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. 2023 Oct;622(7984):718-723.
doi: 10.1038/s41586-023-06586-4. Epub 2023 Oct 25.

Evidence for a liquid silicate layer atop the Martian core

A Khan  1   2 , D Huang  3 , C Durán  4 , P A Sossi  5 , D Giardini  4 , M Murakami  5
Affiliations

Evidence for a liquid silicate layer atop the Martian core

A Khan et al. Nature. 2023 Oct.

Abstract

Seismic recordings made during the InSight mission1 suggested that Mars's liquid core would need to be approximately 27% lighter than pure liquid iron2,3, implying a considerable complement of light elements. Core compositions based on seismic and bulk geophysical constraints, however, require larger quantities of the volatile elements hydrogen, carbon and sulfur than those that were cosmochemically available in the likely building blocks of Mars4. Here we show that multiply diffracted P waves along a stratified core-mantle boundary region of Mars in combination with first-principles computations of the thermoelastic properties of liquid iron-rich alloys3 require the presence of a fully molten silicate layer overlying a smaller, denser liquid core. Inverting differential body wave travel time data with particular sensitivity to the core-mantle boundary region suggests a decreased core radius of 1,675 ± 30 km associated with an increased density of 6.65 ± 0.1 g cm-3, relative to previous models2,4-8, while the thickness and density of the molten silicate layer are 150 ± 15 km and 4.05 ± 0.05 g cm-3, respectively. The core properties inferred here reconcile bulk geophysical and cosmochemical requirements, consistent with a core containing 85-91 wt% iron-nickel and 9-15 wt% light elements, chiefly sulfur, carbon, oxygen and hydrogen. The chemical characteristics of a molten silicate layer above the core may be revealed by products of Martian magmatism.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Seismic properties of the core as seen with InSight and from first-principles simulations.
Comparison of seismic profiles from InSight with results from first-principles simulations. a,b, Density (a) and P-wave velocity (b) profiles of liquid Fe and liquid Fe–Ni–S–C–O–H senary mixtures in Mars’s core obtained from AIMD simulations and InSight observations. The blue shaded profiles represent the predicted seismic core properties based on magnetic (that is, spin-polarized) and non-magnetic (that is, non-spin-polarized) AIMD simulations, including the effects of variations in temperature (±200 K), for the core composition (comprising 67.7% Fe, 5.5% Ni, 7.2% S, 15.1% C, 3.8% O and 0.7% H by weight) that best fits the mean of the InSight seismic profiles within ±2σ at the CMB (indicated by the light orange vertical bar labelled ‘CMB’), corresponding to a radius in the range 1,790–1,840 km (ref. ). The orange shaded profiles represent the predicted seismic core properties based on magnetic and non-magnetic AIMD simulations, including the effects of variations in temperature (±200 K), for the core composition (comprising 69.9% Fe, 5.7% Ni, 14.6% S, 4.3% C, 4.7% O and 0.8% H by weight) that best fits the mean of the InSight seismic profiles within ±2σ at approximately 800 km in the core (indicated by the light orange vertical bar labelled ‘Inside core’). The labelled C values indicate the C contents of the best-fitting solutions at the CMB and at a depth of around 800 km in the core, respectively. As the orange and blue profiles do not overlap, no single composition exists that matches the InSight observations simultaneously at the CMB and in the core (Methods and Extended Data Fig. 4). Note that the width of the vertical light orange bars has no physical significance.
Fig. 2
Fig. 2. Summary of Mars’s interior structure.
a, Inverted S- and P-wave velocity and density profiles. For comparison, black solid and dashed lines represent the range of core profiles determined previously using seismic core-transiting (SKS) data. b, Body wave ray path geometry for all events (labelled with stars) considered in this study. Colour bar denotes ray path density, that is, the number of rays passing through a given area, based on the inverted models shown in a, which explains the diffuseness of the ray paths and source locations. The horizontal column below ‘InSight’ is the radial sensitivity and computed as the integrated ray path density with epicentral distance. Note that the SKS phase for event S0976a is only predicted and not inverted (see Supplementary Information section 1 for details). c, Inverted molten silicate layer (LSL) properties (in blue): mean density (ρ¯LSL), mean P-wave velocity (V¯PLSL) and thickness (ΔZ). d, Inverted core properties (in blue): mean density (ρ¯core) and core radius (Rcore). The probability contours shown in orange in c and d have been obtained by downsampling the models to additionally match the observed diffracted P-wave reverberation (Pdiff^LSLPdiff) in the LSL (see Fig. 3c and main text for details). Blue- and orange-shaded distributions on top and to the right of c and d indicate sampled probability distributions for the various parameters shown in the plots.
Fig. 3
Fig. 3. Molten silicate layer  and core seismic signatures.
a, Ray paths for LSL- and core-interacting phases: P wave and S wave reflected from the top of the LSL (grey layer) (PdP and SdS), P wave diffracted around the mantle–LSL interface (PdiffLSL), P wave reflected from the liquid core (PDcDP), P wave diffracted around the LSL–liquid core interface (PdiffCMB) and reverberating in the LSL (Pdiff^LSLPdiff), and liquid-layer and core-transiting P wave (SDKDS). b, Vertical-component synthetic waveform section showing the diffracted P wavetrain for epicentral distances similar to S1000a (126°, see Supplementary Fig. 7 for a larger section). c, Vertical-component observed polarized waveforms (filtered between 0.2–0.7 Hz) and envelopes showing the Pdiff arrivals, marked by red, blue and black lines, respectively. The vertical-component template trace employed for waveform matching (e) is shown in magenta and consists of a 10-s-long window including the observed PdiffCMB arrival. Red- and grey-shaded rectangles represent the travel time predictions for PdiffLSL and Pdiff^LSLPdiff, respectively, based on the inverted models shown in Fig. 2, and the yellow-shaded rectangle spans the range satisfying the observed differential travel time (−113 ± 5 s) of Pdiff^LSLPdiff relative to PP. d, Three-component scalogram illustrating the temporal change in frequency content. PdiffLSL, PdiffCMB and Pdiff^LSLPdiff arrivals are indicated by arrows following the colour scheme in c. e, Similarity between event trace and template trace. The horizontal line designates the threshold employed for the waveform matching detections. Coloured arrows as in d. f, Polarization attributes for the three-component seismic data, showing the temporal change in azimuth between 0.2–0.7 Hz. The azimuth across the observed diffracted P wavetrain is consistent with the imaged meteorite impact location of 34° (horizontal cyan line). Supporting seismic waveform processing information is provided in Supplementary Information section 6. S0173a, S1000a and S1094b: locations of a marsquake and two imaged meteorite impacts (Extended Data Table 1).
Fig. 4
Fig. 4. Mars’s core composition and light-element budget.
AIMD-predicted core density profiles (AIMD models) for senary Fe–Ni–S–C–O–H mixtures (light blue lines) that match the inverted seismic core density profiles obtained here (dark blue lines). For comparison, the InSight density profiles from ref. based on the larger core radius of 1,780–1,840 km are also shown (light grey lines). The inset shows the senary Fe–Ni–S–C–O–H core compositions after additional application of cosmochemical constraints (see main text for details). For comparison, the entire range of core compositional models before application of the cosmochemical constraints (corresponding to all the light blue AIMD-predicted density profiles) is shown in Supplementary Fig. 13. Each core composition in the inset is further labelled (coloured circle) by its residual (misfit) between observed and AIMD-computed differential core- and LSL-transiting (SDKDS) travel time (ray path is shown in Fig. 3a). The corresponding density profiles are colour-coded accordingly.
Extended Data Fig. 1
Extended Data Fig. 1. Seismic core properties in the binary system.
Density (a,c) and bulk sound velocity (b,d) of liquid Fe-X, where X = Ni, S, O, C and H as a function of alloying element (X) concentration (cX) at conditions equivalent of the Martian core-mantle-boundary (19 GPa and 2100 K) and inside Mars’s liquid core (35 GPa and 2400 K) based on the ab inito molecular dynamics simulations (AIMD) of (solid circles). Experimental data for liquid Fe (K20 and N20), Fe-S (B03, M13, K17, N20 and K22) and Fe-C (N15) are shown using solid and open angular markers. Error bars are reported using 2σ for both calculations and experiments, and are visible when exceeding symbol size. For purposes of illustration, only non-spin-polarised AIMD simulations are shown. Similar results are obtained using the spin-polarised AIMD simulations as indicated in Fig. 1 in the main text. Only experiments performed at high pressures (19–35 GPa) are compared through interpolation. Modified from.
Extended Data Fig. 2
Extended Data Fig. 2. Matching core seismic properties in the quaternary system at the core-mantle-boundary.
Range of core compositions in the liquid Fe-Ni-S-X (X = C, O, H) quaternary system (blue regions) that match InSight observations (yellow regions) of core density (left column, panels a, d, g), core P-wave velocity (middle column, panels b, e, h), and both properties simultaneously (right column, panels c, f, i) at a location just below the core-mantle-boundary. For purposes of illustration, only non-spin polarised ab initio molecular dynamics (AIMD) simulations are shown. Similar results are obtained using the spin-polarised AIMD simulations as indicated in Fig. 1 in the main text.
Extended Data Fig. 3
Extended Data Fig. 3. Matching core seismic properties in the quaternary system inside the core.
Range of core compositions in the liquid Fe-Ni-S-X (X = C, O, H) quaternary system (blue regions) that match InSight observations (yellow regions) of core density (left column, panels a, d, g), core P-wave velocity (middle column, panels b, e, h), and both properties simultaneously (right column, panels c, f, i) inside the core. For purposes of illustration, only non-spin polarised ab initio molecular dynamics (AIMD) simulations are shown. Similar results are obtained using the spin-polarised AIMD simulations as indicated in Fig. 1 in the main text.
Extended Data Fig. 4
Extended Data Fig. 4. Matching core seismic properties in the senary system.
Distribution of liquid Fe-Ni-S-O-C–H senary mixtures that match both density and P-wave velocity in the core immediately below the core-mantle-boundary (CMB) (blue-green-yellow colour bar) and inside the core (blue-purple-orange colour bar), comprising 1.28 ⋅ 105 (1.3‰) and 3.09 ⋅ 105 out of 100 million randomly generated compositions (3.1‰), respectively. The inset shows the distribution of Fe versus Ni at the CMB (white) and inside the core (grey). For purposes of illustration, only non-spin polarised ab initio molecular dynamics (AIMD) simulations are shown. Similar results are obtained using the spin-polarised AIMD simulations as indicated in Fig. 1 in the main text.
Extended Data Fig. 5
Extended Data Fig. 5. Martian mantle geotherms.
Inverted lithospheric and mantle geothermal profiles for high– (blue), and low (red) FeO mantle compositions. The insets show the distributions of sampled potential temperature (Tpot) and lithospheric geothermal gradient (dT/dz) for high (light blue) and low (light red) FeO compositions.
Extended Data Fig. 6
Extended Data Fig. 6. Seismic and geodetic data misfit.
(a,b) Differential body wave travel time misfits for all sampled models shown in Fig. 2a. Blue and red lines denote differential travel times computed using the inverted models, and squares, circles, and triangles indicate the observations including error bars. Note that the SKS pick for S0976a is only predicted. Event picks are aligned by the observed S-P (panel A) and SS-PP (panel B) differential travel time. For the travel time calculations performed here, we always pick the first arrival. (c) P-to-s receiver function waveform (Ps RF) misfit. (d,e) Geodetic data misfit in the form of mean Martian density (ρ¯Mars) and mean normalised moment of inertia (MoI).
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