The mantle of Mars is the rocky layer situated beneath the planet’s crust and above its metallic core. This region accounts for the largest volume of the planet’s interior, extending from the base of the crust down to a depth of approximately 969 miles (1,560 kilometers). Understanding the composition and physical state of this layer is fundamental because it governs the planet’s thermal history, volcanic activity, and overall geological evolution. The mantle is the source of all magmas that have erupted onto the Martian surface.
Primary Mineral and Chemical Composition
The chemical makeup of the Martian mantle differs significantly from Earth’s, primarily by being richer in iron oxide (FeO). This high iron content means the Martian mantle is more oxidized than Earth’s. The mantle is dominated by silicate minerals, specifically iron-magnesium silicates like olivine and pyroxene.
Olivine forms a significant portion of the Martian upper mantle, influencing the planet’s density and internal structure. Pyroxene is also abundant, existing in various forms depending on the presence of calcium and iron. The presence of these two mineral groups, rich in iron and magnesium, is consistent with the basaltic nature of the Martian crust, which is formed from melted mantle material.
In comparison to Earth, the Martian mantle is also enriched in certain trace elements, such as potassium and phosphorus. However, it appears relatively depleted in lighter, volatile elements. This depletion affects the composition of the magmas that rise to the surface, influencing the overall chemical profile of the planet’s crust and atmosphere.
Internal Structure and Physical State
The Martian mantle is divided into an upper and a lower section, based on increasing pressure and temperature with depth. While the entire mantle is considered solid rock, its size allows for extremely slow movement known as convection, which drives heat from the core toward the surface.
The planet’s thick, rigid outer layer is the lithosphere, which includes the crust and the uppermost mantle. Estimates of the lithosphere’s thickness are highly variable, but in geologically cold regions, it can be hundreds of kilometers thick. This suggests Mars is a much more rigid planet than Earth. This great thickness implies the near-absence of the asthenosphere, a soft, partially molten layer characteristic of Earth’s upper mantle.
The boundary between the upper and lower mantle is marked by a seismic discontinuity at a depth of around 1,000 kilometers. This discontinuity is caused by olivine transforming into its denser, high-pressure polymorphs, such as wadsleyite and ringwoodite. Near the core-mantle boundary, recent data suggest the presence of a molten silicate layer. This layer acts as a thermal blanket, insulating the core and affecting the planet’s magnetic field evolution.
How Scientists Determined the Mantle’s Makeup
The most direct and recent insights into the Martian mantle came from the NASA InSight mission, which operated a seismometer on the planet’s surface. This instrument detected numerous Marsquakes, allowing scientists to use seismology to map the planet’s interior. By analyzing how seismic waves travel through the planet, researchers determined the depths and densities of the internal layers.
The speed and reflection of compressional (P) waves and shear (S) waves were used to identify the precise depth of the crust-mantle boundary and the mid-mantle mineral phase transition. Wave speeds are directly related to the rock’s density and rigidity, providing constraints on the iron-rich composition of the silicates. These seismic observations were integrated with mineral physics models to confirm the dominance of olivine and pyroxene.
A second line of evidence comes from the analysis of Martian meteorites, known collectively as the SNC group (Shergottites, Nakhlites, and Chassignites). These samples are igneous rocks that represent solidified magmas originating from the Martian mantle. Their FeO-rich and Al2O3-poor chemical signatures provide a direct chemical constraint on the mantle’s source material.
Furthermore, remote sensing from orbiting spacecraft provides complementary data by mapping surface minerals that are byproducts of mantle melting. The global distribution of surface olivine and pyroxene offers clues about the composition of the mantle regions that produced the volcanic materials. Together, these distinct methods—seismology, meteorite analysis, and remote sensing—have converged to paint a detailed picture of the Martian mantle.