What Is Mars Made Of? Core, Crust, and Mantle Explained

A Planet Born from Stardust

About 4.6 billion years ago, in the swirling chaos of a forming solar system, Mars took shape from dust, rock, and gas. Gravitational forces caused material to coalesce into the rocky body we now know as Mars. As with other terrestrial planets—Earth, Venus, and Mercury—Mars underwent a process called planetary differentiation, where heavier materials sank to the center, forming a dense metallic core, while lighter silicates rose to form the mantle and crust. Unlike gas giants like Jupiter, which lack a solid surface, Mars boasts a rigid outer shell and a defined inner structure. These internal layers are not just physical boundaries; they hold clues about the planet’s geological activity, its magnetic field (or lack thereof), and its potential to have once supported life.

Peering Into the Martian Core

At the very heart of Mars lies its core, a dense, metallic sphere buried beneath nearly 2,000 miles of rock. Scientists believe the Martian core extends from the planet’s center out to about 1,100 to 1,500 miles in radius. Based on seismic data collected by NASA’s InSight lander and earlier gravity field studies, the core is likely composed primarily of iron, with significant amounts of sulfur and lighter elements such as oxygen, carbon, and hydrogen. This composition is different from Earth’s core, which is mostly iron and nickel, with fewer light elements.

The presence of these lighter materials suggests the Martian core is less dense than Earth’s. Another important distinction is that Mars’s core appears to be entirely liquid, unlike Earth’s, which has both a solid inner core and a liquid outer core. The core’s state was inferred from how seismic waves travel through the planet during marsquakes—natural tremors that provide planetary scientists with an invaluable window into Mars’s internal composition.

Perhaps most intriguingly, Mars’s core does not generate a global magnetic field today. Unlike Earth’s dynamic geodynamo, which spins a magnetic shield protecting us from solar radiation, Mars’s magnetic field faded billions of years ago. Scientists believe this may have occurred as the core cooled and convection ceased. Without this magnetic shield, Mars’s atmosphere was stripped away over eons by solar winds, turning a once wetter, possibly habitable planet into the arid world we see today.

The Martian Mantle: A Silent Powerhouse

Above the liquid core lies the Martian mantle, a rocky layer that extends roughly 900 to 1,200 miles thick. It is composed mostly of silicate minerals such as olivine and pyroxene—materials also found in Earth’s mantle. However, the Martian mantle is much less dynamic than Earth’s. While our own mantle churns with convection currents that drive plate tectonics, volcanoes, and earthquakes, Mars’s mantle has been largely inactive for hundreds of millions of years.

This dormancy is one of the major reasons Mars lacks tectonic plate movement, a key driver of geological activity on Earth. That said, the mantle was not always so quiet. In Mars’s distant past, it was the engine behind the planet’s once-powerful volcanic eruptions. The massive volcanoes of the Tharsis region, including Olympus Mons—the tallest volcano in the solar system—owe their existence to mantle plumes, where hot material once rose toward the surface in towering columns.

Even without modern plate tectonics, Mars’s mantle still tells a rich story. Variations in rock composition detected by orbiters and rovers reveal a planet that once underwent extensive melting and differentiation. Some of these rocks, especially the ones ejected by impacts and eventually collected as meteorites on Earth, offer insights into Martian mantle chemistry and history. They hint at a time when Mars was geologically active, forming crust and reshaping its surface in violent episodes of volcanic and tectonic upheaval.

Crust of a Cold Desert World

Capping off the structure of Mars is its crust—a relatively thin outer shell averaging 25 miles thick but reaching up to 45 miles in the southern highlands. This rocky exterior is what we see in rover images, satellite photos, and artist impressions. Unlike Earth, whose crust is divided into constantly moving plates, Mars’s crust is one single rigid plate. Its stillness, shaped primarily by ancient forces, is part of what makes Mars’s surface a kind of time capsule. The Martian crust is composed mainly of basaltic rock, formed from the cooling of lava flows. Basalts are rich in iron and magnesium, which give the surface its signature reddish coloration due to oxidized iron, similar to rust. This oxidized iron is what makes Mars appear red to the human eye and has inspired centuries of speculation and myth.

Interestingly, the crust varies significantly between the northern and southern hemispheres—a phenomenon known as hemispheric dichotomy. The northern lowlands are smoother, younger, and lie several miles below the planet’s average elevation. In contrast, the southern highlands are older, heavily cratered, and rise significantly above the mean planetary level. The reason for this dichotomy is still debated: some scientists suggest it was caused by a massive impact early in Mars’s history, while others argue it formed through internal geological processes.

The crust is also home to ancient riverbeds, dried-up lakes, and sedimentary rock layers that hint at a wetter, possibly habitable past. Instruments onboard NASA’s Curiosity and Perseverance rovers have found clays, sulfates, and carbonates—minerals that typically form in the presence of water. These discoveries suggest that Mars’s crust may have once sheltered liquid water beneath the surface, and potentially, primitive life.

Seismic Revelations: What Marsquakes Tell Us

A major breakthrough in understanding Mars’s interior came with NASA’s InSight mission, which landed in Elysium Planitia in 2018. Equipped with a highly sensitive seismometer, InSight detected hundreds of marsquakes, some of which originated deep within the planet. These seismic waves acted like X-rays, helping scientists “see” the structure of Mars from its crust to its core. The data from these marsquakes confirmed that Mars has a low-density, fully liquid core, a mantle with compositional gradients, and a thicker crust than previously estimated. 

The seismic signals also revealed layering within the crust itself—possibly indicating episodes of resurfacing or shifts in volcanic activity over billions of years. This seismic mapping has transformed our view of Mars from a static, inert planet to one with a more nuanced and evolving internal structure. While Mars is not tectonically active like Earth, the fact that it still experiences quakes tells us that internal forces are still at play. These could be due to mantle contraction, cooling, or lingering volcanic activity. Whatever their origin, they serve as critical tools for planetary scientists trying to decode the Red Planet’s past.

The Vanished Magnetic Field: A Core Concern

One of the most important implications of Mars’s interior structure is its connection to the planet’s lost magnetic field. Billions of years ago, Mars did have a magnetic field, as evidenced by magnetized rocks in its southern highlands. But that field disappeared, leaving the atmosphere vulnerable to solar wind. Why did Mars’s magnetic field vanish? The answer likely lies in the dynamics of its core. On Earth, the motion of liquid iron in the outer core generates our magnetic field through a process known as the dynamo effect. Mars’s core, although liquid, may lack the necessary convection or motion to sustain a similar dynamo.

 It could be that the core cooled too quickly, became stratified, or simply lacked the energy gradients needed to maintain a magnetic field over the long term. This failure of Mars’s geodynamo is one of the most important factors that shaped its destiny. Without a magnetic field, solar radiation stripped away much of the planet’s atmosphere. This led to surface water evaporating or freezing and made the planet far less hospitable for life. Understanding the dynamics of Mars’s core could help us better understand planetary habitability—not just here, but across the galaxy.

Comparing Mars to Earth: Familiar Yet Foreign

Mars and Earth share some internal similarities, such as a layered structure with a core, mantle, and crust. But the differences are profound. Mars is only about half the diameter of Earth and has only about 11 percent of Earth’s mass. This smaller size means it lost its internal heat faster, which in turn contributed to the early shutdown of its geologic and magnetic activity. Earth’s crust is dynamic, constantly reshaped by tectonics, erosion, and sedimentation. Mars’s crust, in contrast, preserves features that are billions of years old, including impact craters and ancient river valleys. Its mantle no longer drives large-scale surface changes, and its core sits quiet, no longer powering a protective magnetic field. Yet, despite these differences, Mars offers a valuable comparison. Studying how it evolved differently from Earth helps scientists understand what makes a planet habitable, what keeps it geologically active, and what signs to look for in exoplanets orbiting other stars.

Beneath the Dust: The Quest for Subsurface Life

One of the most tantalizing possibilities raised by studying Mars’s internal layers is the potential for subsurface life. While the surface is harsh—cold, irradiated, and barren—the underground environment may be more stable. Heat from the core, combined with water ice trapped beneath the surface, could create pockets of liquid water in the subsurface. These briny reservoirs might provide the kind of protected environment where microbial life could survive. 

Recent radar data from orbiters have hinted at the presence of subsurface lakes or aquifers near the Martian poles. While this evidence is still debated, it fuels the scientific imagination. Could there be a hidden biosphere beneath Mars’s dusty crust, warmed by the fading heat of its mantle and core? This is one reason why future missions may include drilling systems capable of reaching deep underground. By exploring below the surface, scientists hope to find organic molecules, chemical biosignatures, or even direct evidence of life—past or present.

Looking Ahead: Unlocking the Red Planet’s Inner Secrets

Mars continues to surprise us. As rovers traverse its plains, landers listen to its tremors, and orbiters scan its geology, a clearer picture of the planet’s inner life is emerging. We now know that Mars, though quiet on the surface, still harbors secrets deep within its core, mantle, and crust. Each layer of Mars tells a piece of the story: a core that cooled too quickly to sustain a magnetic field, a mantle that once forged monumental volcanoes, and a crust that captures the echoes of a wetter, more vibrant world. The composition and structure of these layers not only shape the planet’s geology but also its potential for habitability.

Understanding what Mars is made of is not just an academic exercise—it’s a step toward answering some of humanity’s greatest questions. Could Mars have once supported life? Could it still? And what does its history tell us about Earth’s own evolution, and the nature of planets across the cosmos? In the years ahead, with new missions, more advanced instruments, and perhaps even human explorers, we will continue to dig deeper—both literally and figuratively—into the mysterious interior of the Red Planet. Beneath its rusty shell, Mars still has much to reveal.