Based on new simulations, a team of geoscientists suspects Mercury may have a 10-mile-thick layer of diamonds buried beneath its graphite grey surface.

While we can't yet go digging to prove it, a new study reveals the likely presence of a layer of diamond nearly ten miles thick at the boundary between Mercury's core and mantle.

When viewed by spacecraft from 2011 to 2015, Mercury appeared exceptionally grey due to the high concentrations of graphite, which is pure carbon, on the planet's surface. Given Mercury's carbon coating, it stands to reason that diamonds may have formed beneath the surface under the right conditions.

Diamond formation was initially ruled out because it was believed that the necessary pressures did not exist close to Mercury's core. However, if the boundary between the core and the mantle were deeper than previously thought, the necessary pressure conditions may have existed after all.

Mercury, the closest planet to our Sun, is the least understood despite being one of our closest neighbors. Unlike Earth and the other rocky planets, Mercury's core makes up a much larger part of its mass. It also has a mysteriously persistent magnetic field that scientists still cannot explain.

According to new simulations of Mercury's early evolution, however, a team of geoscientists have discovered evidence that Mercury may indeed have a solid layer of diamond beneath its crust, sandwiched between the core and the mantle hundreds of miles beneath the surface.

"We know there's a lot of carbon in the form of graphite on the surface of Mercury, but there are very few studies about the inside of the planet," said Yanhao Lin, a staff scientist at the Center for High Pressure Science and Technology Advanced Research in Beijing and co-author of the latest study published in Nature Communications.

The original data

This new study was inspired by a previous team's research in reexamining Mercury's gravity field based on the radio tracking measurements taken by NASA's Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) mission, which allowed scientists to gain a better understanding of Mercury's interior.

The data presented by the earlier team from MIT, NASA's Goddard Space Flight Center, and prominent universities led to theories that Mercury's internal structure consisted of a metallic outer core layer, a liquid core layer, and a solid inner core.

While the composition of the core is still uncertain, it seemed likely that it contained abundant iron, nickel, silicon, and possibly sulfur and carbon.

The MESSENGER data further led scientists to believe that the dark patches observed on Mercury's surface were largely made up of graphite that was likely turned up from the interior. This suggests that enough carbon could have crystallized in Mercury's interior between the core and mantle boundary and floated up to the surface as graphite.

The new theory

The new international team comprises researchers from the Center for High-Pressure Science and Technology Advanced Research, the School of Earth Sciences and Resources at the China University of Geosciences, the Department of Earth and Environmental Sciences at KU Leuven, and the Department of Geology at the University of Liege.

For their study, the researchers relied on thermodynamic modeling to recreate these pressure conditions based on the existence of a deeper core-mantle boundary. These experiments allowed them to simulate what conditions were like for Mercury as it slowly cooled.

The team further found that diamonds would form a layer that could remain stable enough to rise along with graphite toward the mantle. Thus, a theory was reborn.

Simulations suggest that, over time, a layer of diamond could form around 15 to 18 kilometers (roughly 9 to 11 miles) thick. Considering diamond is an exceptional thermal conductor, the presence of this layer would change the way astrogeologists model the interior dynamics of Mercury and may finally shed light on its lasting and mysterious magnetic field.

The way heat rises from a core like Earth's significantly affects the cooling and evolution of rocky planets, and the movement of material in the interior is responsible for the generation of magnetic fields.

Not only is Mercury the only rocky planet other than Earth to have a magnetosphere, but there is evidence that it may be far older than our own. As such, revised models of Mercury's interior could explain how the planet's magnetosphere has persisted for so long.

The experiments

The researchers placed graphite under pressure along with silicon, titanium, magnesium, and aluminum, elements believed to be in Mercury's mantle layer. They subjected the mixture to 70,000 times the pressure of Earth at sea level and 2,000 degrees C (3,630 degrees F) – conditions believed to have been present at the core-mantle boundary layer while Mercury was forming 4.5 billion years ago when it coalesced from clouds of gas and dust.

NASA / Johns Hopkins University

Image of Mercury taken by NASA's MESSENGER mission.

Electron microscopy revealed that the mixture melted, and the graphite had turned into diamond crystal.

By examining data from the MESSENGER mission about the mineral composition and depth of Mercury's crust, mantle, and planetary core in the context of their experiment, the authors estimate a thick layer of diamond hundreds of miles below the surface, a bit too far down for a pickaxe and shovel to find.

"However, some lavas at the surface of Mercury have been formed by melting of the very deep mantle. It is reasonable to consider that this process is able to bring some diamonds to the surface, by analogy with what happens on Earth," said Bernard Charlier, head of the department of geology at the University of Liège in Belgium and a coauthor of a study.

Though mining equipment would have to withstand Mercury's surface temperature swings from -290°F (-180°C) at night to 800°F (430°C) during the day, humanity has already extracted material from two asteroids using automated robots, and the companies TransAstra and Astro Mining Corp. are both predicated on a suite of asteroid mining craft for just such an occasion.