The modeling study proposes a diamond layer at the core-mantle boundary on Mercury

A recent study in Nature Communications by scientists from China and Belgium suggests that Mercury’s core-mantle boundary (CMB) includes a layer of diamond, potentially up to 18 kilometers thick, deep within the planet’s interior.

Mercury, the smallest and innermost planet in our solar system, has long puzzled scientists with its extremely dark surface and high core density. Previous missions, such as NASA’s MESSENGER spacecraft, had discovered that Mercury’s surface contains significant amounts of graphite, a form of carbon.

This led researchers to believe that the planet’s early history included a carbon-rich magma ocean. Phys.org spoke with one of the study’s co-authors, Dr. Yanhao Lin, from the Advanced Research Center of High Pressure Science and Technology in Beijing.

“Many years ago, I noticed that Mercury’s extremely high carbon content could have important implications. It made me realize that something special might have happened inside it,” said Dr. Lin.

What we know about Mercury

The most detailed information on Mercury comes from NASA’s MESSENGER and Mariner 10 missions.

Previous observations by the MESSENGER spacecraft had revealed that Mercury’s surface is extremely dark due to the widespread presence of graphite.

The abundance of carbon on the surface is believed to have 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 that contained a significant amount 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 phase of carbon during the crystallization of Mercury’s magma ocean. This is when the planet’s mantle (middle layer) cools and hardens.

Early assumptions about the graphitic crust were based on lower temperature and pressure predictions in the CMB. But newer studies suggest that the CMB is deeper than once thought, prompting researchers to reevaluate graphite’s crust.

Furthermore, another study has also suggested the presence of sulfur in Mercury’s iron core. The presence of sulfur may have had an effect on the crystallization of Mercury’s magma ocean, casting doubt on the original claim of only graphite being present during that stage.

Recreating the internal conditions of Mercury

To recreate the interior conditions of Mercury, the researchers used a combination of high-pressure and temperature experiments and thermodynamic modeling.

“We use large-volume compression to simulate the high-temperature and high-pressure conditions of Mercury’s core-mantle boundary and combine this with geophysical models and thermodynamic calculations,” explained Dr. Lin.

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

Pressure levels of up to 7 Giga Pascal (GPa) were reached by the researchers, roughly seven times the pressure found in the deepest parts of the Mariana Trench.

Under these conditions, the team studied how minerals (those found in Mercury’s interior) melt and reach equilibrium phases and characterized these phases, focusing on those of 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 a planetary interior. Sometimes it’s a challenging thing; you have to push the equipment to suit your needs. The experimental setups have to be very precise to simulate these conditions.” Dr. Lin explained.

They also used geophysical modeling to study observed data about Mercury’s interior.

“Geophysical models come mainly from data collected by spacecraft and they show us the basic 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 experimental data with geophysical simulations, the researchers were able to estimate Mercury’s CMB pressure at about 5,575 GPa.

With about 11% sulfur content, the researchers observed a significant temperature change of 358 Kelvin in Mercury’s magma ocean. The researchers suggest that although 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 liquid of Mercury’s magma ocean. If diamond forms in the magma ocean, it can sink to the bottom and be deposited in the CMB. In turn, sulfur also helps form a layer of iron sulfide in the CMB, which binds with carbon content during planetary differentiation,” explained Dr. Lin.

Planetary differentiation refers to the process by which a planet is structured internally, ie, the center or core, in which the heavier minerals sink, and the surface or crust, in which the lighter minerals rise.

According to their findings, the diamond layer in the CMB has an estimated thickness of between 15 and 18 kilometers. They also suggest that the current temperature in Mercury’s CMB is close to the point where graphite can transition to diamond, stabilizing the temperature in the CMB as a result.

Carbon-rich exoplanetary systems

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

Dr. Lin explained, “Carbon from the molten core becomes supersaturated as it cools, forming diamond and floating in the CMB. Diamond’s high thermal conductivity helps effectively transfer heat from the core to the mantle, causing temperature stratification and temperature change. convection in Mercury’s liquid core, and thereby influencing the generation of its magnetic field.”

In simpler terms, as heat is transferred from the core to the mantle, it affects the temperature gradient and convection in Mercury’s liquid outer core, which affects the generation of its magnetic field.

Dr. Lin also noted the crucial role that carbon plays in the formation of carbon-rich exoplanetary systems.

“It may also be important for understanding other terrestrial planets, especially those of similar size and composition. The processes that led to the formation of a diamond layer on Mercury may also have occurred on other planets, potentially leaving similar signatures,” he concluded. Dr. Lin.

© 2024 Science X Network