9 miles of solid diamonds may lurk beneath Mercury's surface, new study finds

Mercury may have a thick layer of diamonds hundreds of miles below its surface, a new study shows. The findings, published June 14 in the journal Nature Communications, may help solve mysteries about the planet's composition and peculiar magnetic field.

Mercury is filled with mysteries. For one, it has a magnetic field. Although it's much weaker than Earth's, the magnetism is unexpected because the planet is tiny and appears to be geologically inactive. Mercury also has unusually dark surface patches that NASA's Messenger mission identified as graphite, a form of carbon. 

That latter feature is what sparked the curiosity of Yanhao Lin, a staff scientist at the Center for High Pressure Science and Technology Advanced Research in Beijing and co-author of the study. Mercury's extremely high carbon content "made me realize that something special probably happened within its interior," he said in a statement.

Despite Mercury’s oddities, scientists suspect it probably formed the way other terrestrial planets did: from the cooling of a hot magma ocean. In Mercury's case, this ocean was likely rich in carbon and silicate. First, metals coagulated within it, forming a central core, while the remaining magma crystallized into the planet's middle mantle and outer crust. 

For years, researchers thought the mantle's temperature and pressure were just high enough for carbon to form graphite, which, being lighter than the mantle, floated to the surface. But a 2019 study suggested that Mercury's mantle may be 80 miles (50 kilometers) deeper than previously thought. That would considerably ramp up the pressure and temperature at the boundary between the core and the mantle, creating conditions where the carbon could crystallize into diamond. 

To investigate this possibility, a team of Belgian and Chinese researchers, including Lin,  whipped up chemical soups that included iron, silica and carbon. Such mixtures, similar in composition to certain kinds of meteorites, are thought to mimic the infant Mercury's magma ocean. The researchers also swamped these soups with varying amounts of iron sulfide; they figured the magma ocean contained loads of sulfur, as Mercury's present-day surface is also sulfur-rich. 

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