What Is Mercury Made Of? The Planet’s Core, Crust, and Composition

Mercury at a Glance: A Planet of Extremes

Mercury is the smallest terrestrial planet in the solar system, with a diameter of just 4,880 kilometers—about 38% the size of Earth. It has no substantial atmosphere, no moons, and no plate tectonics. Yet this unassuming planet contains a massive secret: over 80% of its volume is core. That statistic alone sets Mercury apart. The planet’s unusually high density, second only to Earth’s, makes it a standout in planetary science. Understanding what Mercury is made of means understanding not just what lies on its surface, but what’s hidden deep beneath.

The Structure of a Planet: Crust, Mantle, and Core

Like Earth and other rocky planets, Mercury is made up of three main internal layers: the crust, mantle, and core. Each layer serves a distinct role in the planet’s composition and evolution. While scientists once thought Mercury might be a simple, inert ball of rock, modern observations—especially from NASA’s MESSENGER mission—have revealed a surprisingly dynamic interior structure. To understand Mercury’s composition, we’ll start from the outside and work our way in.

Mercury’s Crust: Ancient and Impact-Scarred

Mercury’s crust is the planet’s outermost layer—a rugged shell of ancient rock that has been battered by billions of years of meteor impacts. It is between 35 and 70 kilometers thick, thinner than Earth’s crust, yet still a significant structural layer. The crust is primarily composed of silicate rocks rich in minerals such as plagioclase, pyroxene, and olivine. These minerals are also found on the Moon and Earth, hinting at shared building blocks in the inner solar system. However, what makes Mercury’s crust particularly unique is its low iron content. While Mercury’s massive iron core might suggest a crust loaded with iron-rich minerals, this is not the case. Instead, Mercury’s surface is chemically distinct from other rocky planets, and it exhibits unusually high sulfur levels, more than any other terrestrial body.

This sulfur-rich composition challenges traditional models of planetary formation. It implies that Mercury may have formed under much more reducing conditions than Earth—meaning with far less available oxygen during its early development. Such chemistry allows sulfur to combine with metal elements in ways not usually seen on Earth, resulting in exotic surface compounds like magnesium sulfide. Evidence from spectroscopy and surface reflectance studies also indicates that the crust has a dark, low-reflectivity coating, possibly caused by carbon-rich material delivered by comets or embedded in the crust itself from Mercury’s early days. Despite its small size, Mercury’s crust holds a surprisingly diverse geologic history, including lava plains, long escarpments called lobate scarps, and signs of ancient volcanic activity.

The Mantle: Thin, Depleted, and Mysterious

Beneath the crust lies Mercury’s mantle, a rocky layer that would, on other planets, serve as the engine of tectonics, volcanism, and convection. However, Mercury’s mantle is very thin—perhaps only 300 to 400 kilometers thick—and geologically inactive today. The composition of the mantle is believed to be ultramafic, meaning it contains high levels of magnesium and iron silicates. But unlike Earth’s mantle, which is dynamic and convective, Mercury’s appears to have cooled rapidly, forming a rigid shell around its core. This swift cooling likely shut down any large-scale volcanic or tectonic activity early in the planet’s history, contributing to its desolate appearance.

However, that doesn’t mean the mantle was always quiet. Ancient volcanic plains—some of which cover vast areas of Mercury’s surface—suggest that during its early formation, the mantle did experience partial melting and volcanic outflows. These events likely occurred during the planet’s first billion years and have not recurred in the recent geologic past. The lack of tectonic plates and large volcanoes means the mantle no longer shapes the planet’s surface actively, but its presence remains a crucial part of the planet’s internal composition. It serves as a buffer between the surface and the core, and although thin, it played a major role in Mercury’s thermal history.

The Core: Mercury’s Massive Heart of Metal

Mercury’s core is its defining feature. Accounting for nearly 85% of the planet’s radius, Mercury’s core is massive, dense, and still partially molten. It’s believed to be composed mostly of iron, with some nickel and sulfur, and possibly silicon—a blend of materials that makes it even more intriguing to planetary scientists. One of the biggest surprises about Mercury is that its core isn’t completely solid. In fact, data from the MESSENGER spacecraft revealed that the core is partly liquid, much like Earth’s. This means that, despite its small size and quick cooling, Mercury has retained enough internal heat to maintain a liquid outer core. This is a key reason Mercury still has a global magnetic field, although much weaker than Earth’s.

Mercury’s magnetic field is offset and asymmetrical, with the north magnetic pole positioned higher than the south. Scientists believe this is a result of the planet’s thin mantle and the geometry of its core, which may be layered with a solid inner region and a fluid outer layer. It’s a structure somewhat similar to Earth’s core but compressed into a much smaller planetary body. The size and structure of the core have raised fascinating questions about Mercury’s origin. Why does such a small planet have so much iron? One theory suggests that Mercury may have once been larger and was stripped of much of its mantle by a massive impact early in the solar system’s history. Another theory is that Mercury formed in a part of the solar nebula that was rich in metallic materials. Either way, the core’s dominance is one of the most remarkable facts about Mercury’s makeup.

Gravity and Density: Confirming What Lies Beneath

How do we know so much about Mercury’s interior when no spacecraft has landed on its surface? Much of what we understand comes from measuring Mercury’s mass, radius, and gravitational field. Mercury has a density of about 5.43 grams per cubic centimeter, nearly as high as Earth’s. Since it’s significantly smaller than Earth, this high density implies a much higher metal-to-rock ratio. Scientists used observations from spacecraft flybys and orbital missions—most notably Mariner 10 and MESSENGER—to analyze Mercury’s gravity field, track orbital motion, and detect how the planet responds to solar tides.

These measurements allow scientists to model Mercury’s internal structure with surprising precision. Variations in Mercury’s gravitational pull across its surface reveal subtle wobbles that hint at a molten outer core. Combined with surface imaging and magnetic field data, these findings paint a picture of a planet with a dense, active interior and a surprisingly dynamic past.

Mercury’s Surface Composition: More Than Just Dust and Stone

While the planet’s interior gets much attention, the surface composition also tells a crucial part of the story. Mercury’s regolith—the layer of broken rock and dust covering the crust—is rich in magnesium, calcium, and potassium, but low in iron oxide. This makes Mercury’s surface appear dull and dark in color, despite its close proximity to the bright Sun.

One of the more intriguing surface features is the presence of hollows—shallow, irregular depressions found on crater floors. These hollows are unique to Mercury and are thought to form when volatile materials near the surface sublimate in the intense solar heat, causing collapse and erosion. Their existence supports the idea that Mercury’s surface still contains unexpected chemical ingredients, including volatile elements like sulfur and chlorine, once believed to be rare on such a sunbaked world.

MESSENGER’s spectrometers detected unexpectedly high levels of these volatiles, challenging previous assumptions that proximity to the Sun would strip a planet of such materials. Instead, Mercury’s composition suggests it either formed in a chemically unique region of the solar system, or it accreted materials from across a wider range of solar environments.

What Mercury’s Composition Tells Us About Planetary Formation

Mercury’s composition—particularly its oversized metallic core and sulfur-rich crust—challenges many of the traditional models of how terrestrial planets form. Most early theories assumed that planets closer to the Sun would be poor in volatiles and rich in refractory materials like iron and rock. While that’s partially true for Mercury, the presence of sulfur, carbon, and other low-temperature elements complicates the picture.

These features suggest that Mercury either formed in a cooler, more volatile-rich region of the solar system and migrated inward, or it formed where it is now under unusual chemical conditions that favored iron and sulfur accumulation. Alternatively, it’s possible that Mercury was once much larger, with a silicate mantle that was blasted away by a colossal impact, leaving behind the metal-rich remnant we see today.

Each theory offers different insights into the early dynamics of the solar system. Mercury, despite its small size, may hold the key to understanding how planets differentiate and evolve under extreme conditions. Its composition is a kind of cosmic puzzle piece, hinting at forces and events that shaped all the rocky planets—including Earth.

A Planet Unlike Any Other

So, what is Mercury made of? At its heart lies a giant metallic core, layered with a thin silicate mantle and capped by a sulfur-rich crust. Its surface bears the marks of volcanic activity, asteroid bombardment, and solar radiation. Its interior hums with magnetic energy, even as the mantle lies still and the crust remains frozen in time. It is a planet that defies expectations—a metal-heavy world formed in the furnace of the early solar system, shaped by heat, impacts, and gravitational tides.

Despite its proximity to the Sun, Mercury’s unique composition continues to surprise and inform. As new missions like BepiColombo head toward this enigmatic world, we expect to uncover even more about the core chemistry and ancient processes that make Mercury so distinct. Understanding Mercury isn’t just about cataloging another planet—it’s about expanding the boundaries of planetary science itself. In the structure of this small, dense world, we find echoes of Earth’s own story, and perhaps the stories of countless other rocky planets orbiting distant stars.