Mercury has a 10-mile-thick layer of diamonds under its surface

Mercury does not look like a world built for extravagance. It is small, battered, sun-scorched and gray. Yet far below that dark surface, the innermost planet may hold one of the stranger planetary treasures in the solar system: a layer of diamond formed under conditions unlike those on Earth.

That possibility emerges from a new analysis of Mercury’s interior, built on data from NASA’s MESSENGER mission and laboratory experiments designed to recreate the planet’s deep past. The work suggests that carbon inside Mercury may not be sitting only in the form of graphite, the soft mineral long tied to the planet’s unusually dark crust. Some of it may have ended up as diamond at the boundary between Mercury’s mantle and core.

“We calculate that, given the new estimate of the pressure at the mantle-core boundary, and knowing that Mercury is a carbon-rich planet, the carbon-bearing mineral that would form at the interface between mantle and core is diamond and not graphite,” said Olivier Namur, an associate professor at KU Leuven.

The proposed layer is not a scattering of gemstones. It is a deep, buried zone that the researchers estimate could average roughly 14.9 to 18.3 kilometers thick, with substantial uncertainty. That is about 9 to 11 miles.

Scientists have known for years that Mercury’s surface holds clues to an unusual carbon story. Spectral data from MESSENGER showed that the planet’s low reflectivity, its broad darkness, comes from widespread graphite. Neutron and gamma-ray measurements placed carbon in the crust at about 2 to 4 weight percent, although a more recent reanalysis suggested the concentration may be under 1 percent.

Either way, the carbon appears to be native to Mercury itself, not mainly delivered by outside impacts. The close link between graphite and lower crustal material exposed in deep craters points to an internal origin. That matters because it suggests Mercury once had a carbon-saturated magma ocean, and that carbon stayed important through the planet’s earliest differentiation.

For a long time, graphite seemed like the obvious outcome. Under earlier models, Mercury’s mantle and magma ocean were not thought to reach the pressure and temperature conditions needed to stabilize diamond. Graphite, being less dense than molten silicate, would have floated upward and helped form a primordial crust, much as light minerals helped build the Moon’s early crust.

The new work reopens that question because estimates of Mercury’s internal structure have shifted.

Using newer gravity-based models, the team recalculated the depth and pressure at Mercury’s core-mantle boundary. A deeper boundary means higher pressure, and higher pressure changes which form of carbon is favored. The researchers found that Mercury’s core-mantle boundary pressure likely falls around 5.38 to 5.77 gigapascals, with the highest possible estimate reaching 7 gigapascals.

That is enough to make the carbon problem more interesting.

To test the idea, the team used a large-volume press to reproduce the extreme conditions expected deep inside early Mercury. They heated Mercury-like materials to temperatures up to about 3,950 degrees Fahrenheit and examined how those materials melted and crystallized under high pressure.

The experiments focused on mantle compositions resembling the silicate portion of enstatite chondrites, meteorites considered relevant analogs for Mercury’s primordial makeup. They also accounted for sulfur, which appears in significant amounts on Mercury and plays a major role under the planet’s chemically reduced conditions.

That sulfur turned out to matter a great deal.

By lowering the liquidus temperature, the temperature at which the magma ocean would begin to crystallize, sulfur nudged some models into the diamond stability field. In sulfur-free cases, graphite remained favored. But with 7 to 11 weight percent sulfur in the silicate melt, a small fraction of the pressure-temperature models supported diamond instead, especially as the magma ocean cooled.

Even so, the study found that diamond forming directly from Mercury’s magma ocean was probably limited.

“We believe that diamond could have been formed by two processes,” Namur said. “First is the crystallization of the magma ocean, but this process likely contributed to forming only a very thin diamond layer at the core/mantle interface. Secondly, and most importantly, the crystallization of the metal core of Mercury.”

That second mechanism is the heart of the new argument.

When Mercury formed about 4.5 billion years ago, its core was fully molten. As the planet cooled, an inner solid core began to crystallize inside the liquid metal. Because the solid phase is poor in carbon, that process would have concentrated carbon in the remaining liquid outer core.

“The liquid core before crystallization contained some carbon; crystallization, therefore, leads to carbon enrichment in the residual melt,” Namur said.

Once the melt could no longer hold all that carbon, a carbon-rich phase would have to form. Under Mercury’s low-pressure core conditions, the study argues, diamond is more likely than iron carbides to be the stable product. Because diamond is far less dense than the surrounding liquid iron-rich alloy, it would float upward until it reached the core-mantle boundary.

There, over time, it could accumulate into a distinct layer.

The authors estimate that this process could have produced a present-day diamond layer averaging between about 14.9 and 18.3 kilometers thick, depending on which moment-of-inertia model is used. The uncertainty is large, about 10.6 kilometers, and the researchers stress that these numbers are upper-limit style estimates in some respects. Early-formed carbon may have shifted phase, and later convection could have redistributed some material.

Still, the work argues that most of the diamond layer, or its graphite precursor, likely formed after strong lower-mantle convection had already faded, which would limit major disruption.

Mercury’s chemistry sets it apart from Venus, Earth and Mars. Namur said the planet likely formed closer to the Sun from a carbon-rich dust cloud, leaving it poorer in oxygen and richer in carbon than the other rocky planets. That difference shaped how carbon moved through the planet, from magma ocean to crust to metallic core.

Interestingly, the comparison does not stop there.

Namur noted that Earth’s core also contains carbon, and some researchers have suggested diamond formation there as well. But Mercury offers a more favorable natural case because of its strongly reduced composition, silicon-rich core, sulfur-rich silicate portion and evidence that the whole planet was saturated in carbon early on.

The findings also touch on Mercury’s magnetic field. A conductive diamond layer at the core-mantle boundary could change how heat escapes from the liquid outer core. The study suggests that, unlike a thick insulating FeS layer, a diamond-rich boundary could support heat transfer in ways that favor thermal stratification near the top of the core, with possible implications for how Mercury generates its magnetic field.

That does not mean the case is closed.

The researchers note that a diamond layer this thin could not yet be confirmed unambiguously by current interior models. They also point out that if an FeS layer exists at the core-mantle boundary, the diamond would need to be placed relative to that layer depending on whether the sulfide is solid or liquid.

Diamonds have been speculated to exist in various locations within the solar system due to extreme pressure and temperature conditions. Here are some notable examples:

These discoveries highlight the diverse and extreme environments in our solar system and beyond where diamonds could potentially form.

Research findings are available online in the journal Nature Communications.

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