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The smallest planet in the solar system may be hiding a big secret. Using data from NASA’s MESSENGER spacecraft, scientists have determined that a 10-mile-thick diamond mantle may lie beneath the crust of Mercury, the planet closest to the sun.
Mercury has long puzzled scientists because it has many characteristics not common to other planets in the solar system. These include its very dark surface, extremely dense core, and the premature end of Mercury’s volcanic era.
Also among these puzzles are patches of graphite, a type (or “allotrope”) of carbon on the surface of the innermost planet in the solar system. These patches have given scientists the impression that the tiny planet had a carbon-rich magma ocean in Mercury’s early history. This ocean would float to the surface, creating patches of graphite and the dark pigment of Mercury’s surface.
The result of the same process would be the establishment of a carbon-rich mantle below the surface. The team behind these results think that this mantle is not graphene, as previously suspected, but is made up of another much more valuable allotrope of carbon: diamond.
“We calculate that, given the new estimate of the pressure at the mantle boundary, and knowing that Mercury is a carbon-rich planet, the carbon-carbon mineral that would form at the mantle-core interface is diamond and not graphite. ,” team member Olivier Namur, associate professor at KU Leuven, told Space.com. “Our study uses geophysical data collected by the NASA MESSENGER spacecraft.”
MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Range) was launched in August 2004 and was the first spacecraft to orbit Mercury. The mission, which ended in 2015, mapped the entire tiny world, finding abundant water ice under shadows at the poles and gathering vital data about Mercury’s geology and magnetic field.
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Under pressure!
This new study also relates to a big surprise that came a few years ago when scientists re-evaluated the mass distribution on Mercury, discovering that the mantle of this tiny planet is thicker than previously thought.
“We just thought that this must have huge implications for speciation [the distribution of an element or an allotrope amongst chemical species in a system] carbon, diamond vs graphite, on Mercury,” said Namur.
The team investigated this here on Earth by using a large volume press to replicate the pressures and temperatures within Mercury’s interior. They applied incredible amounts of pressure, over seven gigapascals, to a synthetic silicate acting as a proxy for the material found in Mercury’s mantle, reaching temperatures of up to 3,950 degrees Fahrenheit (2,177 degrees Celsius).
This allowed them to study how minerals similar to those found in Mercury’s mantle changed in its early life under these conditions. They also used computer modeling to estimate details about Mercury’s interior, which gave them clues as to how Mercury’s diamond mantle might have formed.
“We believe that diamond could have been formed by two processes. The first is the crystallization of the magma ocean, but this process probably helped to create a very thin layer of diamond at the core/mantle interface,” Namur explained. “Secondly, and most importantly, the crystallization of Mercury’s metal core.”
Namur said that when Mercury formed about 4.5 billion years ago, the planet’s core was completely liquid, gradually crystallizing over time. The exact nature of the solid phases in the inner core is currently not well known, but the team believes that these phases must have been low in carbon or “almost carbon.”
“There was some carbon in the liquid core before crystallization; therefore, an enrichment of carbon in the residual melt as a result of crystallization,” he continued. “At some point, a solubility threshold is reached, which means the liquid can no longer dissolve carbon, and diamond forms.”
Diamond is a dense mineral but not as dense as metal, meaning it would have floated to the top of the core during this process, stopping at the boundary between Mercury’s core and its mantle. This would have resulted in the formation of a diamond layer about 0.62-mile (1 km) thick that continued to grow over time.
The discovery highlights the differences between the birth of the planet closest to the sun when compared to the creation of the other rocky planets of the solar system, Venus, Earth and Mars.
“Mercury was much closer to the sun, probably from a cloud of carbon-rich dust. As a result, Mercury has less oxygen and more carbon than other planets, which caused the formation of a diamond layer,” said Namur. “However, the Earth’s core also contains carbon, and the formation of diamonds in the Earth’s core has already been proposed by various researchers.”
The researcher hopes that this discovery could help reveal clues to some of the other mysteries surrounding the smallest planet in the solar system, including why its volcanic phase was cut off around 3.5 billion years ago.
“A big question I have about the evolution of Mercury is why the major phase of volcanism lasted only a few hundred million years, much shorter than other rocky planets. This must mean that the planet cooled down very quickly,” said Namur. “This is partly to do with the small size of the planet, but we are now working with physicists to try to understand whether a diamond layer could have contributed to very rapid heat removal, which ended major volcanism very early.”
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Namur said the team’s next step will be to investigate the thermal effect of a diamond layer at the mantle/core boundary. This study could be supported by data from a mission that will continue in the MESSENGER phases.
“We are also eagerly awaiting the first data collected by BepiColombo, hopefully in 2026, to refine our understanding of Mercury’s internal structure and evolution,” Namur concluded.
The team’s research was published in the journal Nature Communications.