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.

Figure 1: Radius-density relation of natural satellites larger than 200 km diameter and Pluto. Several groups are distinguished with respect to size and rock content. Source: Hussmann, H., F. Sohl, and T. Spohn (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.

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