Ice Rafts

Io’s density and moment of inertia constrain its internal structure. The satellite has a large metallic core whose exact size is uncertain because we do not know the composition of the core. The core radius could be as large as about half of Io’s radius and is surrounded by a silicate mantle. Io’s extensive volcanism suggests that the satellite has a crust and a partially molten asthenosphere. Much of the tidal dissipative heating probably occurs in this asthenosphere. Io is known to be in hydrostatic equilibrium under the action of the Jovian tidal forces and its rotation. It is not known if Io has its own magnetic field.

Europa is only slightly smaller and less massive than the Moon, but it also looks quite different from our satellite. Figure 1–79 is a Voyager 2 picture of Europa that shows the surface to consist of two major terrain types: a uniformly bright terrain crossed by numerous dark linear markings and a somewhat darker mottled terrain. Relatively few impact craters exist on Europa indicating that the surface is geologically young. The linear markings are ridges and fractures; they have little or novertical relief. They extend over distances as large as thousands of kilometers and vary in width from several kilometers to about 100 km. Europa’s density and moment of inertia indicate that, although it is composed mainlyof silicates, it must contain a large fraction (about 20% by mass) of water. The water is believed to be in a surface layer about 100 km thick surrounding a silicate mantle and metallic core. The water layer may be completely frozen or it may consist of ice above liquid. Infrared spectra of Europa and its high albedo indicate that the surface is covered with water ice or frost. High-resolution Galileo pictures show features such as ice rafts that have rotated and separated from each other over an underlying soft ice layer or an internal liquid ocean. The relative absence of craters on Europa may have resulted from the freezing of a competent ice layer only after the termination of the early phase of severe bombardment or it may be due to geologically recent resurfacing of the satellite; the global fracture pattern may be a consequence of tidal stresses and nonsynchronous rotation of Europa’s outer shell of ice. The surfaces of Europa and, as we shall see, Ganymede and Callisto are shaped by processes occurring in a predominantly ice shell. Although large ice-covered regions of the Earth give us some clues about what surface features to expect, the icy Galilean satellites provide a unique example of surfaces shaped by global-scale ice tectonic processes at extremely low temperatures (the surface temperatures of the Galilean satellites are about 150K). The geologist studying Io must be mainly a volcanologist; the geologist 110 Plate Tectonics investigating Europa, Ganymede, and Callisto, on the other hand, must be mainly a glaciologist! If there is an internal ocean on Europa, the satellite must then be considered a possible site for extra-terrestrial life. Some tidal heating of Europa is necessary to prevent the freezing of an internal liquid water ocean.