A molten layer at the base of the Martian mantle
A study published on October 26 in the journal Nature involving scientists from the CNRS, IPGP, ISAE-SUPAERO and Université Paris Cité proposes a new model for the interior of the red planet. Its mantle is not homogeneous, as previously assumed, but rather consists of a layer of molten silicates overlying the core. This new discovery explains all the geophysical data observed through a common model.
Analysis of the initial data from the InSight mission served to make an initial estimate of Mars’ internal structure, the results of which were published during the summer of 2021. Like its neighboring Earth, Mars is made up of a crust, a silicate mantle and a metal core. The recording of a meteorite impact on September 18, 2021, cast doubt on these initial results. By studying the propagation times of the waves generated by this impact, an international team led by Henri Samuel, CNRS researcher at the IPGP - Institut de Physique du Globe de Paris, and involving scientists from CNRS, ISAE-SUPAERO and Université Paris Cité and supported by CNES and ANR, highlighted the presence of a molten layer at the base of the Martian mantle.
A smaller and denser core than previously envisaged
This discovery has significant consequences for our understanding of the inside of Mars. In previous studies, the interface between the solid and liquid regions of Mars was interpreted as the boundary between the silicate jacket and the metal core. However, this new study shows that it is actually the interface between the solid mantle and the liquid basal layer of this mantle. The presence of such a basal layer thus implies a metal core 150 to 170 km smaller (i.e., a radius of 1650±20 km), more dense (i.e., 6.5 g/cm3 ) and with fewer light elements (oxygen and sulfur, carbon and hydrogen) than in previous estimates. This hypothesis correlates more closely with data from the analysis of Martian meteorites and experiments conducted at high pressures to reproduce and study minerals present within planets.
The discovery of this layering of the mantle, which contrasts with the structure of the Earth, is an additional step in understanding the evolution of Mars. Mélanie Drilleau, research engineer at ISAE-SUPAERO and co-author of the study, explains that “Mars probably experienced an early magmatic ocean stage whose crystallization produced a stable layer at the base of the mantle, highly enriched with iron and radioactive elements. The heat from this latter generated a basal layer of molten silicates above the core, covered by a thinner partially molten layer.”
This discovery also provides an answer to one of the great enigmas surrounding the planet Mars: why did its magnetic field disappear 500 to 800 million years ago, when the Earth’s field is still active? Henri Samuel says: “This liquid layer at the base of the mantle insulates the metal mantle from the rest of the planet, preventing it from cooling down and creating a dynamo. If the source of the magnetic field cannot come from inside the planet, then external sources have necessarily existed in the distant past of Mars, such as energy impacts, or even core movements generated by gravitational interactions with ancient satellites that would have since disappeared”.
Converging data on the evolution of Mars
This new structure model, published on October 26, 2023 in the journal Nature, is not only more consistent with all available the geophysical data, but also explains better the evolution of Mars since its formation.
In particular, the evidence of this stratification of the Martian mantle elucidates the abnormally slow propagation, hitherto unexplained, of diffracted waves from the September 2021 meteorite impact by their trajectory in the lower and fully molten part of the basal layer, where seismic velocities are low.
Furthermore, for several older seismic events, the arrival times of waves 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 explains the observed trajectory of Phobos, Mars’s closest moon.
Indeed, the upper and partially molten part of the basal layer efficiently dissipates the deformations generated by Phobos’s gravitational attraction. In contrast, the solid mantle above this layer is more rigid and seismically poorly attenuating, as suggested by the detection at the surface of Mars of waves associated with seismic events of relatively low magnitude.
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