The evolutionary history of the earth is a story of constant cooling.
4.5 billion years ago, the surface temperature of the newly formed Earth was very extreme, and it was covered by the deep sea of magma. This period is known to geologists as Hadean Eon, a term derived from the Greek myth of Hades, and you can imagine the lava-like purgatory of earth at that time.
Artists of the Underworld. | Image credit: Tim Bertelink via Wiki under CC BY-SA
But over time, the Earth's surface gradually cooled, forming a brittle crust. However, the enormous amount of thermal energy emitted from the Earth's interior is still driving dynamic processes such as mantle convection, plate tectonics, and volcanic activity.
But the question that remains unanswered is how fast the Earth is cooling, and how long will it take for this sustained cooling to stop the thermally driven processes in the Earth's interior?
Now, an international team of researchers has developed a sophisticated measurement system that allows them to simulate the high- and high-pressure conditions prevalent in the Earth's interior and measure the thermal properties of specific minerals under such conditions in the laboratory, opening the door to exploring the ultimate answer to this question.
Thermal conductivity under extreme conditions
Scientists have long suspected that the answer to Earth's cooling may lie in the thermal conductivity of the minerals that form the boundary between the core and the mantle.
This is because, it is here that the sticky rocks of the Earth's mantle come into direct contact with the hot iron-nickel melt of the Earth's outer core. The temperature gradient between these two layers is very pronounced, so there may be a lot of heat flow here.
The boundary layer is mainly formed by a mineral called Bridgmanite. It is a mineral that was officially named only a few years ago in honor of Percy Bridgman, nobel laureate in physics who made his contribution to the foundation of high-pressure physics.
Although this mineral is widespread in the Earth's interior, it wasn't until the past period of time that researchers had the opportunity to study it up close and understand its chemical makeup. But researchers still have a hard time estimating how much heat the mineral transfers from the center of the earth to the mantle because experimental verification is extremely difficult.
In the new study, the research team used a recently developed optical absorption measurement system in a diamond device heated with pulsed lasers to make measurements.
A measuring instrument that confirms the thermal conductivity of Brinell rock under extreme high temperature and pressure conditions. | Image credit: Murakami M, et al, 2021
This system allowed them to discover that the thermal conductivity of Brinellite was about 1.5 times higher than previously assumed. This suggests that the heat flow from the core to the mantle should also be greater than previously thought.
Larger heat flows, in turn, increase mantle convection, accelerating the Earth's cooling. This could lead to a plate tectonic sustained by the convective movement of the mantle, slowing down faster than the researchers had expected based on previous thermal conductivity values.
The team also found that rapid cooling of the mantle would alter the stable mineral phase of the core-mantle boundary. As it cools, the Brunite turns into a post-perovskite.
But the researchers estimate that once post-perovskites appear at the core-mantle boundary and begin to dominate, the cooling of the mantle may indeed be further accelerated, as the mineral's thermal conductivity efficiency is even higher than that of Brinite.
A new perspective on the dynamic evolution of the Planet
Scientists say the new results could show us a new perspective on the dynamic evolution of the Planet.
It also shows that Earth, like other rocky planets Mercury and Mars, is cooling and becoming inactive, a process much faster than expected.
However, it is difficult to say how long it will take for processes like mantle convection to actually stop, because we still lack sufficient awareness of these types of events.
To truly evaluate these aspects, scientists first need to better understand how mantle convection works in space and time. In addition, scientists need to clarify how the decay of radioactive elements in the Earth's interior, one of the main sources of heat, affects the dynamics of the mantle.
#创作团队:
Written by: M Ka
Typography: Wenwen
#参考来源:
https://ethz.ch/en/news-and-events/eth-news/news/2022/01/earth-interior-is-cooling-faster-than-expected.html
https://www.nationalgeographic.com/science/article/140618-bridgmanite-new-mineral-meteorite-geology-earth-science
#图片来源:
Cover image: OpenClipart
首图：University of Bern