Mars is a terrestrial planet that formed through planetary differentiation, resulting in distinct layers: a dense, metallic core, a mantle, and a crust. Scientists seek to determine the precise nature of this deep interior, as it holds the record of the planet’s formation and geophysical evolution. The internal structure of Mars is now being precisely mapped, moving beyond theoretical models to reveal specific details about its size, state, and chemical makeup.
The Core’s Physical State
Recent seismic analyses suggest the Martian core consists of a liquid outer layer surrounding a solid inner core, a structure analogous to Earth’s. Earlier estimates favored an entirely molten core due to Mars’s smaller size and faster cooling rate. However, seismic wave travel times now provide evidence for a solid component at the center.
This solid inner core has an estimated radius of 600 to 613 kilometers, roughly a third of the total core’s radius. The surrounding liquid outer core is an iron-alloy mixture that is less dense and more compressible than pure iron. The presence of this solid inner core indicates that the planet may have cooled and crystallized faster than previously modeled.
Composition and Dimensions
The Martian core is primarily an alloy of iron and nickel, distinguished by a high concentration of lighter elements mixed in with the metal. These elements—including sulfur, oxygen, carbon, and hydrogen—significantly lower the core’s density and melting point. Their proportion is estimated to be between 9 and 14 percent by weight of the core’s total mass, a much greater fraction than found in Earth’s core.
This chemical makeup results in a core that is less dense and more compressible than Earth’s, suggesting different formation conditions. The core-mantle boundary indicates a total core radius of approximately 1,780 to 1,810 kilometers. This dimension means the core makes up about half the entire radius of Mars.
Scientific Evidence and Discovery
The direct measurement of Mars’s deep interior structure became possible through the NASA InSight mission, which landed on the planet in 2018. InSight carried the Seismic Experiment for Interior Structure (SEIS), a sensitive seismometer designed to record vibrations from marsquakes and meteorite impacts. These seismic events generate compressional P-waves and shear S-waves.
The way these waves travel through the planet acts as a geophysical X-ray, allowing scientists to map the interior. P-waves travel through both solid and liquid materials, but S-waves cannot propagate through liquids. By observing the travel times and paths of seismic waves that bounced off the core-mantle boundary, scientists confirmed the core was liquid.
Specific wave types, such as PKiKP and PKKP, which travel through the core and reflect off its boundaries, were used to detect the solid inner core. The arrival time of these waves, compared to models of a fully liquid core, indicated a faster travel speed consistent with a solid inner mass. Analyzing these faint signals required stacking data from numerous low-frequency marsquakes.
Core Dynamics and Magnetic History
The core’s activity is intrinsically linked to the planet’s magnetic history, known as the dynamo effect. A global magnetic field requires the movement of electrically conductive fluid, such as a liquid iron-alloy, within the core. Evidence from magnetized patches in the crust confirms that Mars once possessed a global magnetic field, active from approximately 4.3 to 3.6 billion years ago.
The dynamo eventually ceased, leaving Mars without the magnetic shielding Earth enjoys today. Research suggests this cessation was caused by the core’s surprisingly high thermal conductivity, which led to efficient cooling. This rapid heat loss diminished the thermal convection necessary to sustain the dynamo, causing the liquid metal to cease churning and leading to the end of the global magnetic field.