Mars is a rocky planet made primarily of iron, silicate rock, and volcanic basalt, with a thin atmosphere of carbon dioxide. Its average density is 3.9 grams per cubic centimeter, noticeably lower than Earth’s 5.5, which tells scientists that Mars has a smaller, less dense core relative to its size. From the inside out, the planet is built in layers: a metallic core, a thick silicate mantle, a basaltic crust, and a wispy atmosphere that barely qualifies as one.
The Iron-Sulfur Core
At the center of Mars sits a core made of iron, nickel, and a surprisingly large amount of sulfur, roughly 18 to 19 percent by weight. That sulfur content is far higher than Earth’s core and has major consequences: it lowers the core’s melting point and density, which is why the Martian core is mostly liquid despite the planet being smaller and cooler than Earth. The core’s radius falls somewhere between 1,620 and 1,870 kilometers, making it proportionally larger than you might expect for a planet about half Earth’s diameter.
Data from NASA’s InSight lander, which measured seismic waves traveling through the planet from 2018 to 2022, confirmed that the outer core is liquid. More recently, scientists identified seismic waves reflecting off a boundary deep inside the core, revealing a solid inner core roughly 613 kilometers in radius. That inner core contains a distinct mix of light elements like carbon, oxygen, and hydrogen that separated out as the core slowly crystallized over billions of years.
A Mantle Built on Olivine
Surrounding the core is a thick mantle of silicate rock. The dominant mineral is olivine, a green, iron-and-magnesium-rich crystal that also makes up much of Earth’s upper mantle. Deeper down, increasing pressure transforms olivine into denser crystal structures. The mantle also contains pyroxene (another silicate mineral common in volcanic rock) along with garnet varieties that form under high pressure.
Martian meteorites found on Earth, known as SNC meteorites, have given scientists a direct look at this mantle chemistry. Their mineral signatures show that the Martian mantle has a roughly chondritic composition, meaning its mix of heavy elements closely resembles the ancient building blocks of the solar system. This suggests Mars never went through the same degree of internal processing that stripped and redistributed elements inside Earth.
A Basaltic Crust
The outermost solid layer is a basaltic crust, similar in broad chemistry to the dark volcanic rock found on ocean floors on Earth. Orbital and rover measurements have mapped a wide range of minerals across the surface. The most abundant categories are silicates, oxides, and sulfides. Specifically, missions have identified mafic minerals like pyroxene and olivine, iron oxides like hematite and goethite, clay minerals including various smectites and kaolinite, sulfates such as gypsum and kieserite, and opaline silica.
The clay minerals and sulfates are particularly important because they only form in the presence of liquid water. Their widespread detection across the surface is some of the strongest evidence that Mars once had a much wetter environment, with water interacting with rock over long periods.
Why Mars Is Red
The planet’s signature color comes from iron-rich dust that coats nearly everything. For decades, scientists debated exactly which iron compound was responsible. Recent spectroscopic work points to ferrihydrite, a poorly crystalline iron mineral, as the dominant iron-bearing phase in Martian dust. This dust is a fine mixture of ferrihydrite, ground-up basalt, and sulfate particles. The iron concentration in the dust correlates with sulfur and chlorine levels, suggesting these elements were chemically linked during whatever process created the dust.
Scattered across certain regions are also tiny hematite spherules, nicknamed “blueberries” by the rover teams that first spotted them. These millimeter-sized balls formed when iron-rich water percolated through sedimentary rock and minerals precipitated out, further evidence of Mars’s watery past.
The Thin Carbon Dioxide Atmosphere
Mars has an atmosphere, but it exerts less than 1 percent of Earth’s surface pressure. Carbon dioxide makes up 95.9 percent of it by volume. The remaining slivers are argon, nitrogen, oxygen, and carbon monoxide. This atmosphere is too thin to trap much heat or shield the surface from solar radiation, which is one reason surface temperatures average around minus 60 degrees Celsius.
Mars wasn’t always this exposed. Strong magnetization locked into ancient crustal rocks, particularly in the southern hemisphere, shows that the planet once generated a global magnetic field from its core. Some of these crustal magnetic fields are among the most intense measured on any planetary body, approaching Earth’s own surface field strength in places. The rocks preserving this magnetization contain up to 10 percent ferromagnetic minerals by weight, mostly magnetite with some hematite, pyrrhotite, and goethite. When the core dynamo shut down, likely during the planet’s first billion years, the magnetic shield disappeared and solar wind gradually stripped away much of the atmosphere.
Ice at the Poles
Both poles of Mars have permanent ice caps, but they aren’t made of the same stuff as Earth’s. The south polar cap’s bright surface layer is roughly 85 percent carbon dioxide ice (dry ice) and 15 percent water ice. Beneath that dry ice veneer, radar measurements have revealed a massive deposit of water ice. The north polar cap is primarily water ice with a seasonal coating of carbon dioxide frost that appears each winter and sublimates each spring. Together, these caps lock up a significant portion of Mars’s remaining volatile inventory, the lightweight molecules that once contributed to a thicker atmosphere and possibly surface lakes or rivers.