Mars has three main layers, just like Earth: a thin rocky crust on the outside, a thick mantle in the middle, and a liquid iron core at the center. But thanks to NASA’s InSight lander, which spent four years listening to marsquakes and measuring how seismic waves traveled through the planet, scientists now have a remarkably detailed picture of what’s going on beneath the red surface. Some of the findings were surprising, including a mysterious molten layer that nobody expected.
How Mars Got Its Layers
When Mars formed about 4.5 billion years ago, it assembled from dust and chunks of rock orbiting the young Sun. All that material smashing together generated intense heat, enough to melt the entire planet. Over the first tens of millions of years, heavier elements like iron sank toward the center while lighter rocky material floated upward. This process, called differentiation, sorted Mars into three distinct layers by density: a metallic core, a rocky mantle, and a lightweight crust.
The Crust: Thinner Than Expected
The outermost shell of Mars turned out to be surprisingly thin. InSight’s seismic data revealed that the crust may have two or three sub-layers, with a total thickness of about 12 miles (20 km) if there are two, or 23 miles (37 km) if there are three. Averaged across the whole planet, estimates place the crust somewhere between 30 and 72 km thick.
What’s interesting is that the crust appears to be less dense than the volcanic basalt rocks found on the surface. That suggests the deeper parts of the crust are made of lighter, more silica-rich rock, similar to what geologists call felsic material. The dark basalt lava flows that cover much of Mars are essentially a veneer on top of something chemically different underneath.
The Mantle: Rocky, Iron-Rich, and Still Active
Below the crust, the mantle extends about 969 miles (1,560 km) down toward the core. It makes up the bulk of the planet’s volume. Compared to Earth’s mantle, the Martian mantle is enriched in iron oxide, which reflects the more oxygen-rich conditions present when Mars was forming. The upper portion of the mantle, down to roughly 200 km, forms a rigid layer that acts like a thick lid over the hotter, more pliable rock below.
For a long time, scientists assumed Mars was essentially a dead planet on the inside, slowly cooling with no significant geological activity. That view changed dramatically when researchers found geophysical evidence for an active mantle plume beneath a region called Elysium Planitia. This plume, a broad column of unusually hot rock rising from deep in the mantle, spans roughly 4,000 km in diameter. It explains why Elysium Planitia sits slightly higher than its surroundings, why the region has seen volcanic eruptions within the last few million years (geologically recent), and why InSight detected so many marsquakes in that area. The plume’s characteristics are comparable to the deep mantle plumes on Earth that have built features like the Hawaiian Islands.
This discovery means Mars is not the cold, inert rock it was long assumed to be. Its interior still has enough heat to drive mantle convection and, at least in some places, push hot rock toward the surface.
A Surprise Molten Layer
One of InSight’s most unexpected findings was a layer of molten silicate rock sitting right on top of the core. This layer is about 150 km (93 miles) thick, with a density of roughly 4.05 grams per cubic centimeter, heavier than the solid mantle above it but much lighter than the metallic core below. It’s essentially a shell of liquid rock sandwiched between the two.
This layer matters because it changed how scientists interpret the size of the core itself. Early InSight results suggested the core was larger and less dense than expected, which created a puzzle: the core would have needed an implausibly high percentage of light elements to match the data. The molten silicate layer resolved this problem. Seismic waves that appeared to bounce off the core were actually bouncing off the top of this molten layer, making the core seem bigger than it really is.
The Core: Liquid Iron With a Twist
At the center of Mars sits a liquid core made primarily of iron and nickel. After accounting for the molten silicate layer above it, the revised core radius is about 1,675 km (1,040 miles), smaller and denser than initial estimates suggested. Its density comes in around 6.65 grams per cubic centimeter.
The core is roughly 85 to 91 percent iron and nickel by weight. The remaining 9 to 15 percent consists of lighter elements: sulfur, carbon, oxygen, and hydrogen. About a fifth of the core, by one NASA summary, is made up of these lighter ingredients. Sulfur is the most abundant of the group, which makes sense given Mars’s formation in a region of the solar system where sulfur-rich materials were plentiful.
Unlike Earth, Mars does not appear to have a solid inner core. The entire core remains liquid, though it has cooled significantly over billions of years.
Why Mars Lost Its Magnetic Shield
Mars once had a global magnetic field, generated by churning liquid metal in its core, the same type of dynamo process that protects Earth today. This magnetic field was active from at least 4.3 billion years ago until roughly 3.6 billion years ago. Then it shut down.
The reason comes down to how efficiently the core lost its heat. The core’s thermal conductivity turns out to be quite high, meaning heat flowed out of the core and into the mantle rapidly. This efficient cooling eventually suppressed the vigorous convection currents needed to sustain a magnetic dynamo. Within about 500 to 800 million years of operation, the dynamo ran out of energy and switched off. Researchers modeling this process found that a core thermal conductivity in the range of 16 to 35 watts per meter per kelvin was enough to kill the dynamo within the timeframe preserved in the rock record.
Earth’s dynamo has survived because our planet’s core is larger, hotter, and benefits from the crystallization of a growing solid inner core, which releases energy that keeps convection going. Mars, being smaller, simply cooled too fast. The loss of its magnetic field left the atmosphere exposed to the solar wind, which gradually stripped away much of the air and water that once made the surface more hospitable.
How Scientists Mapped the Interior
Nearly everything known about Mars’s deep interior comes from NASA’s InSight mission, which landed in November 2018 and operated until December 2022. The lander carried an ultra-sensitive seismometer that detected over 1,300 marsquakes during its mission. By tracking how seismic waves from these quakes bent, slowed, or reflected as they passed through different materials inside the planet, scientists could map out the boundaries between layers, much like how a medical ultrasound builds an image of what’s inside the body.
Two particularly strong marsquakes in 2021 proved critical. They generated seismic waves powerful enough to travel all the way through the core and reach the seismometer on the other side of the planet. These core-transiting waves gave scientists their clearest look at the size, density, and composition of the core, leading to the revised, smaller core model published in 2023.