The current scientific consensus, refined by data from the NASA InSight mission, confirms that Mars possesses a core that is largely molten. This iron-alloy mixture remains in a liquid state, significantly reshaping the understanding of the planet’s thermal and chemical history. While early models suggested a completely liquid interior, recent analysis points toward a differentiated structure, likely featuring a solid component within the vast fluid layer.
The Internal Structure of Mars
Like Earth, Mars is a differentiated terrestrial planet, meaning its interior is separated into distinct layers based on density. The outermost layer is the crust, which has a highly varied thickness across the planet’s surface. In the northern lowlands, the crust is relatively thin, but it thickens significantly in the southern highlands, reaching a maximum depth of approximately 117 kilometers in the Tharsis plateau region.
Beneath this crust lies a massive silicate mantle, composed primarily of silicon, oxygen, iron, and magnesium. This rocky layer extends downward for an estimated 1,240 to 1,880 kilometers and is thought to be largely dormant today, with little to no ongoing tectonic activity. The mantle material transitions abruptly at the Core-Mantle Boundary (CMB) to the much denser, metallic core.
The core itself is estimated to have a radius of approximately 1,780 to 1,810 kilometers. The material within this boundary is an iron-nickel alloy, which is significantly denser than the surrounding mantle.
Unlocking the Core’s Secrets with Seismic Data
The most definitive insights into the Martian core’s physical state came from the NASA InSight mission, which deployed the first dedicated seismometer, SEIS (Seismic Experiment for Interior Structure), to the planet’s surface. This instrument recorded “marsquakes” and seismic waves generated by large meteoroid impacts, acting as natural probes of the deep interior.
Seismic body waves come in two main forms: P-waves (Primary, or compressional waves) and S-waves (Secondary, or shear waves). P-waves can travel through both solid and liquid material, but S-waves cannot propagate through a fluid, as liquids cannot sustain the shear stress that S-waves require. The initial analysis of data showed a strong attenuation of S-waves at the core-mantle boundary depth, which provided the first direct evidence that the Martian core was predominantly molten.
Further analysis involved the detection of core-transiting waves, specifically SKS waves, which start as S-waves in the mantle, convert to P-waves as they pass through the liquid core, and then convert back to S-waves upon exiting into the mantle. By precisely measuring the travel times of these waves, researchers constrained the core’s physical properties. These data indicated a core density of 6.2 to 6.3 grams per cubic centimeter, which is lower than that of pure iron, confirming the presence of lighter elements.
While the bulk of the core is liquid, later analyses of the InSight data suggest the presence of a solid inner core. This solid heart, estimated to have a radius of about 600 kilometers, is surrounded by the larger liquid outer core, giving Mars a layered structure similar to Earth’s.
Composition and the Missing Dynamo
The core is composed primarily of iron and nickel, but its low density indicates that it is alloyed with a significant percentage of lighter elements. These light elements include sulfur as the major component, along with smaller fractions of oxygen, carbon, and possibly hydrogen. The total proportion of these lighter elements is high, with estimates ranging from 9 to 22 weight percent.
The presence of these light elements acts to lower the melting temperature of the iron-nickel mixture, which is the primary reason the core remains in a largely liquid state today. For a planetary body, a liquid core is a prerequisite for a magnetic field, which is generated by a process called a dynamo. This process requires the movement and convection of electrically conductive fluid metal within the core.
Mars currently lacks a global magnetic field, but evidence from highly magnetized regions of its crust confirms that it possessed a dynamo early in its history, operating between 4.3 and 3.6 billion years ago. The fact that the core is still liquid, yet the dynamo has ceased, suggests that the necessary convection stopped long ago. Scientists hypothesize that the core cooled too quickly, or that the high thermal conductivity of the iron-sulfur alloy inhibited the strong thermal convection needed to sustain the dynamo.
This early cessation of the magnetic field is thought to have contributed to the stripping of the Martian atmosphere, fundamentally altering the planet’s evolution.