Mercury formed under intensely reducing conditions that retained carbon in its magma ocean rather than sinking it into the planet's metallic interior, according to research published in Nature Communications.
The study used high-pressure, high-temperature experiments to explain surface readings from the MESSENGER spacecraft, which detected low-reflectance regions containing 1 to 3 weight percent graphite. Researchers tested natural chondrite rocks and synthetic powders matching Mercury's crust and mantle inside graphite capsules. Temperatures ranged from 1523 to 2443 Kelvin, and pressures spanned 100 kilopascals to 26 gigapascals across an oxygen fugacity range of IW − 3 to IW − 9.
Carbon shifted chemical behavior under these reducing conditions. In oxidized systems, carbon acts as a siderophile element, binding tightly into molten iron. In the reducing environments representative of Mercury, carbon becomes less siderophile and increasingly lithophile, dissolving into silicate melts as carbon-hydrogen species.
Mantle partitioning
Measurements at pressures between 5 and 6 gigapascals, matching Mercury's core-mantle boundary, showed a steep decline in the metal-silicate partition coefficient for carbon. The ratio fell from roughly 900 under milder reduction to 1.5 at IW − 8. This shift allowed carbon to concentrate in the molten silicate layer instead of draining into the core.
Model calculations showed that graphite began crystallizing at IW − 6.9. Conditions between IW − 6 and IW − 6.5 best reproduced the flotation crust thickness inferred from MESSENGER observations. Between IW − 3 and IW − 6.9, the modeled crust reached a thickness of only 0 to 300 meters, but it expanded to 4600 meters at IW − 8 as primordial graphite accumulated atop the magma ocean.
Core composition
Mercury's core retains less than 5000 micrograms per gram of carbon across these modeled oxygen fugacities. At IW − 8, carbon levels in the metal melt fell to approximately 1200 micrograms per gram as silicon concentrations in the alloy rose.
Geodetic data indicate the core accounts for roughly 70 percent of Mercury's mass and requires several weight percent of light elements to match observed density deficits. Because carbon cannot supply this mass under reducing conditions, silicon and sulfur must account for the deficit. Both elements also depress the melting point of iron, helping explain why the planet's core has remained molten over 4.5 billion years.
Simulations run under hydrogen-poor assumptions altered the timeline of crust development. Without hydrogen to form carbon-hydrogen bonds in the melt, the calculated graphite crust remained thinner than 1 meter at oxygen fugacities more oxidizing than IW − 6.5, before expanding to 4500 meters at IW − 8.
