Mars has a layered interior much like Earth: a rocky crust on the outside, a thick silicate mantle in the middle, and an iron-rich core at the center. But thanks to NASA’s InSight lander, which placed a seismometer on the Martian surface in 2018 and recorded 1,319 “marsquakes” before its mission ended in 2022, scientists now have a surprisingly detailed picture of what lies beneath the red dust. The results reveal a planet that cooled faster than Earth, lost its magnetic shield billions of years ago, and hides some unexpected features deep below the surface.
Three Main Layers
Like Earth, Mars is built in concentric shells. The outermost layer is the crust, which is thinner than scientists originally expected. Beneath that sits the mantle, stretching 969 miles (1,560 km) down from the base of the crust. And at the very center is the core, with a radius of 1,137 miles (1,830 km). That core takes up a larger fraction of the planet’s radius than you might guess, since Mars itself is only about 2,106 miles across at the equator.
A Surprisingly Thin Crust
InSight’s seismic readings showed that the Martian crust has either two or three distinct sub-layers, and the total thickness depends on which interpretation is correct. If the crust has two sub-layers, it extends about 12 miles (20 km) deep. If there are three, it reaches roughly 23 miles (37 km). Extrapolating from InSight’s single landing site to the rest of the planet using gravity and topography data, the average crustal thickness falls somewhere between 24 and 72 kilometers.
That range is wide partly because Mars has a dramatic split personality. The southern hemisphere is heavily cratered and sits at a higher elevation, with a thicker crust. The northern hemisphere is smoother, lower, and thinner-skinned. This “hemispheric dichotomy” is one of the most striking features of Martian geology, and it extends below the surface too. Estimates of the rigid outer shell (the lithosphere, which includes both crust and the stiffest upper mantle) suggest it can be over 300 km thick beneath the north polar cap but only around 110 km in the south. On Earth, shifting tectonic plates keep the lithosphere in constant motion. Mars has no plate tectonics, so its outer shell is a single, rigid piece that has thickened over billions of years as the planet cooled.
A Rocky Mantle With Ancient Lumps
The mantle makes up the bulk of Mars by volume. It is composed primarily of silicate rock, with iron, magnesium, silicon, and oxygen accounting for more than 90% of the planet’s total mass. In broad terms, this is similar to Earth’s mantle, though the Martian version is enriched in certain elements relative to the primitive building blocks of the solar system (a class of meteorites called CI chondrites) while being depleted in more volatile elements that evaporate at lower temperatures.
A 2025 study revealed something unexpected buried in this mantle: lumps. By analyzing eight marsquakes whose high-frequency seismic waves traveled deep into the mantle, researchers noticed the waves were being slowed and scrambled as they passed through small, localized regions. Computer simulations showed these regions are clumps of material with a different composition than the surrounding rock, each as large as 2.5 miles (4 km) across and scattered throughout the mantle. The leading explanation is that these lumps are the remains of massive ancient impacts, rocky debris from collisions so violent that the material was driven deep into the planet’s interior and never fully mixed in.
Heat still moves through the Martian mantle by convection, the same slow churning of hot rock that drives plate tectonics on Earth. Three energy sources power this process: heat flowing up from the core, heat from the decay of radioactive elements within the mantle itself, and residual heat released as the planet gradually cools. As Mars has cooled and lost water from its interior to the surface over billions of years, the mantle has become more viscous and convection has slowed. But it hasn’t stopped entirely. The relatively recent volcanic activity at places like Olympus Mons suggests convection is still moderately vigorous.
An Iron Core With a Solid Center
For years, the Martian core was understood to be a ball of liquid iron alloy. InSight confirmed that the core is at least partially liquid by detecting seismic waves reflecting off the core-mantle boundary and even passing through the core itself. Geodetic measurements (precise tracking of how Mars wobbles as it rotates) ruled out a fully solid core.
But a study published in Nature in 2025 went further: Mars has a solid inner core, much like Earth does. The analysis of InSight’s seismic data constrains this inner core to a radius of about 613 km (plus or minus 67 km), sitting inside the larger liquid outer core. Seismic waves speed up dramatically when they cross into this solid region, with a roughly 30% jump in velocity at the inner core boundary.
The core is made primarily of iron and nickel, alloyed with lighter elements. Sulfur is the most significant of these, estimated at 10 to 20% of the core’s weight. Smaller amounts of oxygen (up to about 3.5%) are also likely dissolved in the molten metal. Silicon, by contrast, is present in only trace amounts. This high sulfur content is one reason the Martian core behaves differently from Earth’s. Sulfur lowers the melting point of iron, which helped keep much of the core liquid even as the planet cooled.
A Lost Magnetic Field
One of the most consequential differences between Mars and Earth is magnetic. Earth’s liquid outer core generates a global magnetic field through a self-sustaining dynamo effect: convection in the molten iron creates electrical currents that produce magnetism. Mars once had the same thing, but it shut down.
Data from the MAVEN orbiter shows the Martian dynamo was active as far back as 4.5 billion years ago and was still running around 3.7 billion years ago. Rocks magnetized during that era still carry the signature of the ancient field, detectable from orbit. But at some point after 3.7 billion years ago, the dynamo stopped. The core’s convection became too sluggish to sustain it, likely because the planet’s smaller size allowed it to cool more quickly than Earth.
Without a global magnetic field, Mars lost its primary shield against the solar wind. Over hundreds of millions of years, charged particles from the Sun stripped away much of the Martian atmosphere. This is a major reason Mars went from a warmer, wetter world to the cold, thin-aired desert it is today. The low present-day heat flow from the core is consistent with a dynamo that can no longer operate, even though the outer core remains liquid.
How Mars Compares to Earth Inside
The broad architecture is the same: crust, mantle, core. But the proportions and activity levels are very different. Mars is about half Earth’s diameter, so everything is scaled down. Its crust is relatively thin, its mantle is cooler and stiffer, and its core takes up a proportionally larger share of the planet’s interior. Earth’s inner core is solid iron and growing as the outer core slowly crystallizes. Mars appears to have the same structure, but its inner core is smaller relative to the total core size, and the high sulfur content in the outer core changes how the whole system behaves thermally.
The biggest functional difference is that Earth’s interior is still highly active: vigorous mantle convection drives plate tectonics, and a powerful dynamo generates the magnetic field that protects our atmosphere. Mars still has some internal heat and convection, but it is a far quieter planet geologically. Its single-plate lithosphere, extinct dynamo, and gradually stiffening mantle all point to a world that is slowly winding down, though not yet geologically dead.