Mercury is essentially a giant ball of iron wrapped in a thin crust

An Unexpectedly Dense Inner World

Mercury is the smallest of the eight recognized planets in the solar system, only slightly larger than Earth's Moon. Yet its physical heft defies its modest proportions. Mercury is the second-densest major body in the solar system, with an average density of roughly 5.43 grams per cubic centimeter, trailing only Earth. Because Earth is substantially larger, much of its high density results from gravitational self-compression, where the immense weight of the overlying rock crushes the deep interior into denser states. When scientists calculate uncompressed density—the density a planet would have if gravity were not squeezing it—Mercury easily ranks as the densest planet in the solar system.

This exceptional density is direct evidence of a skewed internal composition. In most terrestrial worlds, such as Earth, Venus, and Mars, a silicate rock mantle and crust make up the vast majority of the planet's total volume, surrounding a comparatively modest metallic core. Earth's core accounts for just under 20 percent of its volume and roughly half its radius. In contrast, Mercury's interior is dominated almost entirely by iron. Its metallic core measures around 4,000 kilometers across, spanning roughly 85 percent of the planet's total radius and occupying more than half of its total volume.

Surrounding this colossal metallic engine is a remarkably thin rocky shell. Mercury's silicate mantle and solid outer crust together measure only about 400 to 600 kilometers in thickness. To put this into perspective, if Earth had proportional layers, our solid rocky mantle would be only a few hundred kilometers deep, and we would be walking on a thin crust perched directly above a planetary-scale iron sphere. This striking architecture makes Mercury an outlier among rocky planets and poses fundamental questions about how planets formed and evolved near the young Sun.