2023-09-18 21:36:17
Precisely estimating the exact composition of the Earth’s core remains a major scientific challenge. One of the keys to achieving this is having a good idea of the pressures at the heart of our Planet. However, a new study shows that previous pressure estimates might be significantly overestimated. Result: the earth’s core might be much lighter than we thought.
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[EN VIDÉO] Seismology, the stethoscope that listens to the heart of the Earth “The Earth is opaque but it is transparent to seismic waves” explains to us…
Understanding the composition of the Earth’s core is essential in order to discuss certain issues, such as the formation and evolution of planets, or the heat flow that fuels the convectionconvection of the mantle. But how can we know exactly the composition of this area which is located at the heart of our Planet, more than 5,000 kilometers beneath our feet? If seismology has made it possible to establish, thanks to the study of the propagation of seismic waves, that the internal core was composed mainly of solid-solid iron, its exact constitution still remains uncertain.
A core composed mainly of iron but also of lighter elements
Because these same seismological observations revealed that the density of the earth’s core was lower than that of pure iron. This deficit would amount to 3 to 5%. The internal core would therefore contain a part, albeit minimal, of light elements. Estimating the proportion of these light elements, however, requires having a good estimate of the pressure reigning in the depths of the Earth. However, until now, the pressure scales proposed in previous studies have proven to be far too approximate.
A team therefore set regarding defining a new pressure scale, particularly for the deepest regions of our Planet. And it was no easy feat. The researchers thus carried out measurements of the velocities of seismic waves and the density in a sample of rheniumrhenium maintained under extreme increasing pressure in a diamonddiamond cell. The measurements were carried out in one of the most powerful synchrotrons currently, Spring-8 (located in Japan), in particular thanks to the analysis of scattering and diffraction of X-rays.
Twice as many lightweight elements as previously thought
Rhenium is a chemical elementchemical element (Re), belonging to the family of transition metals whose behavior under very high pressure facilitates this type of study. Determining the wave speed and density as the experiment progressed made it possible to determine the evolution of the pressure experienced by the metal sample. Scientists were thus able to reach a pressure similar to that prevailing in the Earth’s core, i.e. 230 GPa. However, this new value is 20% lower than previous estimates. A significant difference which might have important consequences on the composition of the earth’s core.
Thanks to this new pressure scale obtained in the laboratory, the researchers were able to interpret the metallic behavior of iron at high pressure and compare it with the seismic model of the Earth. The results, published in the magazine Science Advances reveal that the Earth’s inner core is much less dense than previously thought. The density deficit compared to pure iron would therefore amount to around 8%. The inner core might thus contain up to twice as much light material as previous studies suggested. Another hypothesis would be that the temperature at the center of the Earth is much higher than previously thought, of the order of 8,700°C, or around 2,700°C higher than what current models give. A combination between these two points of view is also not excluded.
These new results therefore call into question the composition and conditions currently given for the inner core. They might also make it possible to revise the structure of other planets.
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