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 [Formula: see text] 40 km by modeling the triplicated waveforms. From this depth range, we infer a mantle potential temperature of 1,605 [Formula: see text] 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/m².

Keywords: interior of Mars; mantle transition zone; thermal evolution of Mars.

Fig. 1.

Ray path geometry and predicted travel times of mantle triplications in Mars. Teleseismic ray paths of (A) P waves and (B) S waves interacting with the discontinuities in the midmantle, colored according to their bottoming depths. Triplications formed between 60° and 85° from the InSight lander (blue triangle). Pink stars indicate the locations of five LF and BB events which are calculated from $t_S-t_P$ measurements using the EH45coldCrust1 model (38) and assuming a 30-km source depth. Reduced travel time curves (time–distance/velocity) of (C) P and (D) S triplications predicted from the EH45TcoldCrust1 model. Reduction velocities are 12 and 6.5 km/s for P and S waves, respectively. Branches A to C correspond to triplications from ∼800 km depth (associated with the opx to HP-cpx transition), and branches C to F correspond to triplications from ∼1,000 km depth (associated with the postolivine transition). The gray dashed lines highlight the epicentral distances of five LF and BB events.

Fig. 2.

InSight seismic observations of P and S triplications and synthetic waveform fits. Polarization filtered (A) P waveforms on the vertical component (BHZ) and (B) S waveforms on the transverse (BHT) or radial (BHR) components are shown in black curves. We selected either the BHT or BHR component based upon the SNR of the S waves. The bandpass filters for each waveform are listed to the right. Red curves denote the synthetic waveforms from the best-fitting models, with the CCs listed to the right. The parameters of the best-fitting models are shown on the top, and KC08 (46) and YM20 (20) represent the assumed composition models. Background colors denote the amplitudes of P-wave synthetics on the BHZ component and S-wave synthetics on the BHT component, assuming a 30-km source depth. We only consider the fits between data and synthetics in specific time windows (−5 to 8 s for P waves and −5 to 20 s for S waves) to avoid contamination from other seismic phases and noise. Note that the time windows for the S waves in S0185a and S1094b events are extended to account for possible later arrivals of triplications.

Fig. 3.

Depth of the 1,000 discontinuity in Mars from mineral physics and synthetic waveform fits. (A) V_P and (B) V_S profiles from mineral physics models colored by the misfits of P and S triplications, respectively. Abbreviations for mantle minerals are as follows: olivine (ol), wadsleyite (wad), ringwoodite (rw), garnet (gt), clinopyroxene (cpx), orthopyroxene (opx), and high-pressure clinopyroxene (HP-cpx). The red dashed lines in A and B highlight the best-fitting models for P and S triplications, respectively. The velocity jump at ∼1,000 km depth is associated with the postolivine transition. A shallower minor discontinuity at ∼800 km corresponds to the opx to HP-cpx transition. (Insets) Crustal and upper mantle structures of the best-fitting model. Misfits of (C) P triplication, (D) S triplication, and (E) total misfits (weighted sum of the misfits of P and S triplications) are shown as a function of the 1,000 discontinuity depth. The colors represent the mantle potential temperature ($T_P$), and the symbols correspond to six composition models: DW85 (14), EH45 (17), KC08 (46), LF97 (15), TAY13 (19), and YM20 (20). Sizes of the symbols denote the mantle adiabatic gradients: 0.125 K/km (small), 0.15 K/km (medium), and 0.175 K/km (large).

Fig. 4.

Implications for the temperature and bulk composition of Martian mantle. (A) Depths of postolivine transitions are shown as a function of temperature. Note that the depths are converted to corresponding pressures using the density profiles. Solid lines in color represent the phase boundaries between olivine and wadsleyite/ringwoodite for various Mg # [molar Mg/(Mg + Fe) $\times$ 100%]. These phase boundaries were derived using the temperature and pressure at the center depth of the phase transition. The gray shaded region represents the observed range of 1,000 depth and corresponding pressures. The colored shaded regions denote the mantle temperature profiles inferred from premission estimates (pink) (56), upper mantle structures (orange) (12), and this study (blue). The dashed lines highlight the upper and lower bounds of these temperature profiles. (B) Thicknesses of postolivine transitions for various Mg # models, which are measured along the isentrope, are shown as a function of temperature. The phase transition thicknesses are converted to the corresponding pressure intervals using the density profiles. Note that the slope change is associated with the switch from olivine–ringwoodite to olivine–wadsleyite transitions as temperature increases. The gray shaded region indicates the constrained range of discontinuity thickness (60 $\pm$ 40 km or 0.76 $\pm$ 0.51 GPa) from mantle triplications. The blue shaded region shows the constrained temperature range (1,670 to 1,892 K) at the 1,000 depth.