Mercury is the smallest planet in the Solar System, but its interior hides a remarkable story. Unlike Earth, Venus and Mars, Mercury has an unusually large metallic core and a surface that appears to contain an unusually high amount of carbon. Scientists have now developed a clearer explanation for how these unusual features may have formed more than four billion years ago.
As the European-Japanese BepiColombo mission approaches the final stage of its journey toward Mercury, researchers from the University of Liège and KU Leuven have been using laboratory experiments to reconstruct the planet's earliest history. Their work suggests that Mercury's extreme lack of oxygen played a crucial role in creating a graphite-rich primitive crust, while also influencing the composition and long-term behavior of its enormous core.
The findings, published in Earth and Planetary Science Letters, Nature Communications and Advances in Geochemistry and Cosmochemistry, provide new clues about how Mercury evolved from a hot, molten world into the planet we see today.
Mercury Was Once a World of Molten Rock
Mercury, Venus, Earth and Mars formed more than four billion years ago from material surrounding the young Sun. During their formation, countless rocky and metallic particles collided and gradually came together through a process known as accretion.
The energy released during this violent process generated enormous amounts of heat. The young planets became so hot that much of their interiors melted, producing enormous oceans of molten rock called magma oceans.
This was a critical stage in planetary evolution.
As a magma ocean gradually cooled, different materials separated according to their chemical and physical properties. Dense metals moved downward and formed the core, while lighter rocky materials remained above and eventually became the mantle and crust.
The way this process happened determined the planet's internal structure for billions of years.
But Mercury appears to have followed a particularly unusual path.
The Mystery of Mercury's Carbon
Scientists have never collected a rock sample directly from Mercury, and no known meteorite has been definitively linked to the planet. As a result, researchers have had to reconstruct Mercury's composition using observations from Earth and data gathered by spacecraft such as Mariner 10 and MESSENGER.
MESSENGER provided especially important information about Mercury's surface and interior. Its observations revealed evidence of an unusually carbon-rich surface, leading scientists to investigate whether graphite—a crystalline form of carbon—could have played a major role in Mercury's early crust.
But one major question remained:
Why would carbon stay near the surface instead of disappearing into Mercury's enormous metallic core?
Researchers from the University of Liège and KU Leuven tackled this question using a technique called experimental petrology.
Scientists Recreated Mercury Inside the Laboratory
Experimental petrology allows scientists to reproduce extreme conditions that existed inside planets billions of years ago.
Researchers led by Bernard Charlier of the University of Liège and Olivier Namur of KU Leuven performed high-temperature and high-pressure experiments designed to simulate conditions inside early Mercury.
The experiments examined temperatures ranging from approximately 1,250°C to 2,170°C while applying pressures comparable to those deep inside planetary interiors.
The scientists focused particularly on the behavior of carbon as Mercury's metallic core separated from its rocky mantle.
What they discovered was surprising: carbon's behavior depended strongly on the amount of oxygen present.
Mercury Was Extremely Oxygen-Poor
Scientists describe the chemical environment using a measurement called oxygen fugacity, or fO₂. In simple terms, it helps indicate how oxidizing or reducing an environment is.
Under relatively oxygen-rich conditions, carbon tends to behave as a siderophile element. That means it prefers metallic materials and can move into an iron-rich core.
But Mercury appears to have formed under extremely reducing, or oxygen-poor, conditions.
Under those conditions, carbon becomes much less attracted to metal. Instead of following iron into the core, much of it remains in the molten silicate material that makes up the planet's mantle.
As Mercury's magma ocean cooled, this carbon could crystallize into graphite.
And this created an extraordinary consequence.
Graphite Floated to the Surface
Graphite is relatively light compared with the molten material beneath it. As graphite crystals formed in Mercury's cooling magma ocean, they did not simply sink toward the interior.
Instead, they could float upward and accumulate at the surface.
Over time, enormous amounts of graphite may have gathered together to form a primitive carbon-rich crust.
According to the researchers' models, Mercury's original graphite layer could have been approximately 40 to 120 meters thick.
That estimate is consistent with observations suggesting that Mercury's surface contains roughly 1% to 3% carbon by mass.
In other words, Mercury's dark, carbon-rich surface may be partly the remains of an ancient graphite crust that formed directly from its primordial magma ocean.
However, this original crust would not have remained perfectly intact.
Billions of years of meteorite impacts and volcanic activity repeatedly modified Mercury's surface, breaking up and redistributing the ancient graphite material.
The Giant Core Has Another Mystery
Mercury's unusual story does not stop at its crust.
The planet has an exceptionally large metallic core, representing roughly 70% of Mercury's total mass. Compared with Earth's structure, this is remarkably large.
Yet measurements indicate that Mercury's core is not as dense as a core made almost entirely of iron would be.
This means the core must contain lighter elements.
Scientists previously considered carbon as one possible explanation. However, the new experiments suggest that this is unlikely to be the main answer.
Under the extremely reducing conditions required to produce Mercury's graphite crust, relatively little carbon would have entered the core. The researchers estimate that Mercury's core contains less than 0.5% carbon.
That amount is far too small to explain the core's lower-than-expected density.
Silicon and Sulfur May Hold the Answer
If carbon is not responsible for Mercury's core density, what is?
The researchers point toward silicon and, to a lesser extent, sulfur as the more likely light elements inside the core.
These elements are important for another reason. They can significantly lower the melting temperature of iron-rich material.
That could help explain one of Mercury's most fascinating characteristics: its core may have remained at least partly liquid for approximately 4.5 billion years.
A partially molten metallic core is essential for generating a planetary magnetic field through the movement of electrically conducting material inside the planet.
Mercury does indeed possess a global magnetic field, despite being a small planet.
A New Picture of Mercury's Birth
Together, the research creates a much clearer picture of Mercury's early evolution.
The young planet was likely extremely hot and chemically unusual. Its environment contained very little oxygen, which changed the way carbon behaved during the separation of its core and mantle.
Instead of following iron into the core, much of the carbon remained in the molten rocky material. As the magma ocean cooled, graphite crystallized and floated upward, eventually forming a primitive graphite-rich crust.
At the same time, silicon and sulfur probably entered the enormous metallic core, helping reduce its density and lower its melting temperature.
This means Mercury's graphite crust and unusual core may have the same underlying cause: the planet's extremely oxygen-poor chemical environment.
Why Mercury Matters Beyond Mercury
These findings could also help scientists understand other worlds.
The research may provide clues about the larger proto-Mercury that existed during the Solar System's early history before losing much of its outer material. It could also help researchers understand hypothetical super-Mercuries, carbon-rich exoplanets and other planetary bodies formed under highly reducing conditions.
Even Earth's earliest history may contain clues from similar chemistry, because some of the material from which Earth formed was also relatively reduced.
The upcoming observations from BepiColombo could provide an important test of these ideas. By studying Mercury's surface, interior and composition in greater detail, the mission may help determine just how much graphite remains and how extensively it covers the planet.
Mercury may look like a small, barren world today. But beneath its surface lies evidence of a dramatic beginning—a molten planet, an oxygen-poor environment, a floating graphite crust and an enormous metallic core that may have remained partly liquid for billions of years.
The smallest planet in the Solar System may therefore preserve one of the most unusual chapters in the story of how rocky planets are born.
References: (1) Xiaofeng Lu et al, Mantle melting and magma ocean dynamics on Mercury impacted by sulfur in reduced mafic magmas, Earth and Planetary Science Letters (2026). DOI: 10.1016/j.epsl.2026.120123 (2) Olivier Namur et al, Carbon distribution in planet Mercury from magma ocean evolution to graphite crust and core composition, Nature Communications (2026). DOI: 10.1038/s41467-026-75458-y (3) Fabrizio Saracino et al, The crystallization of Mercury's magma ocean and the formation of its primordial mantle structure, Advances in Geochemistry and Cosmochemistry (2026). DOI: 10.33063/agc.v2i1.1003
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