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.
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.
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.
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