PROBING EUROPA'S INTERIOR STRUCTURE WITH NATURAL AMBIENT NOISE

Our goal is to use both acoustic echo-sounding and tomographic techniques to determine Europa's interior structure. Echo-sounding reveals the depth and composition of terrestrial seafloor and sub-bottom layers by analysis of the arrival time and amplitude of acoustic reflections from these interfaces. Tomography reveals the temperature structure of terrestrial oceans by the way sound waves are perturbed along forward propagation paths. We plan to exploit natural cracking events on Europa's surface as sound sources of opportunity. Recent work shows that cycloidal cracks on the surface of Europa likely form on a daily basis due to stresses induced by Europa's eccentric orbit which has a period of roughly 3.5 days. We estimate that the acoustic waves radiated from these cracks will be in the 0.1-100 Hz range with typical wavelengths exceeding 1 km. In contrast to ice-penetrating radar, inhomogeneities such as ice fractures should be transparent to such long acoustic wavelengths. Meteor impacts typically occur at a monthly rate and also have potential use as sound sources. According to current plans, the first Europa landing mission will likely carry only a single geophone. Many valuable measurements can be made with a single geophone. For example, a first task should be to determine the level of acoustic activity on Europa by time series and spectral analysis. Correlation should be made of ambient noise level versus tidal stress to determine whether noise levels respond directly to orbital eccentricities. Such analysis conducted for the Earth's Arctic Ocean and showed near perfect correlation between underwater noise level and environmental stresses and moments applied to the ice sheet from wind, current and drift (Makris and Dyer, JASA 1986). Robust estimates can be made of Europa's ice layering structure and ocean depth with a single acoustic sensor if the signal-to-noise ratio is sufficiently high. On Europa, an isolated cracking event will lead to numerous echoes emanating from multiple reflections of compressional, shear and combined compressional-shear waves from the various layers of Europa's ice-water interior. Using 3-D seismo-acoustic propagation models developed for the Arctic Ocean on Earth, we find that the spacing of arrivals in time can used to robustly estimate source range, ice and ocean thickness as well as layering parameters.