The terrestrial or inner planets

Mercury, Venus, Earth, and Mars are characterised by low masses, small radii, and large densities in comparison to the gas giant planets of the outer solar system. Their chemical compositions are dominated by rock-forming elements and metals such as iron and nickel, the latter concentrated in central cores. Gravitational and magnetic field measurements indicate that terrestrial planet interiors usually are strongly differentiated and subdivided like that of the Earth into a partly or entirely liquid metallic core, a silicate mantle and an outermost crust derived from partial melt processes of the underlying mantle. In the case of Earth, Venus, and Mars, mantle pressures are sufficient for mineral phase transitions to occur by compressing foremost olivine and pyroxene minerals in a smaller volume. Since the depth at which such discontinuous density changes occur are also dependent on the ambient temperature and the iron content of the mantle rocks, seismological observations at planetary surfaces have the potential to provide additional information on the thermal states and compositional differences of the terrestrial planets.

Terrestrial-type exoplanets

constitute a particular class of extrasolar solid Earth-sized bodies, presumably several times as massive as the Earth, with relatively large mean densities and high internal pressures. These planets are expected to have similar interior structures and bulk compositions like the terrestrial-type bodies in the solar system. Due to current detection limits of ground-based observational methods, only a handful of these low-mass planets beyond the solar system have been discovered so far, though, according to predictions of planet formation models, they should be quite common in the Galaxy.

Figure 1: Radius-density relation of the terrestrial planets, the Moon, and the Jovian satellite Io. Note the exceptionally high mean density of Mercury, implying that the planet's interior is predominantly composed of heavy elements such as iron. Source: Sohl, F. and G. Schubert (2007): Interior Structure, Composition and Mineralogy of the Terrestrial Planets, In: Treatise on Geophysics (Editor-in-Chief G. Schubert), Volume 10, Planets and Moons (Ed. T. Spohn), p. 27-68, Elsevier.

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