In brief: Mercury is the smallest planet in the solar system and has always been a mystery because of its dark surface and high core density. However, astronomers have long known that its surface contains significant amounts of graphite, a form of carbon. A new study reveals that a thick layer of diamond lies beneath that graphite crust at the core-mantle boundary.
Scientists from China and Belgium recently published a study in Nature Communications that proposes the existence of a diamond layer at Mercury’s core-mantle boundary. It suggests that this layer is up to 18 kilometers (11 miles) thick. The discovery represents a significant advance in understanding planetary differentiation processes – how planets develop different internal layers.
The scientists believe that the diamond layer formed by the crystallization of Mercury’s carbon-rich magma ocean. As the planet cooled, this carbon formed a graphite crust on the surface. However, the study challenges the assumption that graphite was the only stable carbon phase during this period.
“Many years ago, I noticed that Mercury’s extremely high carbon content might have important implications,” the study’s co-author, Dr. Yanhao Lin, of the Center for High Pressure Science and Technology Advanced Research in Beijing, told Phys.org. “It made me realize that there was probably something special happening inside it.”
The researchers used high-pressure and temperature experiments combined with thermodynamic modeling to recreate the conditions of Mercury’s interior. They reached pressures of up to 7 Giga Pascals, allowing them to study the equilibrium phases of Mercury’s minerals.
They found that the presence of sulfur in Mercury’s iron core influenced the crystallization process of the magma ocean. Sulfur lowers the liquidus temperature, facilitating the formation of a diamond layer at the core-mantle boundary. It also formed an iron sulfide layer, which influenced the carbon content during planetary differentiation.
The high thermal conductivity of the diamond layer affects Mercury’s thermal dynamics and magnetic field generation. The diamond layer helps transfer heat from the core to the mantle, which affects temperature gradients and convection in the liquid outer core, which affects the magnetic field.
The findings also have implications for understanding other carbon-rich exoplanetary systems and terrestrial planets with similar sizes and compositions to Mercury. The processes observed on Mercury could also occur on other planets, potentially leaving behind similar signatures. The study concludes that similar diamond layers could exist on other terrestrial planets, although the conditions would have to be just right.