STRUCTURE AND COMPOSITION OF THE DEEP INTERIOR

Prior to the Galileo mission there were two competing models for the internal structure of Europa.24 In one model, Europa consisted of an anhydrous rocky core with a density like that of Io or the Moon, surrounded by a layer of water ice or liquid water that could be more than 100 km thick. In the other model, most of the water in Europa was retained in a hydrated silicate interior surrounded by a thin water-ice layer. The possibility that Europa might have a differentiated metallic core was not considered. Instead, debate centered on the extent to which Europa's interior was dehydrated and, in the fully dehydrated model, whether the outer water layer was completely frozen or had a melted liquid layer beneath an outer solid layer of ice.

Today, as a consequence of the Galileo measurements of Europa's gravitational field, the model of Europa with a thin ice shell above a largely hydrated silicate interior is no longer tenable. Moreover, the density of Europa's deep interior is high enough that it argues strongly for a metallic core at the center of the satellite. Unfortunately, we still do not know with certainty whether there is a liquid-water ocean beneath Europa's icy surface. Neither do we know if the metallic core is solid or liquid.

The gravitational field of Europa has been measured in four flybys of the satellite by the Galileo space-craft.25,26 The measurements have yielded quite accurate determinations of the degree-two spherical harmonic contributions to Europa's gravity. On the assumption that the shape of Europa's gravitational field results from a physical distortion caused by its spin and by the tidal forces it experiences as it orbits around Jupiter in synchronous rotation, the gravitational field can be used to infer Europa's axial moment of inertia C and tell us about the distribution of mass in Europa's interior. Normalized to MR2 (M is the mass of Europa and R is its radius), the moment of inertia is C/MR2 = 0.346 ± 0.005.27 This value of C/MR2 is substantially less than 0.4, the value for a uniform-density sphere, and requires that the density of the interior increase toward the center of Europa.

The implications of Europa's mean density and moment of inertia for the structure of its interior have been explored in terms of simple two-and three-layer models of the satellite.28 Two-layer models of Europa with an ice outer shell and a uniform silicate/metal inner region are possible, but only if the interior density is greater than about 3800 kg m-3. This structure is considered implausible because the interior density would be higher than Io's mean density, and because it is likely that radiogenic heating in such an interior would cause a metallic core to differentiate.29 Therefore, Europa must have a three-layer structure with an Fe or Fe-FeS core at its center, a rock mantle surrounding the metallic core, and a water-ice or liquid-water shell around the rock. The size of the core is between 40 and 50% of Europa's radius, depending on its composition. The thickness of the outer shell of water must lie in the range of about 80 to 170 km, with a value of some 100 km being the most likely.30 The gravity data do not allow any conclusion regarding the physical state (i.e., liquid or solid) of either Europa's metallic core or its outer water shell. Lack of detection of a europan magnetic field also precludes any unique inference about the physical state of Europa's metallic core.31

EVIDENCE FOR THE PRESENCE OF A LIQUID-WATER MANTLE

Thermal models of Europa provide additional insight into the possibility that a liquid-water ocean may exist under Europa's surface ice.32 Modeling suggests that accretional and radiogenic heat sources are large enough to have dehydrated Europa early in its evolution, leaving the satellite covered with a layer of liquid water 100 km or more thick. Early models, which considered only the conductive cooling and freezing with time of the outer layer of water,33 resulted in the presence today of liquid water beneath the ice shell. However, other models showed that the outer layer of ice would become unstable to convection with sufficient thickening, thereby promoting heat transfer through the ice and the cooling and solidification of the underlying water.34 These models predicted the complete freezing of the outer layer of water in a small fraction of geologic time. However, the predicted freezing of Europa's ocean by efficient subsolidus convection (i.e., the slow deformation and flow of a material below its melting temperature in response to a source of heat) assumed only radiogenic heating in Europa's silicate interior. Other models included the additional heating produced by tidal dissipation in the ice shell and indicated that this heat source could offset the subsolidus convective cooling of the ice and prevent complete solidification of the water ocean.35 Basically, a steady state could be achieved in which the balance between the dissipative heat



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