How Mercury formed its graphite crust and core

As the BepiColombo mission prepares to enter the final phase of its journey to Mercury, a series of studies conducted by researchers at the University of Liège and KU Leuven sheds new light on the early stages of the evolution of the planet closest to the sun. Using experimental petrology, the researchers are reconstructing in the laboratory the formation of Mercury’s core, the crystallization of its magma ocean and the formation of its mantle. The studies are published in Earth and Planetary Science Letters, Nature Communications and Advances in Geochemistry and Cosmochemistry.

The terrestrial planets (Mercury, Venus, Earth and Mars) are the result of more than four billion years of evolution, which began with accretion from the disk surrounding the young sun. During the early stages of evolution, the heat released caused these planets to melt, creating what is known as a magma ocean.

This key stage determines the distribution of elements between the metallic core and the mantle. As it crystallizes, this ocean structures the solid mantle, the partial melting of which will subsequently generate the magmas that form the crust. It is also at this stage that an initial atmosphere may form.

To date, no samples have been taken from Mercury, and no meteorites have been linked to it. Our knowledge of its composition therefore relies on telescope observations and data from the American probes Mariner 10 (1973) and MESSENGER (2011).

However, to reconstruct the planet’s history, scientists draw on a specialized discipline: experimental petrology. This branch of geology enables scientists to reproduce, in the laboratory, the temperatures, pressures and chemical conditions that prevailed inside the planet more than four billion years ago, and then to analyze the minerals, metals and gases that make up the planet’s various layers.

It was by employing these techniques that the teams led by Bernard Charlier (ULiège) and Olivier Namur (KULeuven) carried out an extensive series of high-pressure, high-temperature experiments. They simulated and tracked the behavior of carbon during the separation of the metallic core from the silicate magma of the mantle at temperatures between approximately 1,250°C (2,282°F) and 2,170°C (3,938°F) and at pressures equivalent to those found deep within the planets.

Carbon that changes sides depending on the conditions…

From the results obtained, the teams were able to deduce that the behavior of carbon depended heavily on the oxidation state of the environment, as measured by the oxygen fugacity (fO₂). “Under relatively oxidizing conditions, carbon is strongly siderophile,” explains Charlier, a geologist at ULiège.

“In other words, carbon prefers metal and therefore enters the core. But under the highly reducing conditions specific to Mercury, it becomes much less siderophile (attracted to metal), and so remains in the silicate magma, where it eventually crystallizes as graphite.”

Based on these experiments, the team reconstructed how carbon was distributed between Mercury’s core, mantle, crust and primitive atmosphere. By comparing these results with the thickness of the graphite crust calculated from data from the MESSENGER probe, they were able to determine the conditions that prevailed during the planet’s formation—undoubtedly an environment extraordinarily low in oxygen.

… and a graphite crust formed by flotation

“Under these extreme conditions, graphite is too light to sink; it floats on the surface of the magma ocean and accumulates to form a primitive crust,” adds Namur, also a geologist. “The model reproduces a graphite crust approximately 40–120 meters thick (130–390 feet), consistent with the carbon-rich layer (1% to 3% by mass) that observations attribute to Mercury’s surface.” This primordial crust would then have been disrupted and redistributed by meteorite impacts and by the volcanism that built the planet’s more recent crust.

MESSENGER’s geodetic data also indicate that the exceptionally large core (accounting for around 70% of the planet’s mass) must contain several percent of light elements to account for its density deficit. “Carbon had been proposed as a candidate,” continues Namur. “However, under the very conditions that allow the graphite crust to form, the study shows that the core remains low in carbon—less than 5,000 micrograms per gram, or less than 0.5%. Such a concentration is far too low to account for the observed deficit.”

It is therefore primarily silicon and, to a lesser extent, sulfur that must make up the core’s light elements. These elements have another property: They significantly lower the melting point of iron, which could explain why Mercury’s core has remained at least partially liquid for 4.5 billion years—a characteristic necessary to sustain its magnetic field.

By linking Mercury’s extreme chemical reduction to both its graphite crust and the structure of its core, this research provides a useful framework for understanding other highly reduced bodies: the original, more massive proto-Mercury; the hypothetical “super-Mercuries”; certain carbon-rich exoplanets; and the early Earth, which was largely built from reduced materials.

“Future observations, notably by missions such as BepiColombo, could confirm and quantify graphite as a major phase on Mercury’s surface,” concluded Charlier.

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Robert Egan

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