Mars is a rocky planet with three distinct internal layers: a metallic core, a thick silicate mantle, and a relatively thin crust. This layered structure resembles Earth’s in broad strokes, but key differences in size, composition, and activity make Mars a fundamentally different world. Data from NASA’s InSight lander, which operated on the Martian surface from 2018 to 2022, gave scientists their clearest picture yet of what lies beneath the red dust.
The Liquid Iron Core
At the center of Mars sits a liquid iron core that is smaller and denser than scientists initially estimated. By analyzing seismic waves from two marsquakes in 2021 that traveled directly through the core, researchers determined it has a radius of roughly 1,675 kilometers, about half the planet’s total radius. For comparison, Earth’s core takes up a larger proportion of its interior.
The core is not pure iron. About a fifth of it consists of lighter elements, primarily sulfur, oxygen, carbon, and hydrogen. The sulfur content is likely below 7% by weight, with carbon and hydrogen making up some of the remaining light-element budget. These lighter ingredients lower the core’s density, which seismic data places at about 6.65 grams per cubic centimeter. Whether Mars has a small solid inner core within this liquid shell remains an open question, though current data suggest any such inner core would be less than 750 kilometers in radius.
A Molten Layer Above the Core
One of the most surprising findings from InSight is a layer of molten silicate rock sitting directly on top of the core. This layer is roughly 150 kilometers thick with a density of about 4.05 grams per cubic centimeter, placing it between the dense metallic core and the lighter rocky mantle above. Its existence actually explained a puzzle: earlier estimates of the core’s size had been too large, because seismic waves were bouncing off this molten blanket rather than the core itself. The chemical makeup of this layer may be reflected in certain Martian volcanic rocks that erupted from deep sources.
The Silicate Mantle
Above the molten layer, the mantle makes up the bulk of Mars by volume. It is composed of silicate rock, similar in broad chemistry to Earth’s mantle but with some important differences. Martian mantle rock is richer in iron relative to magnesium, which influences the types of minerals that form and the density of the rock overall. This iron-rich composition also feeds into the surface chemistry that gives Mars its color.
Unlike Earth, Mars does not have active plate tectonics churning its mantle today. The mantle appears to convect sluggishly if at all, which means heat escapes slowly and volcanic activity has become extremely rare. Early in the planet’s history, the mantle was far more active, producing the massive volcanic provinces visible on the surface today.
A Lopsided Crust
The Martian crust is one of the planet’s most striking structural features because it is dramatically uneven. The northern lowlands have an average crustal thickness of about 37 kilometers, while the southern highlands average around 63 kilometers. This north-south split, called the crustal dichotomy, is visible even from orbit: the southern hemisphere stands several kilometers higher than the north and is heavily cratered, while the northern plains are smoother and lower.
What caused this lopsidedness is still debated, but a leading explanation involves a giant impact early in Mars’s history that thinned the crust across the entire northern hemisphere. The crust itself is basaltic, made of volcanic rock rich in iron and silicon. Surface dust is chemically similar to this basaltic bedrock but enriched in sulfur, chlorine, and iron compounds. The iron-bearing mineral ferrihydrite has been detected in Martian dust, and its presence suggests the dust formed under cold, wet conditions long ago. This iron-rich dust is what scatters sunlight to produce the planet’s characteristic reddish appearance.
Major Surface Structures
Mars lacks plate tectonics, but it has enormous tectonic and volcanic features that dwarf anything on Earth. Valles Marineris is the most prominent example: a system of canyons stretching up to 2,000 kilometers long, 200 kilometers wide, and 10 kilometers deep. It is not a river-carved canyon like the Grand Canyon. Instead, it formed through a combination of volcanic loading from the nearby Tharsis region, flexing of the crust along the buried dichotomy boundary, horizontal stretching, and gradual subsidence. Sediment filling the troughs over time added further depth, producing partially filled canyons that may reach 12 kilometers deep in places.
The Tharsis volcanic province itself is a massive bulge on one side of the planet, home to Olympus Mons and three other giant shield volcanoes. This region built up over billions of years as magma from the mantle repeatedly erupted through the same spots, something that happens more easily on a planet without moving tectonic plates to shift the crust around.
A Lost Magnetic Field
Mars has no global magnetic field today, but it clearly had one in the past. The evidence is locked in the crust: patches of rock in the southern highlands are intensely magnetized, about ten times stronger than typical magnetized rocks in Earth’s crust, producing fields as strong as 220 nanotesla at satellite altitude. These rocks cooled and solidified billions of years ago while a planetary magnetic field still existed, locking in a permanent magnetic signature like a fossil compass.
The northern lowlands and large impact basins show little to no magnetization, which tells scientists the dynamo (the churning of the liquid core that generates a magnetic field) had already shut down by the time those surfaces formed. Some researchers think Mars may have even experienced a brief period of plate tectonics early on, with the magnetic stripes in the crust preserving a record of that era. The loss of the magnetic field left the atmosphere exposed to the solar wind, which gradually stripped it away over hundreds of millions of years.
Seismic Activity Today
Mars is not geologically dead, but it is very quiet. InSight’s seismometer recorded more than 1,300 seismic events during its four years of operation, from November 2018 to December 2022. Most of these marsquakes were small, far weaker than the earthquakes that regularly shake our planet. The seismic activity appears to come from slow cooling and contraction of the interior rather than from moving tectonic plates. Still, these vibrations proved invaluable. Each marsquake sent waves rippling through the planet’s interior, and by tracking how those waves bent, slowed, and reflected off internal boundaries, scientists mapped the layered structure described above with a level of detail that was impossible before InSight.