The other ocean: probing Europa’s interior with natural ambient noise sources

Abstract

Jupiter’s moon, Europa, possesses a surface composed primarily of water ice. Gravity and magnetic data collected by the NASA Galileo Orbiter over the past five years have provided increasing evidence that an ocean exists underneath this layer, and that the combined ice/water layer thickness is 80–170 km, although the ice shell thickness remains unknown. The surface is covered by numerous fractures and ridges, believed to have formed in response to tidal deformations generated by Europa’s slightly eccentric 85-hour orbit around Jupiter. A recently published model [Hoppa et al., Science 285, 1899–1902] argues that certain cycloidal fractures must form during a single revolution, and propagate horizontally at an average speed of 3 km/h. Considerations from ice mechanics suggest that these propagating fractures would generate significant acoustic energy in the frequency range 0.5–4 Hz, where low attenuation in the ice/water environment is expected. In this presentation an acoustic sound speed profile for Europa is constructed, and standard ocean acoustic techniques are used to demonstrate how an array of geophones on the icy surface could simultaneously localize discrete events and invert for the ice-layer thickness.