Earth and Mars: Comparing Their Internal Structure

Earth and Mars both have a metallic core, a silicate mantle, and a solid crust, but they are not built alike: Earth is larger, and its inner layers interact more actively. Mars is smaller and lost its internal heat more quickly, so its geological activity weakened over time.

Comparing the internal structure of Earth and Mars helps us understand how a planet’s size and cooling affect its evolution. Let’s look at what is known about the cores, mantles, and crusts of these two neighboring worlds—and why their similar composition led to different histories.

• 1795 ± 65 км estimated extent of Mars’s core
• 1780—1810 км median range for the radius of Mars’s core
• 2890 км extent of Earth’s mantle
• 16—17% stated proportion of sulfur in Mars’s core
• 25 августа и 18 сентября 2021 года dates when SEIS recorded events S0976a and S1000a

How are the internal structures of Earth and Mars alike?

Earth and Mars are alike in that both are terrestrial planets: each has rocky outer layers, a mantle, and a metallic core. Earth’s mantle extends approximately 2890 km and consists of silicate rocks containing iron and magnesium; the description of the crust lists iron, magnesium, and granite among its components.

Mars also has a crust, mantle, and core, but the similarity is primarily in this general layered structure. The Martian core consists mainly of iron with nickel and sulfur impurities; seismic waves yield an estimated median radius of 1780—1810 km.

A shared blueprint does not mean identical structure

The presence of a crust, mantle, and core does not mean that Earth and Mars are the same in size, composition, or geological history. For example, information about Earth’s mantle describes silicate rocks containing iron and magnesium, whereas the description of Mars’s core specifically notes nickel and sulfur. The resemblance between terrestrial planets therefore reflects a shared plan for their internal structure, not proof that their interiors are identical.

How large are Mars’s core and Earth’s mantle?

Mars’s core is estimated to extend 1795 ± 65 km, while Earth’s mantle extends 2890 km. These are different measurements of different planets and layers: the radius of Mars’s core cannot be directly compared with the extent of Earth’s mantle as though they were the same kind of measurement.

What the estimates mean

The median range given for the radius of Mars’s core is 1780—1810 km. It was derived by estimating the planet’s internal structure from seismic waves, not by directly imaging the core; the range therefore represents the results of measurements and interpretation, not a boundary visible in an image.

Earth’s mantle, by contrast, is a distinct planetary layer that extends 2890 km. Thus, 1795 ± 65 km and the range 1780—1810 km refer to Mars’s core, while 2890 km describes Earth’s mantle: it would be incorrect to compare these figures as the sizes of equivalent objects.

What is known about the composition of the crust, mantle, and core?

Earth’s mantle is described as a layer of silicate rocks containing iron and magnesium, and Earth’s crust as containing iron, magnesium, and granite; Mars’s core is described as iron and nickel with sulfur impurities. According to the description provided, sulfur makes up 16—17% of the Martian core.

Layer composition: Earth and Mars

• Earth’s mantle: silicates, iron, and magnesium; its stated thickness is 2890 km.
• Earth’s crust: iron, magnesium, and granite are listed; its stated thickness is 40 km.
• Mars’s core: iron and nickel with sulfur impurities, whose proportion is given as 16—17%.

This information lists substances and materials but does not provide a quantitative comparison of the mineral or metal content of the two planets. So the presence of some of the same components—for example, iron, which is listed for Earth’s layers and Mars’s core—does not mean that they occur in the same proportions on Earth and Mars.

How do seismic waves help us study Mars’s interior?

Seismic waves let researchers study Mars’s interior indirectly: by tracking how the waves pass through the core and reach a seismometer, they estimate its size and density. The core could not be observed directly; the radius range of 1780—1810 km was derived from the analysis of how the waves traveled.

What events S0976a and S1000a revealed

The SEIS seismometer recorded two Martian events that were important for this analysis: S0976a on 25 August 2021 and S1000a on 18 September 2021. Waves from these events passed through the core, allowing researchers to compare their characteristics with models of the planet’s internal structure.

Based on the analysis, the median radius of Mars’s core is estimated at 1780—1810 km, and its density at 6.2—6.3 g/cm³. These values are not the result of directly measuring the core: they are inferred from how seismic waves behave as they travel through Mars’s inner layers.

How is internal structure connected to magnetic fields and volcanism?

The internal structure of Earth and Mars helps frame questions about the origins of their magnetic fields and volcanism, but it does not, by itself, reveal the history of these processes. Comparing magnetic properties is especially relevant to the metallic cores: Mars’s core consists mainly of iron with nickel and sulfur impurities, and its median radius is estimated at 1780–1810 km. This information describes the core’s composition and size but does not explain how or when the planet’s magnetic field changed.

The rocky outer shells—the mantle and crust above the core—are also important to the study of volcanism. Earth’s mantle extends 2890 km and consists of silicate rocks containing iron and magnesium; the crust extends about 40 km and includes granite as well as iron and magnesium. These parameters provide context for comparing planetary structures, but on their own they do not establish the nature or history of volcanic activity.

What conclusions can be drawn from the data on the layers

• Mars’s core: its radius and composition help frame questions about magnetic properties, but do not provide a detailed history of the magnetic field.
• Earth’s mantle and crust: their composition and dimensions describe the outer rocky layers, but do not replace specific data about geological processes.

What conclusions cannot be drawn from these estimates?

Estimates of Earth’s and Mars’s structures do not justify inferring exact dimensions without accounting for uncertainty, directly comparing different layers, applying the composition of one core to the other, or concluding that the planets had identical geological histories. Two ways of describing the size of Mars’s core are given: 1795 ± 65 km and a median radius of 1780—1810 km; neither estimate should be presented as an exact value without uncertainty.

• The radius of Mars’s core and the thickness of Earth’s mantle are not the same thing. The range 1780—1810 km refers to the radius of Mars’s core, while 2890 km is the extent of Earth’s mantle. A simple numerical comparison does not show which planet is “larger”: these figures describe different layers and measurements.
• Sulfur does not describe the composition of Earth’s core. The 16—17% proportion applies to Mars’s core. It cannot be applied to Earth: the description provided does not specify this proportion for Earth’s core.
• Being the same general type of planet does not mean having the same evolutionary history. Earth and Mars are rocky planets, but this similarity alone does not prove that their geological activity or development was identical.

In other words, these figures help describe specific properties of internal structure, but they do not justify turning a limited comparison into a conclusion that the planets are identical in every respect.

Frequently asked questions

Written byRodion Bazutkin

Пишет о телескопах и астрономических наблюдениях: как устроены инструменты, что они способны увидеть и как получают изображения далёких объектов. Объясняет методы измерений без лишнего жаргона и отмечает ограничения данных.