Proposed scenario for diamond formation at the core-mantle boundary of Mercury. (a) Crystallization of the carbon-saturated silicate magma ocean and the potential, but unlikely, early production of diamond at its base. Graphite was the major phase formed in the magma ocean and accumulated at the surface to form an ancient graphitic crust. (b) During crystallization of the inner core, diamond dissolved and drifted to the core-mantle boundary. Such a late diamond layer would have continued to grow during core crystallization. Credit: Dr. Yanhao Lin and Dr. Bernard Charlier.
A recent study in Nature communication from scientists in China and Belgium suggests that Mercury’s core-mantle boundary (CMB) contains a diamond layer, possibly up to 18 kilometers thick, deep in the planet’s interior.
Mercury, the smallest and innermost planet in our solar system, has long puzzled scientists with its remarkably dark surface and high core density. Previous missions, such as NASA’s MESSENGER spacecraft, had revealed that Mercury’s surface contains significant amounts of graphite, a form of carbon.
This led the researchers to believe that the planet’s early history involved a carbon-rich magma ocean. Phys.org spoke with one of the study’s co-authors, Dr. Yanhao Lin, of the Center for High Pressure Science and Technology Advanced Research in Beijing.
“Many years ago, I noticed that Mercury’s extremely high carbon content could have important consequences. It made me realize that something special was probably happening inside it,” said Dr. Lin.
What We Know About Mercury
The most detailed information about Mercury comes from NASA’s MESSENGER and Mariner 10 missions.
Previous observations by the MESSENGER spacecraft showed that Mercury’s surface is unusually dark due to the widespread presence of graphite.
The abundance of carbon on the surface is thought to come from an ancient layer of graphite that floated to the surface early on. This suggests that Mercury once had a molten surface layer or magma ocean with a significant amount of carbon.
As the planet cooled and solidified over time, this carbon formed a graphite crust on the surface.
However, the researchers dispute the assumption that graphite was the only stable carbonaceous phase during Mercury’s magma ocean crystallization, when the planet’s mantle (middle layer) cooled and solidified.
Early assumptions about the graphitic crust were based on lower temperature and pressure predictions at the CMB. But newer studies suggest the CMB is deeper than once thought, prompting researchers to reassess the graphitic crust.
Furthermore, another study has also suggested the presence of sulfur in the iron core of Mercury. The presence of sulfur may affect the crystallization of Mercury’s magma ocean, thus questioning the original claim of only graphite being present during that phase.
Recreating the conditions inside Mercury
To reproduce the conditions inside Mercury, the researchers used a combination of high-pressure and temperature experiments and thermodynamic models.
“We use the large-volume press to simulate the high temperature and pressure conditions of Mercury’s core-mantle boundary and combine this with geophysical models and thermodynamic calculations,” explains Dr. Lin.
They used synthetic silicate as a starting material to mimic the composition of Mercury’s mantle, a common method for studying the interiors of planets.
The researchers reached pressure levels of up to 7 Giga Pascals (GPa), about seven times the pressure found in the deepest parts of the Mariana Trench.
Under these conditions, the team studied how minerals (which are found in the interior of Mercury) melt and reach equilibrium phases. They then characterized these phases, focusing on graphite and diamond.
They also analyzed the chemical composition of the experimental samples.
“What we do in the lab is to simulate the extreme pressures and temperatures of the interior of a planet. Sometimes that’s a challenge; you have to adapt the equipment to your needs. Experimental setups have to be very precise to simulate these conditions,” Dr. Lin explains.
They also used geophysical models to study the observed data about Mercury’s interior.
“Geophysical models are mainly based on data collected by spacecraft and they give us insight into the fundamental structures of a planet’s interior,” said Dr. Lin.
They used the model to predict phase stability, calculate CMB pressures and temperatures, and simulate the stability of graphite and diamond under extreme temperatures and pressures.
Diamonds are formed under pressure
By integrating the experimental data with geophysical simulations, the researchers were able to estimate the CMB pressure on Mercury to be about 5.575 GPa.
At a sulfur content of about 11%, the researchers observed a significant temperature change of 358 Kelvin in Mercury’s magma ocean. The researchers propose that while graphite was likely the dominant carbon phase during the crystallization of the magma ocean, the crystallization of the core led to the formation of a diamond layer in the CMB.
“Sulfur lowers the liquidus of Mercury’s magma ocean. When diamond forms in the magma ocean, it can sink to the bottom and be deposited at the CMB. On the other hand, sulfur also helps form an iron sulfide layer at the CMB, which is related to carbon content during planetary differentiation,” Dr. Lin explained.
Planetary differentiation refers to the process by which a planet acquires an internal structure, i.e., the center or core, toward which the heavier minerals sink, and the surface or crust, toward which the lighter minerals rise.
According to their findings, the diamond layer at the CMB is estimated to be between 15 and 18 kilometers thick. They also suggest that the current temperature at Mercury’s CMB is close to the point where graphite can transition to diamond, stabilizing the temperature at the CMB as a result.
Carbon-rich exoplanetary systems
These findings have implications for Mercury’s magnetic field, which is abnormally strong for its size.
Dr. Lin explained: “Carbon from the molten core becomes supersaturated as it cools, forming diamond and floating toward the CMB. The high thermal conductivity of diamond helps transfer heat effectively from the core to the mantle, causing temperature stratification and convection change in Mercury’s liquid outer core, thus affecting the generation of the magnetic field.”
Simply put, the heat transferred from the core to the mantle affects the temperature gradients and convection in Mercury’s liquid outer core, which in turn affects the generation of a magnetic field.
Dr. Lin also pointed out the crucial role carbon plays in the formation of carbon-rich exoplanetary systems.
“It could also be relevant to understanding other terrestrial planets, particularly those with similar sizes and compositions. The processes that led to the formation of a diamond layer on Mercury could have occurred on other planets as well, possibly leaving similar signatures,” Dr. Lin concluded.
More information:
Yongjiang Xu et al, A diamond-bearing core-mantle boundary on Mercury, Nature communication (2024). DOI: 10.1038/s41467-024-49305-x.
© 2024 Science X Network
Quote: Model study suggests a diamond layer at the core-mantle boundary on Mercury (2024, July 10) Retrieved July 10, 2024 from https://phys.org/news/2024-07-diamond-layer-core-mantle-boundary.html
This document is subject to copyright. Except for fair dealing for private study or research, no part may be reproduced without written permission. The contents are supplied for information purposes only.