The determination of the interior structure of our neighbouring planets, their satellites and extrasolar planets, i.e. planetary bodies orbiting around stars other than the Sun, is one of the most important scientific objectives of ongoing and future space missions and ground-based observations. This is due to the fact that many, if not most planetary processes operating on a global scale are immediately affected by the internal constitution of planetary bodies themselves. Valuable information on the interior structure of the Earth and, to a lesser extent, for the Earth's Moon has been obtained from a vast amount of seismological observations. Unfortunately, such data are not available yet for other solar system bodies and extrasolar planets.
Figure 1: The interior structure of some planetary bodies in our solar system.Rc/Rp denotes the ratio between core radius and planet radius. Source: NASA/JPL/DLR/RPIF; Robert G. Strom (1987) Mercury: The Elusive Planet. Smithsonian Inst. Press, Washington, D.C.
is reflected by the chemical composition of the interior, whereas surface geology and tectonic features are foremost affected by mechanisms that dominate the transport of internal heat from the interior to the surface. The occasional existence of self-sustained and/or induced magnetic fields requires reservoirs of electrically conducting fluids at some depth thereby providing additional constraints on the current thermal state. Since a fluid layer within a planetary body mechanically decouples the deep interior from the outer portion, the propagation of seismic waves and the way in which a planet or satellite responds to tides will be severely affected by the physical state of the interior. Furthermore, the shape of the gravitational field is closely related to the radial and lateral density distribution within the planet or satellite. An important parameter is the polar oblateness of the gravitational field as a measure for the concentration of mass towards the centre. The most important parameter that permits an estimate of how the interior is composed in the absence of seismological data is the average uncompressed density that indicates how much mass is contained in a unit volume. It also accounts for self-compression and pressure-induced mineral phase transitions caused by the weight of overlying layers.
Contact: Dr. Frank Sohl, Frank Wagner