huang et al 2022) and core (Stähler et al, 2021;Khan et al, 2022). Recently, the detection of surface waves enabled the characterization of crustal structure variations away from the InSight landing site (Kim et al, 2022).…”

“…The first observed Rayleigh waves originated from two large meteoroid impacts, called S1000a and S1094b (Posiolova et al, 2022; see also Horleston et al, 2022. The photographic identification of the craters associated with the two events simplified the interpretation of surface wave dispersion measurements by fixing the hypocentral depth and location (Kim et al, 2022). However, the limited frequency content and the absence of Love waves in the record prevented crustal structure below 30 km and anisotropy associated with the martian crust to be constrained.…”

“…However, the limited frequency content and the absence of Love waves in the record prevented crustal structure below 30 km and anisotropy associated with the martian crust to be constrained. In addition, the paths of the minor-arc Rayleigh waves (R1) were mostly confined to the crust in the northern lowlands in the vicinity of Elysium (Kim et al, 2022). A weaker major-arc Rayleigh wave (R2) arrival was also reported, but due to unclear polarization of the data, the interpretation of structural constraints provided by this seismic phase was limited.…”