A new study shows that Mercury may have a thick layer of diamonds hundreds of miles below the surface. The findings were published June 14 in the journal The nature of communicationmay help solve the mysteries of the planet’s composition and peculiar magnetic field.
Quicksilver is full of mysteries. For one, it has a magnetic field. Although it is much weaker than the Earthling magnetism is unexpected because the planet is small and appears to be geologically inactive. Mercury also has unusually dark surface spots that NASA’s Messenger mission identified as graphite, a form of carbon.
It was this last feature that sparked curiosity Yanhao Lin, a research fellow at the High-Pressure Science and Technology Advanced Research Center in Beijing and co-author of the study. Mercury’s extremely high carbon content “made me realize that something strange was probably going on inside,” said va declaration.
Despite Mercury’s peculiarities, scientists believe it probably formed like other terrestrial planets: from the cooling of a hot magma ocean. In the case of Mercury, this ocean was probably rich in carbon and silicates. First, metals within it collided to form a central core, while the remaining magma crystallized into the planet’s mid-mantle and outer crust.
For years, scientists thought that the temperature and pressure of the mantle were high enough carbon to form graphite, which, being lighter than the mantle, floated to the surface. But 2019 studies suggested 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 boundary, creating conditions where carbon could crystallize into diamond.
To investigate this possibility, a team of Belgian and Chinese scientists, including Lin, prepared chemical soups that contained iron, silicon dioxide and carbon. Such mixtures have a similar composition to some species meteoritesThey are thought to mimic the magma ocean of newborn Mercury. The researchers also spiked these soups with varying amounts of iron sulfide; they reasoned that the magmatic ocean contained a lot of sulfur because today’s surface of Mercury is also rich in sulfur.
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Using a multi-anvil press, the team subjected the chemical mixtures to a crushing pressure of 7 gigapascals—roughly 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 simulate conditions deep inside Mercury.
In addition, the researchers used computer models to obtain more accurate measurements of pressure and temperature at the interface between Mercury’s core and mantle, in addition to simulating the physical conditions under which graphite or diamond would be stable. According to Lin, such computer models tell us about the basic structures of the planet’s interior.
The experiments showed that minerals such as olivine likely formed in the mantle, which was consistent with previous studies. However, the team also found that adding sulfur to the chemical drink only caused it to solidify at much higher temperatures. Such conditions are more favorable for the formation of diamonds. Indeed, the team’s computer simulations showed that under these revised conditions, diamonds could have crystallized as Mercury’s inner core solidified. Because it was less dense than the core, it floated up to the core-mantle boundary. The calculations also showed that diamonds, if present, form a layer with an average thickness of about 9 miles (15 km).
However, mining these gems is not exactly feasible. In addition to the planet’s extreme temperatures, diamonds are too deep — about 300 miles (485 km) below the surface — to be mined.
But the gems are important for another reason: they may be responsible for Mercury’s magnetic field. Diamonds can help transfer heat between the core and mantle, which would create temperature differences and cause the liquid iron to swirl, creating a magnetic field, Lin explained.
The results could also help explain how carbon is abundant exoplanets evolve. “The processes that led to the formation of the diamond layer on Mercury could have occurred on other planets, potentially leaving similar signatures,” Lin said.
Another clue may come from BepiColombo, a joint mission between the European Space Agency and the Japan Aerospace Exploration Agency. The spacecraft, which was launched in 2018, is expected to begin orbiting Mercury in 2025.