Modeling study proposes a diamond layer at Mercury’s core-mantle boundary

A proposed scenario for diamond formation at the core-mantle boundary of Mercury. (a) Crystallization of a carbon-saturated silicate magma ocean and initial production of diamond at its base is possible, yet unlikely. Graphite is the main phase of magma formation in the ocean and accumulated at the surface to form a primordial graphite crust. (b) During crystallization of the inner core, diamond dissolved and floated to the core-mantle boundary. Such a late diamond layer continues to grow throughout core crystallization. Credit: Dr. Yanhao Lin and Dr. Bernard Charlier.

A A recent study Inside Natural communication Scientists from China and Belgium have discovered a diamond layer at Mercury’s core-mantle boundary (CMB), 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, revealed that the surface of Mercury contains significant amounts of graphite, a form of carbon.

Researchers believed this involved a carbon-rich magma ocean in the planet’s early history. spoke with one of the study’s co-authors, Dr. Yanhao Lin of the High Pressure Science and Technology Advanced Research Center in Beijing.

„Years ago, I noticed that Mercury’s very high carbon content could have significant implications. This made me realize that something special might be going on in its interior,” said Dr. Lin.

What do we know about Mercury?

The most detailed information about Mercury comes from NASA’s MESSENGER and MARINE 10 missions.

Previous observations by the MESSENGER spacecraft revealed that Mercury’s surface is unusually dark due to the prevalence of graphite.

The abundant carbon at the surface is believed to come from an ancient layer of graphite that initially floated to the surface. This suggests that Mercury once had a molten surface layer or magma ocean containing significant amounts of carbon.

Over time, as the planet cooled and solidified, this carbon formed a graphite crust on the surface.

However, the researchers challenge the assumption that graphite was the only stable carbon-bearing phase during the crystallization of Mercury’s magma ocean. This is when the planet’s mantle (middle layer) cools and solidifies.

Early inferences about the graphite crust relied on predictions of low temperatures and pressures in the CMB. But new studies propose that the CMP is deeper than once thought, prompting researchers to reevaluate the graphite crust.

Additionally, Another study It also suggested that Mercury has sulfur in its iron core. The presence of sulfur may have influenced the crystallization of Mercury’s magma ocean, thereby calling into question the original claim that only graphite was present at that stage.

Recreating Mercury’s interior conditions

The researchers used a combination of high-pressure and temperature experiments and thermodynamic modeling to recreate the conditions inside Mercury.

„We use large-scale pressure to simulate the high-temperature and high-pressure conditions of Mercury’s core-mantle boundary and link it to geophysical models and thermodynamic calculations,” explained Dr Lin.

They used a synthetic silicate as a starting material to resemble the composition of Mercury’s mantle. It is the most commonly used method for studying the interiors of planets.

Pressure levels of up to 7 gigaPascals (GPa) were achieved by the researchers, which is about seven times the pressure found in the deepest parts of the Mariana Trench.

Under these conditions, the team studied how minerals (found in Mercury’s interior) melt and reach equilibrium states and characterized these phases, focusing on graphite and diamond.

They also analyzed the chemical composition of the test samples.

„What we do in the lab is simulate the extreme pressures and temperatures of a planet’s interior. That’s sometimes a challenge; you have to push the equipment to suit your needs. Experiment systems have to be very precise to simulate these conditions.” Dr Lin explained.

They also used a geophysical model to analyze observed data about Mercury’s interior.

„Geophysical models come mainly from data collected by spacecraft, and they tell us the basic structures of a planet’s interior,” Dr Lin said.

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 form under pressure

By integrating the experimental data with geophysical simulations, the researchers were able to estimate Mercury’s CMB pressure at about 5.575 GPa.

At approximately 11% sulfur content, the researchers observed a significant 358 Kelvin temperature change in Mercury’s magma ocean. Although graphite was the dominant carbon phase during magma ocean crystallization, the researchers propose that crystallization of the core led to the formation of a diamond layer in the CMB.

„Sulfur reduces the fluidity of Mercury’s magma ocean. If diamond forms in the magma ocean, it can sink to the bottom and be deposited in the CMB. On the other hand, sulfur also helps the formation of an iron sulfide layer in the CMB, which is related to the carbon content during planetary differentiation,” explained Dr. Lin.

Planetary differentiation refers to the process by which a planet’s interior is structured, ie, the core, or core, where heavier minerals sink, and the surface, or crust, where lighter minerals rise.

According to their findings, the diamond layer in the CMB has an estimated thickness of between 15 and 18 kilometers. The current temperature in Mercury’s CMB is close to where graphite can turn into diamond, thereby stabilizing the temperature in the CMB.

Carbon-rich exoplanets

One of the implications of these findings is for Mercury’s magnetic field, which is paradoxically strong for its size.

Dr. Lin explained, „As carbon cools from the molten core, it forms diamond and floats to the CMB. Diamond’s high thermal conductivity enables heat to be efficiently transferred from the core to the crust, causing temperature stratification and convection in Mercury’s liquid core, thereby affecting the generation of its magnetic field.”

In simple terms, heat is transferred from the core to the mantle, affecting the temperature gradient and convection in Mercury’s liquid outer core, which in turn affects the formation of its magnetic field.

Dr. Lin also pointed out the important role that carbon played in the formation of carbon-rich exoplanetary systems.

„This may also be relevant for understanding other terrestrial planets, especially those with similar sizes and compositions. The processes that led to the formation of a diamond layer on Mercury may also have occurred on other planets, leaving similar signatures,” he concluded. Dr. Lin.

More information:
A diamond-bearing core-mantle boundary on Mercury, Yongjiang Xu et al. Natural communication (2024) DOI: 10.1038/s41467-024-49305-x.

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