2023-10-26 10:53:28
After three years of collecting data, in July 2021 NASA’s InSight mission published its conclusions about the internal structure of Mars, but two months later new information collected after the impact of a powerful meteorite questioned those estimates.
Now, after studying the propagation times of the waves generated by that impact. Two teams led by the CNRS researcher at the Institut de Physique duGlobe de Paris, Henri Samuel and by Amir Khanfrom the ETH Zürich, Zurich (Switzerland), have shown that the red planet hides a molten silicate layer at the base of the mantle that covers the Martian metallic core.
The two studies, published this week in Natureconclude that the structure defined now is not only more realistic and coherent with all the geophysical data available, but also better explains the evolution of Mars since its formation.
The team believes that the evidence for this Martian mantle stratification clarify the abnormally slow – and until now unexplained – propagation of the diffracted waves of the September 2021 meteorite impact due to their trajectory in the lower and completely molten part of the basal layer, where seismic velocities are low.
The existence of this basal layer is consistent with other older seismic events that now have an explanation, and helps explain the observed trajectory of Phobos, the moon closest to Mars.
The existence of this basal layer also agrees with other older seismic events that now have an explanation, and helps explain the observed trajectory of Phobos, the moon closest to Mars.
According to the study, the upper and partially molten part of the basal layer effectively dissipates the deformations generated by the Phobos gravitational pullwhile, on the contrary, the solid mantle that is above this layer is more rigid and seismically little attenuating.
Artist’s view of the internal structure of Mars showing the propagation of diffracted waves from the September 2021 meteorite impact to the SEIS seismometer of the InSight / IPGP / CNES / N. Starter mission (via EFE)
Smaller metal core
The presence of this molten layer at the base of the mantle implies a metallic core 150 to 170 km smaller (i.e., radius 1650±20 km) and 5 to 8% denser (i.e., 6.5 g /cm3) than previous seismic estimates.
This denser core would therefore be composed of a alloy with fewer light elements than previously thought, and would be more compatible with cosmochemical data from the analysis of Martian meteorites and high-pressure experiments.
The planet was able to experience an early stage of a magmatic ocean, whose crystallization produced a stable layer at the base of the mantle, very rich in iron and radioactive elements.
The authors suggest that Mars probably experienced an early stage of magmatic ocean whose crystallization produced a stable layer at the base of the mantle, highly enriched in iron and radioactive elements.
The heat released by it generated a basal layer of molten silicates above the core and covered by a thinner, partially molten layer.
Studies further affirm that such stratification of the mantle insulates the metallic core, thus preventing its cooling and the generation of a thermal dynamo.
External sources to generate the magnetic field
“The thermal coating of the metallic core of Mars by the liquid layer at the base of the mantle implies that external sources are necessary to generate the magnetic field recorded in the Martian crust during the first 500-800 million years of its evolution. These sources could be energetic impacts or core movements generated by gravitational interactions with ancient satellites that have since disappeared,” explains Samuel.
This stratified structure of the interior of Mars’ mantle, which contrasts with that of Earth, reveals that both planets had a different internal evolution.
The stratified structure of the interior of Mars’ mantle, which contrasts with that of Earth, reveals that both planets had a different internal evolution
Mélanie Drilleauresearch engineer at ISAE-SUPAERO and co-author of the study, explains that “the discovery of this stratification in the Martian mantle opens new research horizons, since the seismic data recorded by the SEIS seismometer of the InSight mission will now be reconsidered at the light of this new paradigm.”
NASA’s InSight mission officially ended in December 2022 after more than four years of collecting scientific data on Mars.
References:
Amir Khan et al. “Evidence for a liquid silicate layer atop the Martian core”. Nature (October, 2023).
Henri Samuel et al. “Geophysical evidence for an enriched molten silicate layer above Mars’ core”. Nature (October, 2023).
Rights: Creative Commons.
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