Discovery of a diamond-rich core-mantle boundary offers new information into Mercury’s evolution

Scientists discovered that Mercury may have a 16 km (10-mile) thick diamond layer at its core-mantle barrier. This revelation, based on NASA’s MESSENGER mission data and high-pressure laboratory experiments, shed new light on the planet’s complicated interior structure and thermal development.

• Yongjiang Xu and Yanhao Lin of Beijing’s Center for High Pressure Science and Technology Advanced Research spearheaded the study, which was conducted in partnership with a team of planetary scientists and geologists.
• The study showed that Mercury, the Solar System’s smallest planet, may contain a diamond-rich layer near its core-mantle border. This discovery called into question earlier beliefs about Mercury’s core structure, revealing that extreme conditions within the planet might convert carbon into diamond, influencing its thermal and geological history.

The team was led by Yongjiang Xu and Yanhao Lin of the Center for High Pressure Science and Technology Advanced Research, Beijing in collaboration with a team of planetary scientists and geologists. The team combined data from NASA’s MESSENGER mission, which orbited Mercury from 2011 to 2015, with high-pressure experimental models. The results were published in the journal Nature Communications in August 2024.

The research made a new finding that stated that a 16 km (10 miles) thick diamond-bearing layer could exist near the core-mantle boundary of Mercury, our Solar System’s smallest planet.

This discovery shed new light on Mercury’s internal structure which may be more complex than previously assumed, with high-pressure conditions conducive to the production of diamonds deep within the planet. This finding provided new light on Mercury’s thermal and chemical evolution, and therefore on its unique geophysical characteristics.

The study built upon previous findings that revealed the presence of carbon in the form of graphite on Mercury’s surface. Scientists have recalculated the pressure at Mercury’s mantle-core barrier and determined that, at these extreme conditions, carbon might exist as a diamond rather than graphite. This finding is significant because it showed that mercury might have a diamond-rich mantle generated by carbon-bearing minerals at the interface of its mantle and core.

Mercury is the innermost planet of the solar system and is located around 58 million km (36 million miles) from the Sun. The findings are based on high-pressure simulations conducted in laboratories which were specifically designed to recreate the harsh conditions expected to exist inside Mercury. The MESSENGER mission, which contributed important data for this study, was the first to orbit and characterize Mercury, revealing its distinct geological features.

The investigation behind these findings spanned several years with the most recent results published in June 2024.

This discovery is important because it calls into question previously held beliefs about the planet’s internal structure. Scientists have long theorized about the presence of carbon in Mercury’s mantle, but the concept of a diamond-rich layer expands the understanding of the planet. Extreme pressures and temperatures near the boundary between Mercury’s metallic core and silicate mantle may cause graphite to convert into diamond, forming a thick, diamond-rich layer.

This layer is significant in explaining Mercury’s numerous distinguishing characteristics, such as its prematurely ended volcanic phase and the black patches of graphite found on its surface. The new findings suggested that Mercury’s high carbon content, together with its vicinity to the Sun, resulted in the birth of a diamond layer, which played an important role in its early thermal history.

To simulate the conditions prevalent in Mercury’s core-mantle boundary the researchers carried out high-pressure experiments with a six-anvil cubic press. These investigations involved compressing materials to pressures more than 7 GPa and heating them to temperatures as high as 2 273 K (3 950 °F).

The consequent results revealed that under certain conditions, carbon in Mercury’s mantle might crystallize into diamond. Furthermore, thermodynamic models were utilized to calculate the stability of diamond vs graphite at various depths within Mercury, which corroborated the experimental findings.

The possibility of a diamond-bearing core-mantle border on Mercury opens up new paths for studying the planet’s geological history and the processes that sculpted its interior.

References:

¹ A diamond-bearing core-mantle boundary on Mercury – Xu, Y., Lin, Y., Wu, P. et al. – Nat Commun 15, 5061 (2024) – June 14, 2024 – – OPEN ACCESS

Featured image credit: Nature Communications/Authors