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 …