'11 miles thick': Scientists say Mercury’s carbon-rich interior could hide a vast underground diamond layer

Beneath the blistering heat and cratered terrain of Mercury, scientists believe a glittering geological marvel could be hiding — a diamond shell as thick as 11 miles encasing its core. In a study published in Nature Communications, planetary materials scientist Dr Yanhao Lin and his team propose that the planet’s unique conditions may have forged an immense layer of diamond deep underground, a transformation made possible by Mercury’s carbon-rich history and intense internal pressure.

Mercury’s surface is scattered with graphite, a clear sign that its crust once floated atop a carbon-rich magma ocean. As the molten surface cooled, lighter carbon materials floated upward while denser carbon sank, accumulating near the core. According to the study, at depths where pressures surpass 5.5 gigapascals and temperatures near 3,600°F, this buried carbon could have converted into diamond at the boundary between the core and mantle.

“Many years ago, I noticed that Mercury’s extremely high carbon content might have significant implications,” Dr Lin said. “It made me realize that something special probably happened within its interior.”

Lin’s team also factored in the role of sulfur, a common element in Mercury’s composition, which lowers the melting point of magma and further enables diamond formation. Under these conditions, diamond becomes stable, dense, and able to sink — forming what could be a shell up to 18 kilometers (11 miles) thick around the metallic core.

This potential diamond layer may also help solve one of Mercury’s long-standing mysteries: its magnetic field. Unlike other small planetary bodies, Mercury still maintains a robust magnetic field. Diamond’s high thermal conductivity, Lin explained, might be the key.

“Diamond’s high thermal conductivity helps transfer heat effectively from the core to the mantle,” Lin said. “That affects convection in the core and helps sustain the magnetic field.”

This mechanism makes Mercury’s interior unlike any other in the solar system, and could shed light on how magnetic fields form and persist on other rocky worlds, including exoplanets.

Mercury’s carbon retention also distinguishes it chemically from its planetary neighbours. While Earth, Venus and Mars have lost much of their surface carbon through geological processes, Mercury seems to have preserved and concentrated its carbon deep below.

“It also could be relevant to the understanding of other terrestrial planets, especially those with similar sizes and compositions,” Lin added.

If similar formation conditions occurred elsewhere, diamond-rich interiors might not be unique to Mercury. The study opens the door to rethinking the internal structures of carbon-rich asteroids and rocky exoplanets orbiting close to their stars.