Mars, like Earth, is classified as a terrestrial planet, possessing a structure of a crust, a mantle, and a central core. The definitive answer to whether Mars has an iron core is yes, though its composition and state differ significantly from Earth’s. Understanding the precise nature of this deep interior is a major goal for planetary scientists, as the core drives a planet’s geological and magnetic processes, revealing much about Mars’s history.
Composition and Physical State of the Martian Core
The Martian core is an enormous reservoir of metal, primarily iron, alloyed with a significant proportion of lighter elements. Estimates suggest that between 10 to 20 percent of the core’s mass is made up of elements like sulfur, oxygen, carbon, and hydrogen. Sulfur is believed to be the most abundant of these light elements, which drastically lowers the overall density and melting point of the metallic alloy compared to a pure iron core. This large proportion of lighter elements distinguishes Mars’s interior from Earth’s, where light elements constitute a much smaller fraction of the core.
The core is currently understood to be entirely or mostly in a liquid state, a finding confirmed by recent seismic measurements. The planet’s relatively small size and lower internal pressure prevent the core from solidifying into distinct inner and outer layers, as seen in Earth’s structure. Current models place the core’s diameter in the range of 3,560 to 3,620 kilometers, which is nearly half the entire diameter of Mars (6,779 kilometers).
The liquid metal core is surrounded by the planet’s rocky mantle, but some recent evidence suggests an even more complex boundary. Analysis of seismic data indicates the possible presence of a thin, molten silicate layer, approximately 150 kilometers thick, situated directly between the liquid metal core and the solid silicate mantle. This layer of molten rock is unique to Mars among the terrestrial planets studied so far. The presence of this fluid layer would further influence the thermal and chemical interactions between the core and the mantle, affecting the planet’s long-term cooling and evolution.
Mapping the Interior Seismic Evidence
The most direct and detailed information about the Martian core comes from the NASA InSight mission, which deployed a highly sensitive seismometer (SEIS) onto the planet’s surface. This instrument was designed to detect “Marsquakes” and the seismic waves they generate, allowing scientists to map the interior structure of the planet. Just like on Earth, these seismic waves travel through the planet’s layers, and their speed and behavior change dramatically depending on the material they pass through.
Two main types of waves, P-waves (primary) and S-waves (secondary), are used for this mapping. P-waves compress the material and can travel through solids and liquids, while S-waves shear the material and can only travel through solids. By precisely measuring the travel times of these waves from Marsquakes to the lander, researchers can determine the boundaries between the crust, mantle, and core. Crucially, the absence of S-waves passing through the center of the planet provided the definitive evidence that the core is liquid.
The data gathered from several significant seismic events, including “farside quakes” that occurred far from the lander, were particularly valuable. These waves traveled deep into the planet before being detected, allowing for a clearer picture of the core’s dimensions. These measurements refined the core’s radius to an estimated 1,780 to 1,810 kilometers, slightly smaller than initial models based on gravity data alone. This smaller, seismically determined radius suggests the core is denser than previously thought, which translates to a slightly lower percentage of light elements (closer to 9–15 weight percent).
The seismic analysis also provided constraints on the core’s density, estimated to be between 6.2 and 6.3 grams per cubic centimeter. This relatively low density, combined with the liquid state, confirms the presence of lighter elements mixed with the iron and nickel. The ability to directly measure the core’s size, density, and physical state using seismology moved the understanding of Mars’s interior from theoretical models to direct observation.
The Core’s Magnetic History and Present Inactivity
The physical state of the core, specifically its liquid nature, has direct implications for Mars’s magnetic history. A global magnetic field is generated by a process called a dynamo, which requires the convective motion of a liquid, electrically conductive material, like molten iron, within the core. Evidence from the Martian crust shows that the planet once possessed a strong, global magnetic field comparable to Earth’s present-day field.
This ancient field is recorded in the magnetization of crustal rocks, particularly in the older, heavily cratered southern highlands. These rocks act as “fossil magnets,” locking in the direction and strength of the magnetic field that existed when they cooled and solidified billions of years ago. Data from orbiting spacecraft indicate that the core’s dynamo must have been active very early in the planet’s history, possibly starting around 4.5 billion years ago and potentially lasting until about 3.7 billion years ago.
The contrast between this ancient magnetic signature and the planet’s current state is striking. Today, Mars lacks a global magnetic field, possessing only localized patches of crustal magnetism. This inactivity is directly linked to the core’s thermal evolution; the core cooled relatively quickly compared to Earth’s. As the Martian core lost heat, the necessary turbulent convection of the liquid iron alloy slowed down and eventually ceased, shutting down the dynamo mechanism. This left Mars unprotected from the solar wind and contributed to the stripping away of its early atmosphere.