The smallest planet in the solar system may be hiding a big secret. Using data from NASA’s MESSENGER spacecraft, scientists have determined that Mercury, the closest planet to the Sun, may have a 10-mile-thick diamond mantle beneath its crust.

Mercury has long puzzled scientists because it has many characteristics that are not characteristic of other planets in the solar system, including its very dark surface, its extremely dense core, and the early end of Mercury’s volcanic age.

Also among these mysteries are bits of graphite, a type of carbon (or “allotrope”), on the surface of the inner Solar System planet. These spots have led scientists to suggest that the small planet had a carbon-rich magmatic ocean early in Mercury’s history. This ocean may have risen to the surface, producing the graphite spots and the dark tone of Mercury’s surface.

The same process could also lead to the formation of a carbon-rich mantle beneath the surface. The team behind the discoveries believe this mantle is not graphene, as previously suspected, but another, much more valuable allotrope of carbon: diamond.

“Given the new estimate of the pressure at the mantle-core interface, and knowing that Mercury is a carbon-rich planet, we calculated that the carbon mineral that would form at the mantle-core interface is diamond, not graphite,” team member Olivier Namur, an associate professor at KU Leuven, told Space.com. “Our study uses geophysical data collected by NASA’s MESSENGER spacecraft.”

MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) was launched in August 2004 and became the first spacecraft to orbit Mercury. The mission, which concluded in 2015, mapped the entire small world, revealed large amounts of water ice in the polar shadows, and collected important data on Mercury’s geology and magnetic field.

The new study also addresses a major surprise that emerged a few years ago, when scientists reestimated Mercury’s mass distribution and discovered that the small planet’s mantle was thicker than previously thought.

“We directly thought that this must have a very large impact on speciation [розподіл елемента чи алотропу серед хімічних видів у системі] “The differences between carbon, diamond and graphite on Mercury,” Namur said.

The team investigated this on Earth by using a high-volume press to recreate the pressures and temperatures found inside Mercury. They subjected synthetic silica, a substitute for the material found in Mercury’s mantle, to an incredible amount of pressure, more than seven gigapascals, reaching temperatures of 3,950 degrees Fahrenheit (2,177 degrees Celsius).

This allowed them to study how minerals similar to those found in Mercury’s mantle early in its existence would have changed under these conditions. They also used computer simulations to evaluate data about Mercury’s interior, which gave them clues about how Mercury’s diamond mantle may have formed.

“We believe that diamonds could have formed through two processes. The first is the crystallization of the magma ocean, but this process probably contributed to the formation of only a very thin layer of diamond at the core/mantle interface,” Namur explained. “The second, and most important, is the crystallization of Mercury’s metallic core.”

When Mercury formed about 4.5 billion years ago, the planet’s core was entirely liquid, gradually crystallizing over time, Namur said. The exact nature of the solid phases that formed in the inner core is currently unknown, but the team believes these phases were low in carbon, or “carbon-poor.”

“The liquid core contained some carbon before crystallization, so crystallization leads to carbon enrichment of the remaining melt,” he continued. “At some point, a solubility threshold is reached, meaning the liquid can no longer dissolve carbon and a diamond is formed.”

Diamond is a dense mineral, but not as dense as a metal, which means it could float to the top of the core during this process and come to rest at the boundary between Mercury’s core and mantle. This would result in a layer of diamonds about 0.62 miles (1 km) thick, which would continue to grow over time.

The discovery reveals 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 likely formed much closer to the Sun, from a carbon-rich dust cloud. As a result, Mercury contains less oxygen and more carbon than the other planets, which leads to the formation of the diamond layer,” Namur added. “But the Earth’s core also contains carbon, and several researchers have already suggested that diamonds form in the Earth’s core.”

The researcher hopes the discovery could help solve some of the other mysteries surrounding the smallest planet in the solar system, including why its volcanic phase ended about 3.5 billion years ago.

“The real question about Mercury’s evolution is why the main phase of volcanism 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 planet’s small size, but we are now working with physicists to try to understand whether the diamond layer could have contributed to the very rapid removal of heat and thus stopped large-scale volcanism very early.”

The team’s next step, Namur said, will be to investigate the thermal effects of the diamond layer at the mantle-core interface, work that could be confirmed with data from the mission that will follow MESSENGER’s lead.

“We also look forward to the first data collected by BepiColombo in 2026 to improve our understanding of Mercury’s interior structure and evolution,” Namur said.

The team’s research was published in the journal Nature Communications.