First observations of core-transiting seismic phases on Mars

Abstract

We present the first observations of seismic waves propagating through the core of Mars. These observations, made using seismic data collected by the InSight geophysical mission, have allowed us to construct the first seismically constrained models for the elastic properties of Mars' core. We observe core-transiting seismic phase SKS from two farside seismic events detected on Mars and measure the travel times of SKS relative to mantle traversing body waves. SKS travels through the core as a compressional wave, providing information about bulk modulus and density. We perform probabilistic inversions using the core-sensitive relative travel times together with gross geophysical data and travel times from other, more proximal, seismic events to seek the equation of state parameters that best describe the liquid iron-alloy core. Our inversions provide constraints on the velocities in Mars' core and are used to develop the first seismically based estimates of its composition. We show that models informed by our SKS data favor a somewhat smaller (median core radius = 1,780 to 1,810 km) and denser (core density = 6.2 to 6.3 g/cm³) core compared to previous estimates, with a P-wave velocity of 4.9 to 5.0 km/s at the core-mantle boundary, with the composition and structure of the mantle as a dominant source of uncertainty. We infer from our models that Mars' core contains a median of 20 to 22 wt% light alloying elements when we consider sulfur, oxygen, carbon, and hydrogen. These data can be used to inform models of planetary accretion, composition, and evolution.

Keywords: Mars; core evolution; planetary structure.

Fig. 1.

Location map, seismic data, and frequency-dependent polarization analysis for events S0976a and S1000a. (A) Locations of the two farside events, S0976a (red circle) and S1000a (blue star), and the InSight seismometer (orange triangle). The dotted lines show the SKS path in the mantle, and the solid lines depict the part of the SKS path in Mars’ core. [Surface topography model from ref. . Raypaths of seismic phases SKS and PP are shown in the same colors as events. SKS travels through the core; PP remains in the mantle. PP may have multiple arrivals at this epicentral distance (10); we show the path of the first propagating wave. SS, used together with PP as a reference phase, has a very similar path to PP (SI Appendix, Figs. S15 and S16). (B) Radial (blue), transverse (gray), and vertical (orange) component seismograms for S1000a (Left) and S0976a (Right), together with travel time picks. Above the radial component, we show its envelope. (C) Horizontal-vertical summed FDPA intensity as a function of time (analysis method A). The strong horizontally polarized signal is interpreted as the arrival of SKS.

Fig. 2.

Inversion results for the seismic properties of Mars’ core. Geophysical inversion results are shown in blue, geodynamical results are shown in orange. (A and B) Density and seismic velocity models for Mars’ core. In panel (B), the gray area indicates the results of geodynamical inversions carried out without using the SKS differential travel times. (C) Average density and core radius of Mars. The histograms above and to the right display the posterior distributions of the average density and core radius, respectively. (D) Observed (black) and predicted (colors) travel times.

Fig. 3.

Core velocity and density at the CMB compared with equation of state predictions. Results from the geodynamical inversions with and without SKS data are shown, with the lightly and strongly shaded areas indicating 90% and 50% of the models, respectively. The lines correspond to predictions for liquid Fe–S, Fe–O–S, Fe–O–S–C, and Fe–O–S–C–H alloys. Moving along each line corresponds to variations in the amount of sulfur present (wt% S is indicated by the numbers along the line). Lines are dashed where the alloys contain more than 18 wt% sulfur. When present, carbon is at saturation level and hydrogen is fixed at 1 wt%.

Fig. 4.

Schematic showing core-transiting ray-paths through the three seismically explored planetary bodies: Earth (38), Mars, and the Moon (78). Earth’s inner core was discovered some thirty years after the outer core (79). Colors within each body correspond to different dominant minerals and phases. Mars has an upper mantle dominated by olivine (shown in green), and a mantle discontinuity corresponding to the post-olivine phase transition (10) indicated by dark blue. On Earth, below the olivine-rich upper mantle and the transition zone, the lower mantle is predominantly bridgmanite (light blue); the lowermost mantle is not shown. The liquid metallic core of each body is shown in shades of yellow, while on the Moon and Earth, the inner core is shown in gray, and the Moon’s partial melt layer is shown in red.