Internal constitution of Mars

Abstract

Models for the internal structure of Mars that are consistent with its mass, radius, and moment of inertia have been constructed. Mars cannot be homogeneous but must have a core, the size of which depends on its density and, therefore, on its composition. A meteorite model for Mars implies an Fe‐S‐Ni core (12% by mass of the planet) and an Fe‐ or FeO‐rich mantle with a zero‐pressure density of approximately 3.54 g/cm^3. Mars has an iron content of 25 wt %, which is significantly less than the iron content of the earth, Mercury, or Venus but is close to the total iron content of ordinary and carbonaceous chondrites. A satisfactory model for Mars can be obtained by exposing ordinary chondrites to relatively modest temperatures. Core formation will start when temperatures exceed the eutectic temperature in the system Fe‐FeS (∼990°C) but will not go to completion unless temperatures exceed the liquidus throughout most of the planet. No high‐temperature reduction stage is required. The size and density of the core and the density of the mantle indicate that approximately 63% of the potential core‐forming material (Fe‐S‐Ni) has entered the core. Therefore, Mars, in contrast to the earth, is an incompletely differentiated planet, and its core is substantially richer in sulfur than the earth's core. The thermal energy associated with core formation in Mars is negligible. The absence of a magnetic field can be explained by lack of lunar precessional torques and by the small size and high resistivity of the Martian core.