A molten layer covers the metallic core of Mars

2023-10-25 17:17:20

Artist’s representation of the liquid silicate layer that surrounds the Martian core. – IPGP-CNES.

MADRID, 25 Oct. (EUROPA PRESS) –

Studying the propagation times of waves after a meteorite impact The presence of a layer of molten silicate has been demonstrated at the base of the mantle that covers the metallic core of Mars.

This new model of structure, described in a new study published in ‘Nature’is not only more realistic and consistent with all available geophysical data, but also better explains the evolution of Mars since its formation.

The first data from the ‘InSight’ mission made it possible to determine the internal structure of Mars in a series of articles by the scientific team published in the summer of 2021. However, since then, the analysis of new data generated by the impact of a powerful meteorite that occurred on September 18, 2021, questioned early estimates about the internal structure of the Red Planet.

In particular, evidence of this stratification of the Martian mantle elucidates the anomalously slow, until now unexplained, propagation of diffracted waves from the September 2021 meteorite impact by their trajectory in the lower and completely molten part of the basal layer, where seismic velocities are low.

Furthermore, for several older seismic events, wave arrival times measured at the surface of Mars are compatible with reflections of shear waves at the top of the molten layer (located several tens of kilometers above the metallic core). and not at the core-mantle interface, as previously assumed. Finally, the presence of this basal layer helps explain the observed trajectory of Phobos, the closest moon to Mars.

Indeed, the upper and partially molten part of the basal layer effectively dissipates the deformations generated by the gravitational attraction of Phobos. On the contrary, the solid mantle located above this layer is more rigid and seismically little attenuating, as suggested by the detection on the surface of Mars of waves associated with relatively low magnitude seismic events.

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 an alloy with fewer light elements than what was required until now, and would be more compatible with cosmochemical data from the analysis of Martian meteorites and high pressure experiments.

The authors of the study, including scientists from France, Belgium, the United States, Switzerland, the United Kingdom, Russia and NASA, therefore propose that Mars probably experienced an early stage of a magmatic ocean whose crystallization produced a stable layer in the base of the mantle, highly enriched in iron and radioactive elements. The heat released by it generated a basal layer of molten silicates located above the core, covered by a thinner, partially molten layer.

The study further states that such stratification of the mantle insulates the metallic core, thus preventing its cooling and the generation of a thermal dynamo. “The thermal covering 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 Henri Samuel, CNRS researcher at the Institut de Physique duGlobe in Paris (France) and director of the team.

This stratified structure of the interior of Mars’ mantle, which contrasts with that of Earth, indicates a different internal evolution of these two planets. Mélanie Drilleau, research 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 instrument of the ‘InSight’ mission will now be reconsidered in light of this new paradigm.”