A proposed scenario for the formation of a diamond at the core-mantle interface of Mercury. (a) Crystallization of a carbon-saturated silicate magma ocean and potential but unlikely early diamond production at its base. Graphite was the main phase that formed in the magma ocean and accumulated at the surface to form the primordial graphite crust. (b) During the crystallization of the inner core, the diamond became loose and floated at the core-mantle interface. Such a late diamond layer would continue to grow during core crystallization. Credit: Dr. Yanhao Lin and Dr. Bernard Charlier.
A recent study in The nature of communication scientists from China and Belgium suggest that Mercury’s core-mantle boundary (CMB) includes a diamond layer, potentially up to 18 kilometers deep inside the planet.
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, have revealed that Mercury’s surface contains significant amounts of graphite, a form of carbon.
This led scientists to believe that the planet’s early history involved a carbon-rich magmatic ocean. Phys.org spoke with one of the study’s co-authors, Dr. Yanhao Linem, of the Center for Advanced Research in High Pressure Science and Technology in Beijing.
“Many years ago, I noticed that the extremely high carbon content of Mercury could have significant consequences. This made me realize that something strange was probably going on in its interior,” said Dr. Tench.
What we know about Mercury
The most detailed information about Mercury comes from NASA’s MESSENGER and Mariner 10 missions.
Previous MESSENGER observations revealed that Mercury’s surface is unusually dark due to the widespread presence of graphite.
The amount of carbon on the surface is thought to come from an ancient layer of graphite that soon floated to the surface. This suggests that Mercury once had a molten surface layer or magmatic ocean containing significant amounts of carbon.
Over time, as the planet cooled and solidified, this carbon formed a graphite crust on the surface.
However, scientists question the assumption that graphite was the only stable carbon-bearing phase during the crystallization of Mercury’s magmatic ocean. That’s when the planet’s mantle (middle layer) cools and hardens.
Early assumptions about a graphitic crust relied on predictions of lower temperature and pressure in the CMB. However, more recent studies suggest that the CMB is deeper than previously thought, leading researchers to reconsider the graphitic crust.
In addition, another study also suggested the presence of sulfur in Mercury’s iron core. The presence of sulfur may have influenced the crystallization of Mercury’s magmatic ocean, challenging the original claim that only graphite was present during this phase.
Regenerative conditions of Mercury’s interior
The researchers used a combination of high-pressure and temperature experiments and thermodynamic modeling to recreate the conditions of Mercury’s interior.
“We use a high-volume press to simulate the high-temperature, high-pressure conditions at the core-mantle interface of Mercury and combine these with geophysical models and thermodynamic calculations,” explained Dr. Tench.
They used synthetic silicate as a starting material to resemble the composition of Mercury’s mantle. This is a commonly used method for studying planetary interiors.
The researchers reached pressure levels of up to 7 Giga Pascals (GPa), which is approximately 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 the graphite and diamond phases.
They also analyzed the chemical composition of the experimental samples.
“What we do in the lab is to mimic the extreme pressures and temperatures inside planetary space. It’s sometimes a challenging thing; you have to adapt the equipment to your needs. The experimental setup has to be very precise to simulate these conditions,” explained Dr. Tench.
They also used geophysical modeling to study observed data about Mercury’s interior.
“Geophysical models come mainly from data collected by spacecraft and tell us the basic structures of the planet’s interior,” said Dr. Tench.
They used the model to predict phase stability, calculate CMB pressures and temperatures, and simulate the stability of graphite and diamond at extreme temperatures and pressures.
Diamonds are formed under pressure
By integrating experimental data with geophysical simulations, scientists were able to estimate Mercury’s CMB pressure to be approximately 5.575 GPa.
At roughly 11% sulfur, the researchers observed a significant temperature change of 358 Kelvin in Mercury’s magma ocean. The researchers suggest that although graphite was probably the dominant carbon phase during the crystallization of the magma ocean, crystallization of the core led to the formation of the diamond layer in the CMB.
“Sulfur reduces the liquidus of Mercury’s magma ocean. If a 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 the iron sulfide layer in the CMB, which is related to the carbon content during planetary differentiation,” he explained Dr. Lin.
Planetary differentiation refers to the process by which a planet is structured internally, i.e. a center or core into which heavier minerals sink and a surface or crust into which lighter minerals rise.
According to their findings, the diamond layer in the CMB is estimated to be between 15 and 18 kilometers thick. They also suggest that the current temperature in Mercury’s CMB is close to the point where graphite can transition to diamond, which in turn stabilizes the temperature at the CMB.
Carbon-rich exoplanetary systems
One consequence of these findings is Mercury’s magnetic field, which is anomalously strong for its size.
Dr. Lin explained, “The carbon in the molten core becomes supersaturated as it cools, forms diamond, and floats into the CMB. The high thermal conductivity of diamond helps efficiently transfer heat from the core to the mantle, causing thermal stratification and convection change in Mercury’s liquid outer surface.” core, thereby influencing the formation of its magnetic field.”
More simply, how heat is transferred from the core to the mantle affects temperature gradients and convection in Mercury’s liquid outer core, which affects the formation of its magnetic field.
Dr. Lin also pointed out the key role that carbon plays in creating carbon-rich exoplanetary systems.
“It could also be important for understanding other terrestrial planets, especially those with similar sizes and compositions. The processes that led to the formation of the diamond layer on Mercury could also have occurred on other planets, potentially leaving similar signatures,” he concluded. Dr. Tench.
More information:
Yongjiang Xu et al, Core-mantle boundary with diamonds on Mercury, The nature of communication (2024). DOI: 10.1038/s41467-024-49305-x.
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