Low Shear Velocity Zones (LSVZ) in the Martian Upper Mantle Highlighted by Sound Velocity
Measurements
A collaborative work conducted by researchers at the Geodynamics Research Center (GRC)and Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC) suggest
the existence of low shear seismic velocity regions between 150 and 350 km depth in Mars
mantle. The experimental results presented in this study are consistent with recent
observations from the InSight mission and could give important clues on Mars’ mantle
mineralogy and internal structure. The scientific article presenting and discussing theseresults was published on September 21 in the journal Geophysical Research Letters.
Since its landing on Mars in late 2018, the InSight (Interior exploration using Seismic
Investigation, Geodesy and Heat Transport) mission has been monitoring the red planet’s
seismic activity. To date, the deployed seismometer has recorded more than 600 distinct
events, of which over 60 present characteristics that allow interpreting them as relatively
distant quakes. Around ten of these latter carry information of Mar’s interior (KnapmeyerEndrun et al., Science 2021; Khan et al., Science 2021; Stähler et al., Science 2021).
Interpretation of seismic observations and inversion of P- and S-wave velocities in terms ofplanetary structure and mineralogical composition requires the knowledge of the physicalproperties of the minerals and rocks constituents, at the relevant pressure and temperature
conditions. These data are however still scarce for materials and aggregates expected tocomprise the Martian mantle, especially lacking high-temperature sound velocity
measurements.
The researchers at IMPMC, co-investigators and collaborators of the InSight Mission,
investigated the nature and abundance of the mineral phases stable at Martian mantle P andT conditions. They used a laser levitation furnace to fabricate unique glass starting materials
under various oxidation states representing some possible redox of Mars’ mantle. Theseglasses were then compressed and heated in a multi-anvil apparatus, at pressure andtemperature conditions that approach those of Mars’s interior. Analysis of equilibriummineralogy revealed the stability of magnetite, an Fe3+-rich mineral not reported in previous
studies but a likely candidate in Mars’ oxidized mantle environment.
The researchers at GRC conditioned these Martian rock samples and prepared devices for theultrasonic experiments. The compressional (VP) and shear (VS) velocities, as well as thedensity of the synthesized Martian rock aggregates were then investigated in Japan, within
the framework of the Premier Research Institute for Ultra-high Pressure (PRIUS) program atGRC. The joint GRC-IMPMC team conducted together, several high-pressure experiments
using a combination of ultrasonic techniques and a large volume press apparatus at thebeamline BL04B1 in SPring-8 (Hyogo, Japan). The obtained data allowed researchers topropose forward velocity and density profiles that provide guidance to the interpretation ofInSight seismic observations .
In particular, the measured pressure and temperature derivatives of VP and VS, once coupledwith predicted areotherms (pressure and temperature profiles for increasing depth withinMars) showed a clear temperature-induced reduction of seismic wave speeds between 150

and 350 km depth. The reduction is particularly large for shear velocities (VS) and would
dominate over pressure-induced effects at Mars’ shallow mantle conditions for all thepredicted areotherms. Interestingly, the existence of such low-shear-wave-velocity layer
would be independent from mineralogy as revealed by further thermodynamics modelling
conducted by the study’s co-authors. These findings are consistent with recent observations
from the InSight mission on Mars and should greatly contribute to further constrain theMartian mantle mineralogy and thus understanding the planet’s internal structure anddynamics.

Reference:
Low Velocity Zones in the Martian Upper Mantle Highlighted by Sound Velocity
Measurements, 2021, Xu F., Siersch N.C., Gréaux S., Rivoldini A., Kuwahara H., Kondo N.,
Menguy N., Kono Y., Higo Y., Plesa A.-C., Badro J. and Antonangeli D., Geophysical ResearchLetters, In Press. doi.org/10.1029/2021GL093977
URL: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GL093977