New simulations suggest that a 9-mile-thick layer of diamonds may be hidden deep beneath Mercury’s surface. These gems almost certainly won’t be mined for bling, but they could help solve some of the planet’s biggest mysteries. A new study suggests that a thick layer of diamonds may lie hundreds of miles beneath Mercury’s surface. The findings, published June 14 in the journal Nature Communications , could help solve mysteries about the planet’s composition and unique magnetic field.

Mercury is full of mysteries. First, it has a magnetic field. Although much weaker than Earth’s, the magnetism is unexpected because the planet is small and appears to be geologically inactive. Mercury also has unusually dark spots on its surface that NASA’s Messenger mission identified as graphite, a form of carbon.

That last feature piqued the curiosity of Yanhao Lin, a scientist at the Advanced Research Center for High Pressure Science and Technology in Beijing and co-author of the study. Mercury’s extremely high carbon content “probably made me realize that something special was happening inside,” he said.

Despite Mercury’s oddities, scientists suspect that it likely formed, like the other terrestrial planets, from a hot magma ocean that cooled. In Mercury’s case, that ocean was likely rich in carbon and silicates. Initially, metals coagulated into a central core, with the rest of the magma crystallizing in the planet’s middle mantle and outer crust.

For years, researchers believed that the temperature and pressure of the mantle were high enough for carbon to form graphite, which is lighter than the mantle and floats toward the surface. But a 2019 study revealed that Mercury’s mantle may be 80 miles (50 kilometers) deeper than previously thought. This would greatly increase the pressure and temperature at the core-mantle interface, creating the conditions where carbon can crystallize into diamond.

To investigate this possibility, a team of Belgian and Chinese researchers, including Lin, prepared chemical soups containing iron, silicon, and carbon. These mixtures, which resemble certain types of meteorites, are believed to mimic the magma ocean of newborn Mercury. The researchers also added varying amounts of iron sulfide to the soups; since Mercury’s modern surface is also rich in sulfur, they suggested that the magma ocean contained a lot of sulfur.

Using a multi-anvil press, the team subjected the chemical mixtures to pressures of up to 7 gigapascals (about 70,000 times the pressure of Earth’s atmosphere at sea level) and temperatures of up to 3,578 degrees Fahrenheit (1,970 degrees Celsius). These extreme conditions mimic conditions deep inside Mercury.

In addition, the researchers used computer models to model the physical conditions under which graphite or diamond would be stable, as well as to obtain more accurate measurements of pressure and temperature at the boundary between Mercury’s core and mantle. Such computer models can tell us about the fundamental structures of the planet’s interior, Lin said.

The experiments showed that minerals like olivine likely formed in the mantle, a finding consistent with previous research. But the team also found that adding sulfur to the chemical drink only caused it to solidify at much higher temperatures. These conditions are more conducive to diamond formation. In fact, the team’s computer simulations showed that under these revised conditions, diamonds could also crystallize as Mercury’s inner core solidifies. Because it is less dense than the core, it would rise to the core-mantle interface. The calculations also showed that diamonds, if present, would form a layer with an average thickness of about 9 miles (15 km).

But these gemstones are completely unminable. In addition to the planet’s extreme temperatures, diamonds lie too deep (about 300 miles (485 km) below the surface) to be mined.

But the gems are also important for another reason: They may be responsible for Mercury’s magnetic field. Lin explained that diamonds could promote heat transfer between the core and mantle, which would create a temperature difference and cause the liquid iron to swirl, creating a magnetic field.

The results could also help explain how carbon-rich exoplanets evolved. “The processes that led to the formation of the diamond layer on Mercury may have also occurred on other planets, potentially leaving similar traces,” Lin said.

BepiColombo, a joint mission between the European Space Agency and the Japan Aerospace Exploration Agency, could provide more clues. The spacecraft, which is set to launch in 2018, is expected to enter orbit around Mercury in 2025.