SPACE/COSMOS
How Mercury formed its graphite crust and core
Studies reveal that Mercury had a graphite crust because its carbon did not migrate into the core, which owes its low density to silicon and sulphur
University of Liège
image:
The young planet was characterised by a global magma ocean and a liquid metallic core, before cooling and differentiating to acquire its current internal structure.
view moreCredit: University of Liège / B.Charlier
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 crystallisation of its magma ocean and the formation of its mantle.
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 disc 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 crystallises, 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, in order to reconstruct the planet’s history, scientists draw on a specialised 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 analyse 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 (at temperatures of between approximately 1,250 and 2,170 °C) and at pressures equivalent to those found deep within the planets, in order to simulate and track the behaviour of carbon during the separation of the metallic core from the silicate magma of the mantle.
Carbon that changes sides depending on the conditions...
From the results obtained, the teams were able to deduce that the behaviour of carbon depended heavily on the oxidation state of the environment, as measured by the oxygen fugacity (fO2 ). “Under relatively oxidising conditions, carbon is strongly siderophile,” explains Bernard 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 crystallises 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 Olivier Namur, also a geologist. The model reproduces a graphite crust approximately 40 to 120 metres thick, consistent with the carbon-rich layer (1 to 3 per cent 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 per cent of the planet’s mass) must contain several per cent of light elements to account for its density deficit. “Carbon had been proposed as a candidate,” continues Olivier 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 gramme, or less than 0.5 per cent. Such a concentration is far too low to account for the observed deficit.”
It is therefore primarily silicon and, to a lesser extent, sulphur that must constitute the light elements of the core. 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 also 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 Bernard Charlier.
Journal
Nature Communications
Article Title
Carbon distribution in planet Mercury from magma ocean evolution to graphite crust and core composition
Turbulent times for star formation in Stephan’s Quintet
Mapping interacting galaxies reveals how turbulence can suppress star formation
Osaka Metropolitan University
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The contour lines show radio emissions from carbon monoxide molecules in Stephan’s Quintet, arranged like the contours on a topographic map. Just as higher contours on a map represent higher elevations, higher contour levels here represent stronger CO emission, indicating regions with more molecular gas.
view moreCredit: Osaka Metropolitan University
Star formation in galaxies is closely linked to molecular gas. In the distant past, when galaxy interactions were more common, these encounters compressed molecular gas, causing dense clouds to collapse under their own gravity and form new stars.
However, a curious phenomenon occurs in some regions with abundant molecular gas. Despite having plenty of the raw material needed to make stars, they produce surprisingly few. Why a galactic collision can trigger star formation in some regions while suppressing it in others has long intrigued scientists.
To understand this phenomenon, a research team at Osaka Metropolitan University used the Atacama Compact Array, a network of radio telescopes in Chile, to create the first detailed map of the molecular gas throughout Stephan’s Quintet, a nearby group of interacting galaxies where some regions form stars far less efficiently than expected.
When the researchers compared the amount and motion of gas in different regions with how efficiently each region was forming stars, they found that regions where the molecular gas was moving more violently tended to form stars much less efficiently, even in the presence of abundant gas.
“Interactions between galaxies can both compress and disperse molecular gas, creating dramatic differences in star formation activity,” Misaki Yamamoto of the Graduate School of Science explained. “The findings pointed to turbulence as an important factor in regulating where stars can form.”
They propose a model where the turbulence generated by the galaxies’ interactions prevents the gas from settling and collapsing under its own gravity. If the gas is highly turbulent, its motions spread out the gas, preventing parts of the cloud from settling, becoming concentrated, and collapsing. This creates fewer opportunities to form stars.
“Star formation is one of the most fundamental processes in galaxy evolution. Studies like ours help refine our picture of the universe and encourage us to reflect on our place within it,” Associate Professor Kazuyuki Muraoka said. “Understanding how galaxy collisions and interactions in the early universe enhance or suppress star formation will allow researchers a better tool to trace the history of galaxy evolution across cosmic time.”
The findings were published in The Astrophysical Journal.
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About OMU
Established in Osaka as one of the largest public universities in Japan, Osaka Metropolitan University is committed to shaping the future of society through the “Convergence of Knowledge” and the promotion of world-class research. For more research news, visit https://www.omu.ac.jp/en/ and follow us on social media: X, Instagram, LinkedIn.
Journal
The Astrophysical Journal
Method of Research
Observational study
Subject of Research
Not applicable
Article Title
Molecular Gas Structure and Star Formation Diversity in Stephan’s Quintet Revealed by ACA CO(1-0) Mapping
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