[1] Previous studies have documented the potential for using relatively short-period body waves and intermediate-period surface waves to explore the structure and tectonics of Europa. We show that long-period measurements (0.001 to 0.1 Hz) may have large amplitudes of displacement (millimeters to centimeters) and are potentially measurable from orbit without requiring a lander. To accurately model the long-period response of Europa, we use normal modes calculated from physically self-consistent models of Europa's structure developed in part 1 (Cammarano et al., 2007). On the basis of the geometry of observed faults, we estimate that faulting events of magnitude 5 or larger may occur regularly. Synthetic seismograms show that long-period displacement measurements with millimeter accuracy could detect current tectonic activity and determine the thickness of Europa's ice shell, and confirm the presence of a subsurface ocean. Determination of deeper structure with seismic measurements, however, is more challenging in the presence of a global liquid ocean, which acts to decouple deeper seismic energy from the surface.

[1] In order to examine the potential of seismology to determine the interior structure and properties of Europa, it is essential to calculate seismic velocities and attenuation for the range of plausible interiors. We calculate a range of models for the physical structure of Europa, as constrained by the satellite's composition, mass, and moment of inertia. We assume a water-ice shell, a pyrolitic or a chondritic mantle, and a core composed of pure iron or iron plus 20 weight percent of sulfur. We consider two extreme mantle thermal states: hot and cold. Given a temperature and composition, we determine density, seismic velocities, and attenuation using thermodynamical models. While anelastic effects will be negligible in a cold mantle and the brittle part of the ice shell, strong dispersion and dissipation are expected in a hot convective mantle and the bulk of the ice shell. There is a strong relationship between different thermal structures and compositions. The “hot” mantle may maintain temperatures consistent with a liquid core made of iron plus light elements. For the “cold scenarios,” the possibility of a solid iron core cannot be excluded, and it may even be favored. The depths of the ocean and core-mantle boundary are determined with high precision, 10 km and 40 km, respectively, once we assume a composition and thermal structure. Furthermore, the depth of the ocean is relatively insensitive (4 km) to the core composition used.

[1] We gathered seismic refraction and wide-angle reflection data from several active source experiments that occurred along the Mid-Atlantic Ridge near 35°N and constructed three-dimensional anisotropic tomographic images of the crust and upper mantle velocity structure and crustal thickness. The tomographic images reveal anomalously thick crust (8–9 km) and a low-velocity “bull's-eye”, from 4 to 10 km depth, beneath the center of the ridge segment. The velocity anomaly is indicative of high temperatures and a small amount of melt (up to 5%) and likely represents the current magma plumbing system for melts ascending from the mantle. In addition, at the segment center, seismic anisotropy in the lower crust indicates that the crust is composed of partially molten dikes that are surrounded by regions of hot rock with little or no melt fraction. Our results indicate that mantle melts are focused at mantle depths to the segment center and that melt is delivered to the crust via dikes in the lower crust. Our results also indicate that the segment ends are colder, receive a reduced magma supply, and undergo significantly greater tectonic stretching than the segment center.