Mercury, often overshadowed by its flashier planetary neighbours, is turning out to be far more intriguing than once thought. New research inspired by NASA’s MESSENGER mission has uncovered surprising clues about the planet’s deep interior—hints that Mercury’s history may have shaped it into something far richer and more complex than its barren surface suggests.

A Diamond-Rich Underworld: Scientists Propose Mercury Holds a 10-Mile-Thick Layer of Diamonds

In a revelation that sounds like science fiction but is rooted in rigorous planetary science, researchers have proposed that the planet Mercury may contain a vast subterranean layer of diamonds, potentially stretching as deep as 10 miles beneath its crust. This bold new hypothesis is based on a combination of observational data from NASA’s MESSENGER spacecraft and experimental simulations conducted in Earth-based laboratories.

If confirmed, this would not only make Mercury the most diamond-rich body in the solar system but also radically reshape our understanding of the planet’s composition, history, and internal dynamics.

A Planet Born of Carbon

Mercury is a world of extremes. As the closest planet to the Sun, it endures searing surface temperatures of over 800°F (430°C) during the day, while its lack of atmosphere allows temperatures to plummet to –290°F (–180°C) at night. It is also the densest planet in the solar system, with a massive iron core that makes up over 80% of its radius. But what has caught planetary scientists’ attention in recent years is the planet’s unusually high carbon content.

The surface of Mercury, as revealed by NASA’s MESSENGER mission (which orbited the planet from 2011 to 2015), is rich in graphite—a soft, carbon-based mineral more commonly associated with pencil lead than planetary crusts. Graphite’s presence suggests that early Mercury may have once been enshrouded in a primordial “graphite flotation crust,” a surface layer formed when Mercury’s molten outer layers cooled and solidified. In such a scenario, lighter elements like carbon floated to the top, while denser materials sank inward.

This abundance of carbon raised an intriguing question: under Mercury’s immense internal pressures and high temperatures, could that graphite have undergone a transformation into something more exotic—like diamond?

Diamonds from Pressure and Time

Diamonds are, at their core, simply carbon atoms arranged in a highly ordered lattice structure. What sets them apart from other carbon forms like graphite is the pressure and temperature required to produce them. On Earth, diamonds typically form more than 90 miles (150 km) below the surface, where pressures exceed 5 gigapascals and temperatures soar above 2,000°C (3,600°F).

Although Mercury is a much smaller planet than Earth, its enormous iron core generates extreme pressures near the core-mantle boundary—potentially enough to create the conditions necessary for graphite or other carbon-rich material to compress into diamond.

To test this theory, scientists recreated Mercury’s internal conditions in high-pressure laboratory environments using carbon-rich analog materials. By simulating pressures greater than 7 gigapascals and extreme temperatures, they observed the transition of graphite and other carbon forms into diamond. These experiments provided compelling evidence that such a process is not only theoretically possible but likely, given Mercury’s unique characteristics.

According to these studies, a significant portion of Mercury’s carbon may have been driven deep beneath the surface over billions of years. If conditions were just right, a large-scale metamorphosis could have occurred—resulting in a diamond layer as much as 10 miles (16 kilometers) thick.

Implications for Planetary Science

The possibility of a diamond-rich layer hidden beneath Mercury’s crust is far more than just a geological curiosity. It has wide-ranging implications for our understanding of planetary formation and interior evolution.

First, it adds a new dimension to Mercury’s already anomalous profile. Mercury is distinct in many ways: its enormous core, its magnetic field (unusual for a planet its size), and now, potentially, its carbon-to-silicon ratio. If diamonds are indeed present in such large quantities, it suggests Mercury may have formed in a highly carbon-enriched region of the early solar nebula—perhaps even closer to the Sun than its current orbit.

Second, diamonds are exceptionally good conductors of heat. A thick diamond layer could affect the way heat is transferred from Mercury’s core to its surface, with potential implications for volcanic activity, tectonics, and even the longevity of its magnetic field. It may also alter models of how Mercury’s interior has cooled over time.

Third, the study fuels broader questions about other celestial bodies. Could similar carbon-rich transformations occur elsewhere in the solar system? Are there other small worlds hiding massive, unexpected mineral deposits beneath their surfaces? The discovery encourages scientists to re-examine assumptions about the geochemical processes that drive planetary differentiation across a range of conditions.

What’s Next?

While current spacecraft lack the ability to directly detect subsurface diamond layers, there is hope on the horizon. The European Space Agency’s BepiColombo mission, which launched in 2018 and is scheduled to arrive at Mercury in 2025, will provide unprecedented insights into Mercury’s gravity, magnetism, and surface composition. These data could offer indirect clues supporting or challenging the diamond-layer hypothesis.

In the meantime, scientists continue to refine their models and conduct more precise high-pressure experiments to better simulate Mercury’s interior conditions. Seismology, if ever feasible on Mercury through future lander missions, could one day provide direct evidence of compositional layers beneath the crust.

For now, the idea that Mercury—a blistering, battered world scorched by the Sun—might harbor a hidden treasure trove of diamonds is a dazzling reminder of how much we still have to learn about the planets that share our solar neighborhood.

The full study was published in the journal Nature Communications.