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The solar system’s smallest planet may be hiding a big secret. Using data from NASA’s MESSENGER spacecraft, scientists have determined that a 10-mile-thick diamond mantle may exist beneath the crust of Mercury, the planet closest to the sun.
Mercury has long puzzled scientists because it has many properties that are not common to other planets in the solar system. These include its very dark surface, its remarkably dense core, and the premature end of Mercury’s volcanic era.
Also among these puzzles are chunks of graphite, a type (or “allotrope”) of carbon on the surface of the innermost planet in the solar system. These chunks have led scientists to suggest that the small planet had a carbon-rich magma ocean early in Mercury’s history. This ocean would have floated to the surface, creating the graphite chunks and the dark hue of Mercury’s surface.
The same process would also have led to the formation of a carbon-rich mantle beneath the surface. The team behind these findings think that this mantle is not graphene, as previously suspected, but is composed of another, much more valuable allotrope of carbon: diamond.
“We calculate that, given the new estimate of the pressure at the mantle-core boundary, and knowing that Mercury is a carbon-rich planet, the carbon-bearing mineral that would form at the mantle-core boundary is diamond, not graphite,” team member Olivier Namur, an 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 Ranging) was launched in August 2004 and was the first spacecraft to orbit Mercury. The mission, which ended in 2015, mapped the entire small world, discovered abundant water ice in shadows near the poles, and collected crucial data on Mercury’s geology and magnetic field.
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Under pressure!
This new research also addresses a major surprise that occurred a few years ago, when scientists reevaluated the mass distribution on Mercury and discovered that the small planet’s mantle is thicker than previously thought.
“We immediately thought that this must have major consequences for speciation [the distribution of an element or an allotrope amongst chemical species in a system] of carbon, diamond versus graphite, on Mercury,” Namur said.
The team investigated this here on Earth by using a large press to replicate the pressures and temperatures found inside Mercury. They applied incredible amounts of pressure, more than seven gigapascals, to a synthetic silicate that acted 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 like those found in Mercury’s mantle early in its existence changed under these conditions. They also used computer models to evaluate data about Mercury’s interior, giving them clues about how Mercury’s diamond-like mantle might have formed.
“We believe that diamond could have formed by two processes. First, there is the crystallization of the magma ocean, but this process probably contributed to the formation of only a very thin diamond layer at the core/mantle interface,” Namur explained. “Second, and most importantly, the crystallization of Mercury’s metallic core.”
Namur said that when Mercury formed about 4.5 billion years ago, the planet’s core was completely liquid and gradually crystallized over time. The exact nature of the solid phases that formed in the inner core are not well understood at this time, but the team believes these phases must have been low in carbon, or “carbon-poor.”
“The liquid core contained some carbon before crystallization; crystallization therefore leads to carbon enrichment in the remaining melt,” he continued. “At some point, a solubility threshold is reached, meaning the liquid can no longer dissolve carbon and diamond forms.”
Diamond is a dense mineral, but not as dense as metal, meaning that during this process it would have drifted to the top of the core and stopped at the boundary between Mercury’s core and its mantle. This would have resulted in the formation of a diamond layer about 0.62 miles (1 km) thick that then continued to grow over time.
The discovery highlights the differences between the birth of the planet closest to the sun and the creation of the other rocky planets in the solar system: Venus, Earth and Mars.
“Mercury formed much closer to the Sun, probably from a carbon-rich dust cloud. As a result, Mercury contains less oxygen and more carbon than other planets, leading to the formation of a diamond layer,” Namur added. “However, the Earth’s core also contains carbon, and diamond formation in the Earth’s core has been suggested by several researchers.”
The researcher hopes that this discovery can help solve some other mysteries surrounding the smallest planet in the solar system, including why its volcanic phase ended prematurely about 3.5 billion years ago.
“A big question I have about Mercury’s evolution is why the major volcanism phase lasted only a few hundred million years, much shorter than other rocky planets. This must mean that the planet cooled very quickly,” Namur said. “This is partly due to 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 a very rapid heat dissipation, ending the 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 on the mantle/core boundary. This study could be supported by data from a mission that follows in MESSENGER’s footsteps.
“We also eagerly await 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.