The rocks of the interior of Mars reproduced in the laboratory on Earth

An international team of geophysicists, mainly French and Japanese, has just published in Geophysical Research Letters the results of work that would undoubtedly have pleased the Nobel Prize winner physical Percy Williams Bridgman, and especially to his student Francis birch who, in 1952, demonstrated that the Earth’s mantle is mainly composed of silicates, and that our Planet also has an external core liquid and an internal core solid, both made of iron.

Indeed, Bridgman was one of the pioneers of the high pressure physics that we find at great depth, in the mantle or in the heart of the Earth, even in the center of giant planets like Jupiter. For this, he invented and developed the technique for submitting samples of matter at pressures exceeding 100,000 atmospheres by means of diamond anvil cells.

Birch’s equations and diamond anvil cells

Francis Birch, for his part, will not only refine the technology of these anvils but he will propose an equation of state and a law (Birch-Murnaghan equation of state and Birch’s law) connecting the pressure to the density of the rocks in the first case, and the speed of certain seismic waves to the density and coarse chemical composition of a rock in the second case.

Birch will use all this to compare the data obtained from the analysis of seismic waves propagating inside the Earth with that deduced from the study of similar elastic waves traveling in the laboratory of materials subjected to high pressures and temperatures. This will enable him and his fellow geophysicists, geochemists and geologists to determine the nature of the rocks of the mantle of the Earth according to the depth.

It is exactly the same strategy that planetologists seeking to know or simply to specify the composition and the internal structure ofstars like the moon or March. In the case of the example taken today, researchers from the Institute of Mineralogy, Materials Physics and Cosmochemistry (IMPMC, CNRS / MNHN / Sorbonne University) and the Paris Institute of Globe Physics (IPGP, CNRS / IPGP / Univ. Paris), in collaboration with Japanese colleagues, undertook to make speak with new data obtained in the laboratory the recordings of Martian seismic waves provided by the Seis seismometer, deposited on the surface of Mars in early 2019 by NASA’s InSight mission.

As explained in a CNRS press release, the data collected by Seis was analyzed with a model of the interior of the mantle of Mars largely indirectly inferred from the model of the composition of the mantle of the Earth. But the researchers wanted to have a less indirect determination by trying to reproduce in the laboratory the state of mixtures of minerals likely subjected to credible pressure and temperature pairs at different depths inside the Red Planet.

Seismic waves that propagate according to an unforeseen law

Technically, the press release specifies that, in fact, they are glasses which were synthesized and which were then compressed and heated, either in a piston-cylinder apparatus or in a multi-anvil apparatus, creating the desired pressure and temperature conditions existing in the mantle of Mars, namely 3 GPa and 1,200 ° C at a depth of 250 km, and 8 GPa and 1,300 ° C for a depth of 670 km.

The experiments in the footsteps of Francis Birch were carried out in Japan with the samples studied using ultrasound techniques and diffraction and imaging by X-rays in order to determine their densities and the velocities of the seismic compression and shear waves generated and propagated in materials as a function of pressure and temperature.

Among the main conclusions reached by researchers, the CNRS press release specifies that the experiments ” have highlighted a more complex mineralogy than that considered so far, with the probable existence of minerals rich in ions ferric in the more oxidized regions of the upper mantle. Furthermore, studies have revealed the existence of a region in the upper mantle in which, contrary to conventionally expected behavior, the speed of seismic waves decreases with depth, a finding which is consistent with the observations of the mission. Insight… as temperature and pressure increase with depth, it has been found that the temperature-induced effect is dominant over the pressure-induced effect, leading to a marked reduction in seismic wave velocities , in particular for shear waves, in a layer between 150 and 350 km deep. Remarkably, the existence of such a layer is independent of its mineralogical composition. “.

All these data will of course help to specify the structure, the history and the equivalent of the terrestrial geodynamics in the case of Mars; they are of course also subject to change with the arrival of new seismic data from Seis. We should also be able to study the mantle-nucleus interface of Mars in the long term.