Physics – Geophysics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p22b0552m&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #P22B-0552
Physics
Geophysics
0900 Exploration Geophysics, 6218 Jovian Satellites, 7200 Seismology
Scientific paper
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 surfaces as sound sources of opportunity. Recent work shows that cycloidal cracks on the surface of Europa likely form on a daily basis (Gregory V.Hoppa, Randall Tufts, Richard Greenberg, Paul E. Geissler, SCIENCE, vol 285, pages 1899 1902, 17 Sept 1999) due to stresses induced by Europa's eccentric orbit which has a period of roughly 3.5 days. The tide is expected to initiate cracks that propagate along arcs that extend for hundreds of kilometers at a speed of about 3.5 km/hour. We estimate that along a given active cycloidal feature, cracks will form about every 30 seconds and will extend about 100 meters in depth. We also 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. Correlations should be made of ambient noise level versus tidal stress to determine whether noise levels respond directly to orbital eccentricities. Such an analyses was conducted for the Earth's Arctic Ocean and showed a near perfect correlation between underwater noise level and environmental stresses and moments applied to the ice sheet from wind, current and drift (N. C. Makris and I. Dyer, J. Acoust. Soc. Am. 79, 1434-1440, 1986). Robust estimates can be made of Europa's ice layering structure and potential ocean depth with a single acoustic sensor if the signal-to-noise ratio is sufficiently high. On Europa, an isolated cracking event from a cycloidal feature 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 be used to robustly estimate source range as well as ice and ocean layering parameters. To investigate signal-to-noise ratio issues, we have developed a Europan waveguide noise model that is based on classical ocean acoustic noise models (Kuperman and Ingenito, "Spatial correlation of surface noise in a stratified ocean," J. Acoust. Soc. Am. 67, 1988-1996, 1980). Our present simulations indicate that signal echoes from an ice-ocean interface and ocean-core interface can stand robustly above Europan ambient noise if the spatial distribution of active surface cracks is on the order of a typical cycloidal crack length.
Lee Seongsu
Makris Nicholas C.
Pappalardo Robert T.
Thode A.
Wilson Dalziel J.
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