A big surprise from a small planet – this discovery could explain many of Mercury's strange properties.
According to a study published in the June 14 issue of Nature Communications, Mercury’s surface may be hiding a 15-kilometer-thick layer of diamonds. While it’s unlikely these diamonds will ever be mined, the discovery could help explain some of the planet’s greatest mysteries. Live Science Reports.
The first such mystery is Mercury's magnetic field, which is weaker than Earth's, but surprisingly such a small planet has one at all – all this in addition to the planet's apparent geological inertia. In addition, NASA's Messenger mission identified dark patches of graphite on the surface, which also requires further investigation.
Yanhao Lin, a scientist at the Beijing High-Pressure Research Center for Advanced Science and Technology and a co-author of the study, was interested in the latter, namely Mercury’s high carbon content, which eventually led to the hypothesis that something unusual happened inside the planet during its evolution. Scientists believe that Mercury formed like other rocky planets — an ocean of hot magma rich in carbon and silicates that slowly cooled. Initially, the planet’s core was formed by the coagulation of minerals in the magma, while the remaining magma crystallized into the mantle and crust.
Previously, researchers thought that the temperature and pressure in Mercury’s mantle were high enough for carbon to turn into graphite. Graphite is lighter than the mantle, so it can rise to the surface, which may explain the dark spots on the planet’s surface spotted by NASA Messenger. However, a 2019 study concluded that Mercury’s mantle may actually be 50 kilometers deeper than previously thought. This greater depth would result in significantly higher pressure and temperature at the core-mantle interface. Such extreme conditions would also allow carbon to crystallize into diamonds, not just graphite.
To investigate this possibility, a team of Belgian and Chinese researchers, including Lin, produced different chemical mixtures containing iron, silicon dioxide, and carbon. These mixtures are similar in composition to certain types of meteorites and are thought to resemble the magma ocean of young Mercury. The researchers also spiked these mixtures with varying amounts of iron sulfide, assuming that the magma ocean contained a lot of sulfur, since Mercury’s current surface is also rich in sulfur.
The experiments showed that minerals like olivine likely formed in the mantle, which is consistent with the results of previous studies. However, the research team also discovered that when sulfur was added to the chemical mixture, it only solidified at much higher temperatures. These conditions are more favorable for the formation of diamonds. According to the team’s computer simulations, under these modified conditions, diamonds could have crystallized as Mercury’s inner core solidified. Since diamonds are less dense than the core, they could have floated to the core-mantle boundary. The calculations also showed that if diamonds were indeed present, they could have formed a layer about 15 kilometers thick.
However, diamond mining is not really possible. In addition to the planet’s extreme temperatures, diamonds are found too deep—about 300 miles (485 kilometers) below the surface—to be mined. However, diamonds are important in another way: They could contribute to Mercury’s magnetic field. Diamonds could help transfer heat between the core and mantle, creating temperature differences and swirling liquid iron, processes that generate a magnetic field.
More information about the planet will be provided by the BepiColombo mission, a joint mission between ESA and the Japan Aerospace Exploration Agency, which launched in 2018 and is expected to begin orbiting Mercury in 2025.
(Image: Mercury's surface in false color, Credits: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)
