Researchers: Mercury could have underground layer of diamonds

New research shows that beneath the surface of Mercury, the smallest planet in the solar system and the closest to the sun, there may be a layer of diamonds up to 18 kilometers thick.

The diamonds may have formed shortly after Mercury merged into a planet about 4.5 billion years ago. of a swirling cloud of dust and gas, in the crucible of a high-pressure, high-temperature environment. At that time, the young planet would have had a graphite crust, floating above a deep ocean of magma.

A team of researchers recreated that fiery environment in an experiment using a machine called an anvil press, normally used to study how materials behave under extreme pressure, but also for the production of synthetic diamonds.

“It’s a huge press, which allows us to subject tiny samples to the same high pressures and high temperatures that we would expect deep in Mercury’s mantle, at the boundary between the mantle and the core,” said Bernard Charlier, head of the geology department at the University of Liège in Belgium and co-author of a study reporting the findings.

The team placed a synthetic mix of elements — including silicon, titanium, magnesium and aluminum — into a graphite capsule, mimicking the theoretical composition of Mercury’s early interior. The researchers then exposed the capsule to pressures nearly 70,000 times greater than Earth’s surface and temperatures up to 2,000 degrees Celsius (3,630 degrees Fahrenheit), mimicking the conditions likely found near Mercury’s core billions of years ago.

After the sample melted, the scientists looked at changes in the chemistry and minerals under an electron microscope. They saw that the graphite had turned into diamond crystals.

According to the researchers, this mechanism could not only give us more insight into the secrets hidden beneath Mercury’s surface, but also into planetary evolution and the internal structure of exoplanets with similar characteristics.

Mysterious Mercury

Mercury is the second-densest planet after Earth. A large metallic core takes up 85 percent of Mercury’s radius, and it is also the least explored of the terrestrial planets in the solar system. The last completed mission to Mercury, NASA’s MESSENGER, orbited the planet between March 2011 and April 2015. Also known as the Mercury Surface, Space Environment, Geochemistry and Ranging mission, it collected data on the planet’s geology, chemistry and magnetic field before the spacecraft ran out of fuel and crashed into the surface.

“We know that there is a lot of carbon in the form of graphite on the surface of Mercury, but there are few studies of the interior of the planet,” said Yanhao Lin, a scientist at the Center for High Pressure Science and Technology Advanced Research in Beijing and co-author of the study, which appeared in the journal Nature Communications in June.

“Compared to the Moon or Mars, we know very little about Mercury, also because we don’t have samples from the planet’s surface,” Charlier said. Mercury is different from all other terrestrial planets, he added, because it is so close to the sun and therefore has a very low amount of oxygen, which affects its chemistry.

One of the findings of MESSENGER was the fact that Mercury is rich in carbon and that its surface is gray due to the widespread presence of graphite, a form of carbon. Diamonds are also made of pure carbon, formed under specific pressure and temperature conditions. The researchers wanted to see if this process could have occurred during the formation of the planet.

As Lin, Charlier and their colleagues prepared the experiment to recreate the interior of Mercury shortly after the planet formed, a key element was knowing that sulfur is also present on Mercury, as evidence from previous studies had shown. “We found that the conditions are different from Earth because there is a lot of sulfur on Mercury, which lowers the melting point of our sample,” Charlier said.

“It melted completely at a lower temperature compared to a system without sulfur, which is good for the stability of diamond, because diamond likes high pressure but lower temperatures. And this is mainly what our experiments tell us — Mercury’s magma ocean is cooler than expected, and also deeper, as we know from reinterpretation of geophysical measurements,” he added, referring to data also from MESSENGER.

According to the research, it is these two factors that enable the formation of diamonds.

Diamonds on the surface?

Charlier cautions that the thickness of the diamond layer, between 15 and 18 kilometers (9.3 and 11.1 miles), is only an estimate and could change as the process of forming the diamonds continues as Mercury’s core continues to cool.

It is also impossible to say how big the individual diamonds are. “We have no idea about their size, but a diamond is only made of carbon, so they should be similar to what we know on Earth in terms of composition. They would look like pure diamonds,” he said.

Could the diamonds ever be mined? Charlier said that would be impossible even with future, more advanced technologies, since they are at a depth of about 500 kilometers (310 miles). “However, some lavas on the surface of Mercury are formed by melting of the very deep mantle. It is reasonable to assume that this process is capable of bringing some diamonds to the surface, in analogy to what happens on Earth,” he said.

This diamond-forming process could happen on some of the exoplanets we discover in our galaxy, Charlier explained, if their chemistry is also low in oxygen, like Mercury. “If an exoplanet is smaller than Mercury, the core-mantle boundary would be too shallow and the pressure would be too low, preventing the formation of diamonds,” he said. “But a size between Mercury and Earth, combined with low oxygen, are favorable conditions for getting diamonds.”

Scientists may soon know more. A mission called BepiColombo — consisting of two spacecraft that launched in October 2018 — is expected to enter Mercury’s orbit in December 2025 after a series of flybys. The mission, led by the European Space Agency and Japan Aerospace Exploration Agency, will study the planet from orbit, revealing much more about its interior and characteristics.

The collaboration is named after Italian scientist Giuseppe “Bepi” Colombo, who invented the “gravity assist” maneuver routinely used to send probes to other planets.

“BepiColombo may be able to identify and quantify the carbon on the surface, but also whether there is diamond or more graphite on the surface,” Charlier said. “This was not possible with MESSENGER, and the measurements will also be more precise, giving us better estimates of the depth of the core-mantle boundary. We will be able to test our hypothesis again.”

An important step forward

Sean Solomon, the principal investigator for NASA’s MESSENGER mission to Mercury and an adjunct senior research scientist at Columbia University in New York City, said it presents “an interesting idea” but that it will be a challenge for future missions to Mercury to confirm. “Such a diamond layer is deep and relatively thin,” he said in an email. Solomon was not involved in the study.

“The most promising technique is probably seismology, because the velocities of seismic waves in diamond are much higher than those in mantle rocks or core material, but seismic measurements would require one or more long-lived landers on the surface of Mercury,” Solomon said. BepiColombo, the only mission currently planned to reach Mercury, originally had a lander, but that was scrapped due to budget constraints.

Felipe González, a theoretical physicist in the Department of Earth and Planetary Sciences at the University of California, Berkeley, who was also not involved in the work, said the study is an important step forward in our understanding of planetary interiors and how they form and evolve. He also believes that interdisciplinary studies like this are key to tackling the complex problems we face in science today.

The proposed mechanism by which this diamond layer forms is plausible, González added, but it still largely depends on our assumptions about Mercury’s interior. “Although very good constraints have been imposed over the years as we study this planet more deeply, we can only approximate its composition in our models and experiments based on indirect measurements,” he said via email.

“However, this study is still the best we can do with what we have now,” González said. “Only future missions to the planet Mercury will show whether these predictions were correct. For now, we can focus on improving our understanding of materials under these extreme conditions by performing more and better simulations and experiments in our laboratories.”