Seismic structure of Martian mantle inferred from in situ sound velocity measurements

We report elastic wave velocity (Vp: P-Waves velocity, and Vs: S-wave velocities) measurements of iron-enriched pyrolite aggregates by a combination of ultrasonic interferometry, synchrotron radiation techniques and multi-anvil apparatus experiments. We carried out eight synchrotrons in situ experiments on two different bulk pyrolite compositions with Fe/(Fe + Mg) ratio (Fe#) of 0.17 and 0.27, over a range of pressures from 3 to 16 GPa and temperatures up to 1000 °C. These data are complemented by seven laboratory ex situ experiments between 5 and 16 GPa and temperatures varying from 900 °C to 1300 °C, using the same starting compositions. Based on these results and previous data on pyrolite composition, we modeled the effect of iron on seismic profiles along several possible Martian areotherms.

If, as reported in previous works, the Martian mantle had a very high Fe#, for instance Fe# = 0.27, it would be composed of four layers separated by three seismic discontinuities. They would be located at depths of ∼620 km (ΔVp of +8.25 %; ΔVs of +6.72 %), ∼880 km (ΔVp of +2.35 %; ΔVs of 0.03 %) and ∼ 1130 km (ΔVp of −17.88 %; ΔVs of −13.05 %). However, since there is no major discontinuity inferred by InSight's seismic data at depths shallower than 1000 km, we conclude that the Martian mantle presents a Fe# significantly lower than 0.20. For the Fe#0.17 composition, our experiments show a single discontinuity (ΔVp of +5.62 %; ΔVs of +10.44 %) induced by the transition from olivine to wadsleyite at ∼980 km depth, which is compatible with the seismic data.

Plain language summary

The geochemical and geophysical data available for Mars suggest an interior enriched in iron, but this information alone is insufficient to constrain the mineralogy and the internal structure of the Martian mantle. In this study, we determine experimentally the elastic wave velocities (Vp: P-Waves velocity, and Vs: S-wave velocities) of two different iron-rich mantle compositions, with Fe# ratios (=Fe/(Mg + Fe)) of 0.17 and 0.27, in a range of pressure and temperature covering the Martian mantle conditions. We then calculate the seismic wave velocity profiles expected in the Mars interior for the different thermal profiles recently proposed in the literature. Three complementary arguments arise in favor of a relatively low Fe# of the Martian mantle: (i) the lack of seismic discontinuity at around 600 km depth suggests the absence of a phase transition to ringwoodite and therefore Fe# significantly lower than ∼0.27. (ii) The report of one seismic discontinuity affecting both Vp and Vs is only compatible with the olivine to wadsleyite transition, which was reported to occur for Fe# lower than 0.2. (iii) A seismic transition reported at 1000 km (i.e. ∼13 GPa) is compatible with our measurements with Fe# of 0.17 (± 0.02).