Scientists have unearthed a dazzling secret hidden beneath the surface of Mercury, the solar system's smallest planet. Using data gathered by NASA's MESSENGER spacecraft, they've discovered a massive diamond mantle, stretching 10 miles thick, beneath the planet's crust.
Mercury has long presented a puzzle to scientists, exhibiting characteristics unlike other planets in our solar system. These include its dark surface, dense core, and the premature cessation of its volcanic activity.
One particular intrigue has been the presence of graphite patches on the surface, a form of carbon. This led researchers to theorise that Mercury, in its early history, possessed a carbon-rich magma ocean. As this ocean rose to the surface, it left behind the graphite patches, contributing to Mercury's dark hue.
This same process, the team believes, also led to the formation of a carbon-rich mantle beneath the surface. However, rather than graphene, as previously speculated, they propose this mantle is composed of a much more precious form of carbon: diamond.
"Our calculations, considering the estimated pressure at the mantle-core boundary and Mercury's carbon-rich composition, suggest that the carbon-bearing mineral forming at the interface between mantle and core would be diamond, not graphite," explained Olivier Namur, an associate professor at KU Leuven, in an interview with Space.com. "Our study utilises geophysical data collected by the NASA MESSENGER spacecraft."
MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) launched in August 2004, becoming the first spacecraft to orbit Mercury. The mission, concluding in 2015, comprehensively mapped the tiny world, revealing abundant water ice in the polar shadows and gathering vital information about Mercury's geology and magnetic field.
This new research also sheds light on a surprising discovery made a few years ago, where scientists reassessed the mass distribution on Mercury, revealing a thicker mantle than previously thought.
"We immediately realised the significant implications this had for the speciation of carbon, diamond versus graphite, on Mercury," stated Namur.
To delve deeper, the team recreated the immense pressures and temperatures found within Mercury's interior using a large-volume press on Earth. They subjected a synthetic silicate, representing the mantle material, to pressures exceeding seven gigapascals and temperatures reaching 3,950 degrees Fahrenheit (2,177 degrees Celsius).
This allowed them to observe the transformation of minerals under these extreme conditions. Additionally, computer modelling was used to analyse data about Mercury's interior, offering valuable insights into the creation of its diamond mantle.
"We believe the diamond formation occurred through two processes," elaborated Namur. "Firstly, during the crystallization of the magma ocean, although this process likely contributed to a very thin diamond layer at the core/mantle interface. Secondly, and more significantly, through the crystallization of Mercury's metallic core."
According to Namur, when Mercury formed around 4.5 billion years ago, its core was entirely liquid, gradually solidifying over time. While the exact nature of the solid phases forming in the inner core remains uncertain, the team believes they were low in carbon.
"The liquid core before crystallization contained some carbon; hence, crystallization leads to carbon enrichment in the remaining melt," he continued. "At a certain point, a solubility threshold is reached, where the liquid can no longer dissolve more carbon, resulting in diamond formation."
Diamond, being a dense mineral but less dense than metal, would have floated to the top of the core, settling at the boundary between Mercury's core and mantle. This resulted in the formation of a roughly 0.62-mile (1 km) thick diamond layer, which continued to grow over time.
This discovery highlights the stark differences between the formation of the closest planet to the sun and that of other rocky planets: Venus, Earth, and Mars.
"Mercury formed much closer to the sun, likely from a carbon-rich dust cloud. Consequently, Mercury contains less oxygen and more carbon than other planets, leading to the formation of a diamond layer," Namur added. "However, Earth's core also contains carbon, and diamond formation in Earth's core has already been suggested by various researchers."
Namur hopes this discovery will provide crucial clues to solve other mysteries surrounding the solar system's smallest planet, such as the reason for its shortened volcanic phase, ending approximately 3.5 billion years ago.
"A major question about Mercury's evolution is why the main phase of volcanism lasted only a few hundred million years, significantly shorter than other rocky planets. This implies a rapid cooling of the planet," said Namur. "While partly related to the planet's small size, we're currently working with physicists to understand if a diamond layer might have contributed to very rapid heat dissipation, thereby ending major volcanism early."
The team's next step involves investigating the thermal impact of a diamond layer at the mantle/core boundary. This study could benefit from data collected by a future mission following in MESSENGER's footsteps.
"We eagerly await the first data from BepiColombo, hopefully in 2026, to refine our understanding of Mercury's internal structure and evolution," concluded Namur.
The team's research has been published in the journal Nature Communications.
