Treasures Beneath: Deep within Mercury, there may exist a layer of pure diamond measuring up to 18 kilometers in thickness, as indicated by high-pressure experiments and models. This diamond layer is situated at the interface between the planet’s core and mantle, a result of the transformation of graphite, which is plentiful in Mercury, over time. However, this conversion of carbon into its densest form was possible due to another unique element within Mercury’s interior.
The innermost planet in our solar system is truly eccentric: Mercury is smaller, denser, and older than any other planet, possessing a disproportionately large core. Furthermore, it rotates at an unusually swift pace and appears to be shrinking. Its composition is also unique: Mercury’s crust and interior contain one to three percent carbon in the form of graphite, while Earth’s carbon content is merely 100 parts per million (ppm). This carbon imparts a dark hue to Mercury’s crust.
Dark spots like those found on this Mercury crater are due to higher graphite content. © NASA/ JHUAPL/ Carnegie Institution
Graphite and Magma Ocean
Within Mercury, the graphite may have transformed into a genuine treasure – pure diamond. This theory, led by Yongjiang Xu from the Center for High Pressure Research and Technology in Beijing, has been developed through high-pressure experiments and models that reconstruct the potential impacts of high graphite content on the evolution of Mercury’s interior.
Similar to Earth, Mercury was once covered by a magma ocean in its early history. “Graphite, being less dense than the molten material of the magma ocean, gathered at the surface and contributed to the formation of the planet’s initial crust,” Xu and his team explain. However, graphite also existed in Mercury’s initially mixed interior and became distributed throughout the core and mantle during the layering process.
Mercury Mixture Under High Pressure
This is where Xu and his team come into play: they studied the state the carbon might have adopted at the core-mantle boundary of early Mercury: Did it stay graphite, or was the pressure enough to compress the carbon into the solid crystal form of diamond? “Our calculations indicate that the maximum pressure at Mercury’s core-mantle boundary is seven gigapascals,” the researchers note.
The sulfur content influences the pressure and temperature at which diamond or graphite forms.© Xu et al./ Nature Communications, CC-by 4.0
In their high-pressure experiments, the researchers examined how different compositions of early Mercury material behaved under this pressure and at temperatures ranging from 1,700 to 2,000 degrees. Unlike previous studies, Xu and his colleagues incorporated another element into the carbon-containing mixture of silicates alongside an iron-silicon alloy for the core: sulfur. Recent measurements have shown that this element is also highly abundant in Mercury.
Sulfur as the Key
The key aspect here is that the presence of sulfur lowers the temperature at which the silicate melt of the magma ocean starts to solidify. “At one percent sulfur by weight, the liquidus temperature decreases by 59 Kelvin; furthermore, as more sulfur is added, the temperature continues to drop—though at a slower rate,” explain Xu and his colleagues. Under high pressure and reduced conditions, this shifts the graphite-diamond equilibrium favorably towards diamond, as demonstrated by the high-pressure tests.
At approximately eleven percent sulfur content, the high pressures and temperatures at the bottom of the magma ocean were sufficient for a small fraction of carbon to crystallize into diamonds. “The amount of diamond production during these initial stages of magma ocean crystallization would be minimal; our calculations suggest this would lead to a diamond layer of 0.1 to 200 meters thick above the core-mantle boundary,” the researchers report.
This illustrates how Mercury may have formed a diamond layer: A) In its early days, graphite was converted into diamonds at the bottom of the magma ocean. B) As Mercury’s solid core formed, carbon crystallized from the core into diamonds, which then ascended to the core-mantle boundary. The researchers consider the latter scenario to be more probable.© Xu et al./ Nature Communications, CC-by 4.0
Diamond Crystals from Mercury’s Core
Moreover, additional diamonds may originate from Mercury’s core, which likely contains around three percent carbon, as confirmed by Xu and his team through further testing and modeling simulations. “When the solid inner core of Mercury formed, diamonds crystallized from the molten material,” they explain. Because these diamond crystals were less dense than the surrounding molten metal, they migrated to the core-mantle boundary.
This led to the gradual accumulation of diamonds deposited from below at the core-mantle boundary. Consequently, scientists estimate that this diamond layer could currently reach thicknesses of 15 to 18 kilometers, although it lies more than 600 kilometers beneath Mercury’s surface, making it completely unreachable. Nevertheless, this crystalline layer could play a crucial role in Mercury’sheat balance, as diamonds are excellent conductors of heat—significantly superior to most metals or rocks.
“Future research should explore how this kilometer-thick diamond layer at the core-mantle boundary influences the thermal evolution of the planet and its silicate layers,” Xu and his team conclude.
In Mercury’s “high-magnesium province,” diamonds may have ascended from the depths to the surface. © NASA/ JHUAPL/ Carnegie Institution
Could BepiColombo Discover Diamonds?
But how can we determine whether Mercury contains a diamond layer within? As the researchers indicate, this layer of gemstones is too thin to be detected externally or through indirect measurements. Nonetheless, they propose it is possible that some of these diamonds were brought to the surface by the circulation currents present in Mercury’s early phase, volcanic activity, or large impacts—similar to how diamonds on Earth are transported to the surface through rising magma in volcanoes.
The likelihood of discovering such diamonds would be particularly high in the so-called “high magnesium province” of Mercury. In this region, measurements from the MESSENGER space probe have indicated an unusually high magnesium concentration in Mercury’s crust. This is considered a sign that material from deeper layers has reached the surface, potentially including diamonds. “The BepiColombo space probe could shed light on this by closely examining this high-magnesium province,” Xu and his colleagues state. The European Space Agency’s Mercury probe will enter the planet’s orbit in 2025. (Nature Communications, 2024; doi: 10.1038/s41467-024-49305-x)
Source: Nature Communications
July 24, 2024 – Nadja Podbregar
Treasures Beneath Mercury: The Fascinating Discovery of Diamond Layers
Deep in the interior of Mercury, scientists believe there may be a layer of pure diamond measuring up to 18 kilometers thick. High-pressure experiments and models indicate that this precious stone layer sits on the boundary between the planet’s core and mantle, formed over millennia from the abundant graphite found on the planet.
The Unique Characteristics of Mercury
Mercury, the innermost planet of our solar system, is a fascinating enigma. It is smaller, denser, and older than all other planets, with a core that is disproportionately large. Additionally, Mercury rotates unusually quickly and continues to display signs of shrinking.
One of the planet’s most intriguing traits is its composition. Unlike Earth, which contains around 100 parts per million (ppm) of carbon, Mercury has a carbon content of 1-3%. This significant difference contributes to the dark coloration of its crust.
Graphite and the Formation of a Diamond Layer
A team of researchers, led by Yongjiang Xu from the Center for High Pressure Research and Technology in Beijing, theorizes that the graphite within Mercury could have transformed into diamonds under the extreme conditions present in its early interior. Just like Earth, Mercury was once covered in a magma ocean.
“Graphite, being less dense than molten material, floated on the surface and contributed to the primordial crust,” says Xu. However, the graphite also mixed within the planet’s interior during the layering process.
High-Pressure Experiments and Results
The research team conducted high-pressure tests to analyze the potential states of carbon at the core-mantle boundary of early Mercury. They discovered that pressures could reach up to seven gigapascals, sufficient to compress carbon into diamond.
Experiments revealed how various mixtures of early Mercury material behaved under this immense pressure, particularly at temperatures between 1,700 and 2,000 degrees. A key element in their findings was sulfur, which they found to be abundant in Mercury.
How Sulfur Influences Carbon Transformation
Sulfur plays a crucial role in shifting the graphite-diamond equilibrium at high pressures. When sulfur is present, it lowers the solidification temperature of the silicate melt, enabling more carbon to crystallize into diamond.
At approximately eleven percent sulfur by weight, the extreme conditions at the base of the magma ocean allowed for some carbon to form diamonds, albeit in small quantities. Xu noted that the resultant diamond layer might have thicknesses ranging from 0.1 to 200 meters.
Diamond Crystals Originating from Mercury’s Core
Further research revealed that Mercury’s core likely contains around three percent carbon. As the solid inner core formed, diamonds crystallized from the molten mass and ascended to the core-mantle boundary, leading to the potential creation of a substantial diamond layer, possibly up to 18 kilometers thick.
This diamond layer, hidden more than 600 kilometers beneath Mercury’s surface, could play a significant role in the planet’s thermal dynamics due to diamond’s excellent heat conduction properties.
Implications of Mercury’s Diamond Layer
The existence of a diamond layer may have profound implications for our understanding of Mercury’s heat balance. Future research should focus on analyzing how this layer influences the thermal evolution of the planet and its surrounding silicate layers.
Exploring the Possibility of Detecting Diamonds
Identifying the diamond layer presents challenges, as it is too thin for direct detection from space. However, researchers speculate that some diamonds might have been transported to the surface through volcanic activity or impacts, similar to how diamonds are brought to Earth’s surface.
The “high magnesium province” of Mercury is of particular interest, as it suggests materials from deeper layers may have reached the surface, possibly including diamonds. ESA’s BepiColombo spacecraft, set to enter Mercury’s orbit in 2025, could provide crucial insights into this hypothesis.
Key Findings Summarized
| Finding | Description |
|---|---|
| Diamond Layer Thickness | Potentially 15 to 18 kilometers thick |
| Core Carbon Content | Approximately 3% carbon in the core |
| Sulfur’s Role | Reduces solidification temperature, favoring diamond formation |
| Temperature of Diamond Formation | Between 1,700 to 2,000 degrees Celsius |
| Study Source | Nature Communications |
Potential Future Research Directions
Further studies could examine the dynamics of Mercury’s interior, particularly focusing on the thermal implications of the diamond layer. Understanding these aspects can refine our knowledge of planetary formation and evolution.
Final Thoughts on Mercury’s Hidden Treasures
Mercury continues to reveal astonishing secrets about our solar system’s formation. The possibility of diamonds lying beneath its surface adds another layer of intrigue to a planet that is already known for its unique qualities. Continued exploration, particularly with missions like BepiColombo, will undoubtedly enhance our understanding of this celestial body.
Source: Nature Communications, July 2024, Nadja Podbregar.
