Mercury's First Crust Was Graphite. It Floated Up Out of the Magma, 40 to 120 Meters of It.

Mercury has a carbon problem that nobody has been able to explain. Its surface carries between 1% and 3% carbon by mass, far more than any other rocky planet in the solar system, and the darkest patches on it appear to be graphite. Researchers at the University of Liege and KU Leuven have now reproduced the conditions that would have made that layer, and the answer turns on one word: oxygen, or rather the near-total lack of it.

The team ran high-pressure, high-temperature experiments simulating the inside of Mercury more than four billion years ago, tracking where carbon goes as temperatures sweep from 1,250C to 2,170C and a metallic core separates out of a silicate magma ocean. On Earth, carbon is siderophile — it prefers iron, and follows the metal down into the core. Mercury formed under conditions so chemically reducing, so starved of oxygen, that carbon stops behaving that way.

"Under highly reducing conditions specific to Mercury, it becomes much less siderophile," said Bernard Charlier of the University of Liege, "and so remains in the silicate magma." Left behind in the melt, the carbon crystallized as graphite. Graphite is less dense than molten silicate, so it floated. The experiments produce a primordial graphite crust between roughly 40 and 120 meters thick — thin by planetary standards, but consistent with the carbon abundance NASA's MESSENGER mission measured at the surface.

The same experiments settle a second argument in the opposite direction. Mercury is anomalously dense for its size, and one longstanding proposal held that a carbon-rich core could account for the shortfall between its measured density and what iron alone predicts. The measurements say no: the core holds less than 0.5% carbon, nowhere near enough. Silicon and sulfur are the more likely light elements down there.

That substitution has a consequence that is still observable today. "These elements significantly lower the melting point of iron," said Olivier Namur of KU Leuven, "which could explain why Mercury's core has remained at least partially liquid for 4.5 billion years." A partially molten core is what generates Mercury's magnetic field — weak, roughly 1% of Earth's, but present, and a genuine puzzle for a planet that small, which should have frozen solid long ago.

The findings appear across three peer-reviewed papers, in Earth and Planetary Science Letters, Nature Communications, and Advances in Geochemistry and Cosmochemistry. Their timing is convenient. The European-Japanese BepiColombo mission entered orbit around Mercury after a long cruise, and its instruments are built to map surface composition in far more detail than MESSENGER managed. A prediction of 40 to 120 meters of buoyant graphite is the kind of claim an orbiter can go and test.