Mercury may have a 17-kilometer underground layer of diamonds, researchers say

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A layer of diamonds up to 18 kilometers (11 miles) thick could be hidden beneath the surface of Mercury, the smallest planet in the solar system and the closest to the Sun, according to new research.

Diamonds may have formed shortly after Mercury fused into a planet, about 4.5 billion years ago, from a swirling cloud of dust and gas in the crucible of a high-pressure, high-temperature environment. . Right now, the fledgling planet is believed have a graphite crust floating on a deep ocean of magma.

A team of researchers recreated this scorching environment in an experiment, with a machine called an anvil press, typically 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 small samples to the same high pressure and high temperature that we would expect deep in Mercury’s mantle, at the boundary between the mantle and core,” said Bernard Charlier, head of the geology department at the University of Mercury. Liège, Belgium, and co-author of a study reporting the findings.

The team inserted a synthetic mix of elements — including silicon, titanium, magnesium and aluminum — inside a graphite capsule, mimicking the theorized composition of Mercury’s interior in its early days. The researchers then subjected the capsule to pressures nearly 70,000 times higher than those found on Earth’s surface and temperatures up to 2,000 degrees Celsius (3,630 degrees Fahrenheit), replicating conditions likely found near Mercury’s core billions of years ago.

After the sample melted, scientists observed the changes in chemistry and minerals under an electron microscope and noticed that the graphite had transformed into diamond crystals.

This mechanism, researchers say, could not only give us more information about the secrets hidden beneath Mercury’s surface, but also about 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 occupies 85% of Mercury’s radius and is also the least explored of the solar system’s terrestrial planets. 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 gathered data on the planet’s geology, chemistry and magnetic field. , before release. the spacecraft ran out of fuel and impacted the surface.

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

“Compared to the Moon or Mars, we know very little about Mercury, also because we have no samples from the planet’s surface,” said Charlier. Mercury is different from all the other terrestrial planets, he added, because it is very close to the Sun and therefore has a very low amount of oxygen, which affects its chemistry.

One of MESSENGER’s discoveries was the fact that Mercury is rich in carbon and its surface is gray due to the widespread presence of graphite, which is 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 planet’s formation.

When Lin, Charlier and their colleagues were preparing the experiment to mimic the interior of Mercury shortly after the planet’s formation, a crucial element was the knowledge that sulfur is also present on Mercury, as evidenced by Previous studies It had shown. “We found that the conditions are different from Earth because there is a lot of sulfur on Mercury, which lowered the melting point of our sample,” Charlier said.

“It fully melted at a lower temperature compared to a sulfur-free system, which is good for diamond stability because diamond likes high pressure but lower temperature. And this is mainly what our experiments tell us – Mercury’s magma ocean is colder than expected, and also deeper, as we know from reinterpretation of geophysical measurements”, he added, referring to data also from MESSENGER .

These two factors, according to the study, are what enable the formation of diamonds.

Diamonds on the surface?

Charlier warns 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 because the diamond formation process is still ongoing as the core of Mercury continues to cool. .

It is also impossible to tell the size of individual diamonds. “We have no idea how big they are, but a diamond is made only 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 diamonds be mined? According to Charlier, this would be impossible even with future, more advanced technologies, because they are at a depth of about 500 kilometers (310 miles). “However, some lavas on Mercury’s surface were formed by melting of the very deep mantle. It is reasonable to consider that this process is capable of bringing some diamonds to the surface, by analogy with what happens on Earth,” he said.

This diamond-forming process could be happening on some of the exoplanets we’re discovering in our galaxy, Charlier explained, if their chemistry is also oxygen-poor, like Mercury. “If an exoplanet were smaller than Mercury, the core-mantle boundary would be very shallow and the pressure would be very low, preventing diamonds from forming,” he said. “But a size between Mercury and Earth, combined with low oxygen content, are favorable conditions for obtaining diamonds.”

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

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

“BepiColombo can identify and quantify the carbon on the surface, but also whether there is diamond on the surface or more graphite,” said Charlier. “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, 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 be able to confirm it. “Any 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 seismic wave speeds in diamond are much higher than those in mantle rocks or core material, but seismic measurements would require one or more long-lived probes on the surface of Mercury,” Solomon said. BepiColombo, the only mission currently planned to reach Mercury, originally had a lander, but it was cut due to budget constraints.

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

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

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

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