Europa, one of the four major moons of Jupiter, presents planetary scientists with a set of fascinating questions, not the least of which concerns the presence of a water ocean beneath its icy surface. Recent magnetometer data acquired by the Galileo flybys seem to have confirmed the presence of Europa's ocean (Kivelson et al., 2000). Additionally, detailed images of the planet allowed the observation of cracks in the ice consistent with flow of warm ice or water below the surface (Greeley et al., 2000) and the near-infrared mapping spectrometer experiment probably detected hydrated salts on the surface (Mc Cord et al., 2001). Nevertheless, any quantitative constraints on the ocean depth and the depth of the ice shell above it require further explorations.
Measurements of the seismic response of Europa remotely from an orbiter or using a lander can greatly expand our knowledge of its internal structure. Despite this potential, the feasibility of a seismic experiment that would exploit natural sound sources (e.g., the opening of the cracks in the ice) to investigate the thickness of the ice shell and the ocean depth, has only recently been considered. In order to determine the potential of seismic signals to discriminate between different possible scenarios for the structure of Europa, it is essential to provide a family of reasonable physical models.
We generate a set of physical models by assuming a three-layer composition: water-ice, silicate mantle (either pyrolitic or chondritic), metallic core (either solid iron or iron+sulfur) and different thermal structures. The thermal structures are based on estimations of the internal heating. Two extreme (cold and hot scenarios) have been considered. Thermodynamic properties as a function of pressure and temperature are computed for each layer by using equation of states based on the most recent mineral physics data (e.g. Wagner and Pruss 2002 for water, Stixrude and Lithgow-Bertelloni 2005 for the silicatic mantle). The depth of the ocean and of the core-mantle boundary are constrained by the mass and moment of inertia for each physical model.
Berkeley Seismological Laboratory
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