JPL/NASAProbing Europa's Interior with Natural Sound Sources
JPL/NASA
Europa is one of four Jovian moons. It was discovered by Galileo in one of his first uses of the telescope in 1610. Europa is roughly the size of earth's moon but is much brighter since, with its albedo of about 0.64, it scatters more than half the light it intercepts. Its brightness stems from the fact that its surface is completely covered with frozen water. What is remarkable about Europa is that below this ice sheet may lie a dark ocean of salty water that exceeds the volume of all terrestial oceans combined. This makes Europa probably the most likely world outside of our own for life to exist as we know it.
What are the current hypotheses about Europa's interior composition? Europa is covered with a layer of ice and possibly liquid water that is roughly 100-200 km thick above a silicate and iron core. Its radius is roughly 1500 km, about 1/4 that of the Earth. Most planetary scientists agree that the outermost surface of Europa is a brittle ice sheet that is roughly 5 km thick.
JPL/NASA
What stimulates most of the debate is what lies beneath this layer. At one extreme is a thin shelled model and at the other is a soft convective ice model. Proponents of the former argue that a liquid ocean at least 100 km deep lies directly below the brittle ice. Those in favor of the latter argue that there is no liquid ocean below but only mushy ice that extends to the moons core. The middle ground, which seems to be most popular, is taken by those who believe that about 40 km of soft convective ice lies below the brittle surface and beneath this is an ocean of liquid water that extends for 5 to 100 km to the moons core [1].
What evidence is there for an ocean on Europa? Perhaps the strongest evidence for a liquid ocean on Europa comes from magnetometer data collected during the Galileo probes orbit [2]. Apparently Europa's magnetic pole flips direction like a big compass as the moon orbits around Jupiters immense magnetic field. The magnitude of this field change indicates that the surface of Europa has a conducting layer that is at least 6 km thick. A salty ocean would fit the bill but warm slushy ice would probably not. Other evidence comes from optical images of Europa from the orbiting Voyager probe of the early 1970s and the Galileo probe of the mid 1990s. These revealed a large number of surface cracks and craters on Europa's surface, some extending for thousands of kilometers, that have apparently been healed by liquid water or soft ice oozing up from below.
How does acoustics come into the picture? Electromagnetic waves are quickly attenuated in water, so the primary tool for probing the depth and structure of our terrestial oceans is with sound. At low frequency, sound waves can propagate thousands of kilometers across an ocean. For example, humpback whales vocalizing as far away as Greenland have been readily tracked using acoustic listening stations in Bermuda. Ice-penetrating radar has been used to determine the thickness of glaciers on Earth. Our expectation, however, is that such radar waves will be scattered away long before they reach a potential ice-ocean interface on Europa. This is because the radar wavelengths are on the order of one meter while the scale of surface and subsurface fractures and anomalies is on the order of at least 100 meters.
Our plan is to use both acoustic echo-sounding and tomographic techniques to probe Europa's interior. Echo-sounding reveals the depth and composition of the seafloor and subbottom layers by analysis of the arrival time and amplitude of acoustic reflections from these interfaces. Tomography reveals the temperature structure of the ocean by the way sound waves are perturbed along forward propagation paths. Multiple sources and receivers are typically required to probe a large volume of ocean by tomography. On Europa, our goal is to use echo-sounding to determine the thickness of the ice layer and the depth of the potential ocean. We also hope to use tomography to invert data for the temperature structure of Europa's ice or water layers since this temperature information holds vital clues for the existence of life as we know it.
Europa's surface and interior structure in this figure was taken from Views of the Solar System, © copyrighted by Calvin J. Hamilton, and reproduced by permission.
We plan to exploit natural cracking events on Europa's surfaces as sound sources of opportunity [3]. Recent work shows that cycloidal cracks on the surface of Europa likely form on a daily basis [4] due to stresses induced by Europa's eccentric orbit which has a period of roughly 3.5 days. The resulting tide is expected to have an immense amplitude of roughly 30 meters leading to tensile stresses on the order of 40 kPa, equivalent to the kind of pressure experienced at roughly 400 meters below a terrestial oceans surface. These cracks typically have arcs that extend for hundreds of kilometers and probably propagate at a rate 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, inhomogeneties 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.
Estimates can be made of Europa's ice and ocean depths even with a single acoustic sensor. This can be done by first finding the range to an isolated cracking event by comparing compressional and shear wave arrivals and then exploiting subsequent echoes to determine ice and ocean thicknesses. With later Europa missions, more sensors will inevitably be available and more robust inversions involving triangulation, matched field processing and tomography can be employed.
We have investigated the robustness of these various acoustic sensing techniques with fully elastic 3-D models developed for arctic acoustic propagation and scattering [3]. One of the primary issues is that of signal-to-noise ratio. To determine conditions in which ambient noise from the sum of a large number of cracking events distributed over a wide area may overwhelm signal echoes from a distant ice-ocean or ocean-core interface, we have developed a Europan waveguide noise model that is based on classical ocean acoustic noise models. 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.
References
[1] Pappalardo, R. T., et al., "Does Europa have a subsurface ocean? Evaluation of the geological evidence," J. Geophys. Res. 104, 24015-24055 (1999).
[2] Kilvelson, M. G., K. K. Khurana, C. T. Russell, M. Volwerek, R. J. Walker, C. Zimmer, "Galileo Magnetometer Measurements: A stronger Case for a Subsurface Ocean at Europa," Science 289, 1340-1343 (2000).
[3] Lee, S., M. Zanolin, A. M. Thode, R. T. Pappalardo, N. C. Makris, "Probing Europa's Interior with Natural Sound Sources," Icarus 165, 144-167 (2005).
[4] Hoppa, G. V., B. R. Tufts, R. Greenberg, P. E. Geissler, "Formation of Cyclodial Features on Europa," Science 285, 1899-1902 (1999).