Earth is cooling faster than previously thought

Since its formation 4.5 billion years ago, the earth keeps giving off heat, and therefore to cool down. This heat comes mainly from the radioactive disintegration of the components of the various terrestrial envelopes. Three other energy sources can also be cited: the progressive crystallization of the outer core, the movements gravitational minerals crystallizing inside the nucleus liquid, and theenergy internal tides of the Earth. The assembly produces a significant heat flux which is essential for the generation of convection mantle, which underlies the tectonic and volcanic activity of our Planet. It can be said that it is this flow of heat that makes the Earth geologically alive. However, this heat flow will not be eternal.

As the Earth cools, it gradually depletes its internal heat supply. The day will come (very far away, however) when the heat flow will no longer be sufficient to support mantle convection. It is very likely that at that time, the Earth will become a “dead” planet, and that plate tectonics will cease. Although this very long-term development is not in doubt, it remains difficult to quantify. Because we do not currently know exactly at what speed the Earth is cooling, or how long it will take to exhaust its reserves.

Bridgmanite, a more conductive mineral than previously thought

To answer these questions, it is necessary to understand how heat is transmitted inside the Earth, up to the surface where it is evacuated by volcanic activity, among other things. One of the key areas appears to be the interface between the outer core and the coat. Indeed, it is at this level that the liquid crystalline mush of the outer core, composed of a mixture of iron and of nickel, is in direct contact with the rocks of the mantle. the gradient between these two levels is very high and the heat flux is very high. At this interface, the mantle is mainly composed of a mineral called bridgmanite. Knowing the ability of this mineral to conduct heat would therefore make it possible to better understand the rate of cooling of the Earth. A team of researchers from ETH Zürich and the Carnegie Institution in Washington have thus reproduced in the laboratory the conditions prevailing at the level of the core/mantle interface, in order to measure the radiative thermal conductivity of bridgmanites at the level of the base of the mantle. inferior.

The results show that the average thermal conductivity at the core/mantle interface would be 1.5 times greater than previously estimated. This new value suggests that the heat flux from the core may be greater than previously thought. The reassessment of the heat flow has two consequences. First, the generated mantle convection must be stronger than assumed. As a result, the mantle cools more efficiently, and therefore faster than previous designs predicted.

Towards an acceleration of the cooling of the Earth

These new data could have consequences on the duration life of certain tectonic activities driven by mantle convection. A faster cooling of the mantle could in particular lead to modifications of the mineral phases at the core/mantle interface. Indeed, when bridgmanite cools, it transforms into a new mineral, post-perovskite. However, this mineral conducts heat even more efficiently than bridgmanite. Thus, the more the Earth cools, the more the post-perovskite will become the dominant mineral at the base of the mantle, accelerating the transfer of heat towards the surface and therefore accelerating the cooling.

The results of the study published in Earth and Planetary Science Letters therefore open up new perspectives on the evolution of the Earth’s dynamics. Like Mars and Mercury, the Earth could thus become inactive much more quickly than we thought. But let’s be reassured, if we still have no idea how long it will take, plate tectonics still has a good life ahead of it.