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Interior Structure of Planetary Bodies

Deutsche Version


The determination of the interior structure of our neighbouring planets and their satellites has been and will be an important scientific objective of interplanetary space missions targeted at different classes of solar system objects. 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 the planetary body itself. The most valuable information on the interior structure of the Earth has been obtained from a vast amount of seismological observations. Unfortunately, such data are not available yet for solar system objects other than Earth and, to a lesser extent, for Earth's Moon.

The origin and early evolution of a planetary body ...
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 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.

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.

The natural satellites of the giant outer planets ...
except Io are covered by pure water ice or intimate mixtures of water-, ammonia-, and methane ice as indicated by spectral observations. Io, a volcanically active body similar in size to the Moon but of entirely different composition, may have lost most of its volatile inventory due to intense tidal heating in the proximity of massive Jupiter. The low densities of Ganymede and Callisto, the large icy satellites of Jupiter, and Titan, the largest Saturnian moon, suggest that their interiors are composed of ice and silicates plus metals at nearly equal shares by mass. The densities of the Moon and the inner Jovian satellites Io and Europa, however, imply that their interiors mainly consist of silicates and metal. Europa's deep interior is additionally overlain by a heavily tectonised, less massive water-ice liquid shell. The detection of induced magnetic fields in the vicinity of the Jovian satellites Europa, Ganymede, and Callisto further suggests the existence of electrically conducting reservoirs of liquid water beneath the satellites' outermost icy shells that may contain even more water than all terrestrial oceans combined. The internal differentiation of the terrestrial planets and major satellites probably took place early in their histories only shortly after their violent accretion from colliding planetesimals.

Modelling planetary interiors ...
based on numerical calculations using laboratory data of physical material properties therefore aims at improving our understanding of the origins, evolutions, and current states of planetary bodies. The resultant radial profiles of density and related material properties are required to be consistent with geophysical observations and cosmochemical evidence for the likely compositions of crust, mantle and core as obtained from measurements by interplanetary space probes.

Contact: Dr. Frank Sohl


Publications

  • Van Hoolst, T, F. Sohl, I. Holin, O. Verhoeven, V. Dehant and T. Spohn
    (2007): Mercury's Interior Structure, Rotation, and Tides, Space Science Reviews 132 (2-4), 203-227, doi:10.1007/s11214-007-9202-6
  • Grott, M., F. Sohl and H. Hussmann
    (2007): Degree-one convection and the origin of Enceladus' dichotomy, Icarus, 191, p. 203-210, doi:10.1016/j.icarus.2007.05.001
  • 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
  • Hussmann, H., Sohl, F., Spohn, T.
    (2006): Subsurface Oceans and Deep Interiors of Medium-Sized Outer Planet Satellites and Large Trans-Neptunian Objects, Icarus 185, 258-273, doi: 10.1016/j.icarus.2006.06.005.
  • Sohl, F., Schubert, G., Spohn, T.
    (2005): Geophysical constraints on the composition and structure of the Martian interior. J. Geophys. Res., 110, E12008, doi:10.1029/2005JE002520.
  • Sohl, F., H. Hussmann, B. Schwentker, T. Spohn, and R.D. Lorenz (2003): Interior structure models and tidal Love numbers of Titan. J. Geophys. Res., 108(E12):5130, doi:10.1029/ (2003)JE002044.
  • Sohl, F., T. Spohn, D. Breuer, and K. Nagel (2002): Implications from Galileo observations on the interior structure and chemistry of the Galilean satellites. Icarus, 157:104-119.
  • Spohn T., M. A. Acuña, D. Breuer , M. Golombek, R. Greeley, A. Halliday, E. Hauber, R. Jaumann, and F. Sohl (2001): Geophysical constraints on the evolution of Mars. In: Chronology and Evolution of Mars (Hartmann, B. and R. Kallenbach, Eds.), Chapter 9, Proceedings of the ISSI workshop "The Evolution of Mars" in Bern 2000, Kluwer Academic Publishers, Dordrecht and published in Space Sci. Rev., 96, 231-262.
  • Spohn, T., F. Sohl, K. Wieczerkowski, and V. Conzelmann (2001): The interior structure of Mercury: What we know, what we expect from BepiColombo. Planet. Space Sci., 49:1561-1570.
  • Spohn, T., F. Sohl, and D. Breuer (1998): Mars. Astron. Astrophys. Rev., 8, 181-236.
  • Sohl, F. and T.Spohn (1997): The interior structure of Mars: Implications from SNC meteorites. J. Geophys. Res., 102:1613-1635.
  • Sohl, F., W.D. Sears, and R.D. Lorenz. (1995): Tidal dissipation on Titan. Icarus, 115:278-294.
  • Sohl, F. and H. Weber. (1993): Schroeter's effect and the phase anomaly of Venus. J. Brit. Astron. Assoc., 103:305-308, .

Author: Dr. Frank Sohl WWW-Editor: Nils Müller
Last Changes: Thursday, 10-Jan-2008 11:50:30 CET
URL of this page: http://solarsystem.dlr.de/TP/aufbau_en.shtml
 
Planetary Interiors
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.

Ratio-Density-Diagram
Density vs. radius diagram of the moons in the solar system.
Source: Dr. Hauke Hussmann

Address

Institute of Planetary Research
Department of Planetary Physics
Rutherfordstr. 2
D-12489 Berlin

Internet:  > http://www.dlr.de/pf

Head of Department

 > Prof. Dr. Doris Breuer

Related Links

 > Aufbau und Geologie der Planeten und Satelliten des Sonnensystems (only in german)