Until now considered as solid, the inner core of the Earth would actually be in an intermediate state, between solid and liquid. This is demonstrated by a new study, whose numerical model makes it possible to resolve many seismological observations.
You will also be interested
[EN VIDÉO] The core of the Earth reconstructed in the laboratory By spinning liquid sodium around a 23-tonne sphere, geophysicists at the University of Maryland simulate the conditions around our planet’s core. Goal of the game: to understand why the Earth is the only telluric planet in the Solar System to possess a strong magnetic field which protects us from the wrath of the Sun. Here is this amazing experience in video.
The Earth comprises different geological envelopes: the crustthe coat, the outer core and the inner core, also called “seed”. While the first kilometers of crust are now well known, the deeper levels are still the subject of many questions and debates. If important information is obtained through the study of seismic wave propagation through these deep environments, the exact composition of the nucleus as well as the physico-chemical mechanisms that govern it are still poorly constrained.
A “soft” inner core?
If we have known for several decades now that the outer core is made up of iron in the state liquid by the fact that the speed of S (shear) waves is equal to 0, the nature of the inner core is much less clear. The seed was until now considered as solid. But this hypothesis comes up against a major observation: the speed of the S waves passing through it is abnormally weak (3.6 km/sec), giving the impression that the inner core is “soft”. It therefore seems necessary to consider the presence of a molten fraction within it. However, the conditions physical reigning at the center of the Earth are extreme: the pressure is about 350 GPa, or 3,500,000 times the atmospheric pressure and the temperature more than 5,000 °C. However, the mechanisms allowing the presence of a liquid phase under such conditions are far from being understood.
An inner core neither solid nor liquid
It is difficult to reproduce these conditions in the laboratory. This is where the Numerical simulation. A team of Chinese researchers has indeed succeeded in studying, by calculation, the molecular dynamics for the pressure and temperature prevailing in the inner core. They were able to study the behavior of alloys of iron in this extreme environment. Their results, published in Natureshow that, under these conditions, the iron alloys (FeH, FeO and FeC) composing the inner core are in a particular state, called superionic. It is an intermediate state, between solid and liquid. Thanks to their simulations, the scientists succeeded in showing that the atoms of H, O and C, which are light elements, behave within the inner core like liquids and diffuse within the iron matrix.
The inner core can therefore be seen as a mixture composed of solid iron and liquid light elements, the latter being animated by a movement of convection. The terrestrial seed is therefore not a solid as we thought until now. The researchers show that their hypotheses correlate well with seismic observations: the presence of a superionic phase induces a drastic deceleration in the speed of the shear waves. A small proportion of light elements in this state also allows this observation.
A link with the magnetic field?
It is already known that at the level of the transition between the inner core and the liquid outer core, the solidification of the seed generates a heat latent and leads to separation and the rise of light elements in the outer core, allowing its convective flow. An extremely important process since it is this convection movement that ensures the generation of magnetic field earthly. However, according to the results of the study, a small part of the light elements, in particular H, O and C, become trapped in the inner core. The pressure/temperature conditions mean that they are then in a superionic state, and their behavior can be likened to that of a liquid. This state allows them to diffuse through the iron matrix of the core following a convective movement.
The presence of these elements has a significant effect on the elastic properties of the inner core. This model also explains most of the seismic observations concerning the seed.
The link between magnetic field and inner core is also to be explored. Indeed, it seems that there are certain relations between the seismic structure of the inner core and the magnetic field. The study of these interactions could in particular make it possible to refine our knowledge of its structure and its evolution.
Interested in what you just read?