Probing Europa's interior with natural sound sources

Lee, Sunwoong; Zanolin, M.; Thode, Aaron; Pappalardo, R.; Makris, Nicholas C.

doi:10.1016/s0019-1035(03)00150-7

Cited by 59 publications

(74 citation statements)

References 45 publications

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“…Lee et al, 2003). The resulting crack propagation velocities are still much higher than those of w1ms À 1 inferred on the basis of a cycloid formation model , and suggest that the interrupted crack propagation mechanism of Lee et al (in press) may be correct.…”

Section: Discussionmentioning

confidence: 67%

“…Scholz, 2002). If the same critical strain applies to the larger (L¼30 km) Europan fault, the fault slip during an individual event will be z0.5 m. Assuming a down-dip distance of 4 km and a shear modulus of 0.4 GPa, the maximum moment release (if the entire fault moves) is 10 17 N m. This is equivalent to a seismic magnitude M s ¼5.3 (Scholz, 2002) and is considerably larger than the moments predicted by Lee et al (2003). To generate the observed maximum fault offset of 600 m will take roughly 400 events.…”

Section: Discussionmentioning

confidence: 93%

“…Identification of such faults will affect predictions of seismicity, important for future mission design (Kovach and Chyba, 2001;Lee et al, 2003). Terrestrial faults typically break when the strain exceeds z0.005% (e.g.…”

Section: Discussionmentioning

confidence: 99%

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Normal faulting on Europa: implications for ice shell properties

Nimmo¹,

Schenk²

2006

Journal of Structural Geology

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We identify two likely normal faults on Europa, of lengths z30 and 11 km. A simple flexural model of fault-related topography gives effective elastic thicknesses of 1.2 and 0.15 km, respectively, and the resulting inferred fault strength is of order 1 MPa. The maximum fault displacement: length ratio for each fault is z0.02, comparable with values on silicate planets. We combine this observation with a modified linear elastic fracture mechanics model to conclude that the shear modulus of the Europan surface must be significantly less than that for unfractured ice. The low value of the modulus is probably due to near-surface fracturing or porosity, which will affect the material's radar properties and seismic velocities. For a likely reduction in shear modulus of an order of magnitude, the driving stresses inferred are about 6e8 MPa, much higher than present-day diurnal tidal stresses. However, stresses approaching these values can be generated by non-synchronous rotation or polar wander, while stresses exceeding these values arise during ice shell freezing. If the entire larger fault breaks, it will generate an event of seismic magnitude M s z5.3.

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Section: Discussionmentioning

confidence: 67%

Section: Discussionmentioning

confidence: 93%

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Normal faulting on Europa: implications for ice shell properties

Nimmo¹,

Schenk²

2006

Journal of Structural Geology

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“…However, tidal measurements may not be sufficient to determine accurately the depth and thickness mainly because of the poorly known composition and rheological characteristics of the icy shell (Wahr et al, 2006), and in interpretations of magnetic data the depth of the ocean may be difficult to separate from the ocean conductivity. A clear picture of Europa's interior could be obtained from seismic sounding (Kovach and Chyba, 2001; Lee et al, 2003), but that would require a lander.…”

Section: Introductionmentioning

confidence: 99%

The librations, shape, and icy shell of Europa

Hoolst

Rambaux

Karatekin

et al. 2008

Icarus

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Europa, the smallest of the Galilean satellites, has a young icy surface and most likely contains an internal ocean. The primary objective of possible future missions to Europa is the unambiguous detection and characterization of a subsurface ocean. The thickness of the overlying icy shell provides important information on the thermal evolution of the satellite and on the interaction between the ocean and the surface, the latter being fundamental for astrobiology. However, the thickness is not well known, and estimates range from several hundred of meters to some ten of kilometers. Here, we investigate the use of libration (rotation variation) observations to study the interior structure of Europa and in particular its icy shell. A dynamical libration model is developed, which includes gravitational coupling between the icy shell and the heavy solid interior. The amplitude of the main libration signal at 3.55 days (the orbital period) is shown to depend on Europa's shape and structure. Models of the interior structure of Europa are constructed and the equatorial flattening of the internal layers, which are key parameters for the libration, are calculated by assuming that Europa is in hydrostatic equilibrium. Europa's flattened shape is assumed to be due to rotation and permanent tides, and we extend the classical Radau equation for rotationally flattened bodies to include also tidal deformation. We show that the presence of an ocean increases the amplitude of libration by about 10%, depending mainly on the thickness of the icy shell. Therefore, libration observations offer possibility of detection of a subsurface ocean in Europa and estimation of the thickness of its overlying icy shell.

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“…On the one hand, it needs a source to excite the free motion. Usually, some episodic events such as internal body shakes or impacts from an external object on the ice-I crust can be considered (see Lee et al 2003 for Europa case), but they must be sufficiently energetic to produce an observable amplitude. On the other hand, unless the variations of the gravitational field were large enough to be observed from an orbiter, it would be necessary to deploy a gravimeter or similar device on the surface of the body.…”

Section: Discussionmentioning

confidence: 99%

Free Translational Oscillations of Icy Bodies With a Subsurface Ocean Using a Variational Approach

Escapa

Fukushima

2011

The Astronomical Journal

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We analyze the influence of the interior structure of an icy body with an internal ocean on the relative translational motions of its solid constituents. We consider an isolated body differentiated into three homogeneous layers with spherical symmetry: an external ice-I layer, a subsurface ammonia-water ocean, and a rocky inner core. This composition represents icy bodies such as Europa, Titania, Oberon, and Triton, as well as Pluto, Eris, Sedna, and 2004 DW. We construct the equations of motion by assuming that the solid constituents are rigid and that the ocean is an ideal fluid, the internal motion being characterized by the relative translations of the solids and the induced flow in the fluid. Then we determine the dynamics of the icy body using the methods of analytical mechanics, that is, we compute the kinetic energy and the gravitational potential energy, and obtain the Lagrangian function. The resulting solution of the Lagrange equations shows that the solid layers perform translational oscillations of different amplitudes with respect to the barycenter of the body. We derive the dependence of the frequency of the free oscillations of the system on the characteristics of each layer, expressing the period of the oscillations as a function of the densities and masses of the ocean and the rocky inner core, and the mass of the icy body. We apply these results to previously developed subsurface models and obtain numerical values for the period and the ratio between the amplitudes of the translational oscillations of the solid components. The features obtained are quite different from the cases of Earth and Mercury. Our analytical formulas satisfactorily explain the source of these differences. When models of the same icy body, compatible with the existence of an internal ocean, differ in the thickness of the ice-I layer, their associated periods experience a relative variation of at least 10%. In particular, the different models for Titania and Oberon exhibit a larger variation of about 37% and 30%. This indicates an absolute difference of the order of three and two hours, respectively. This suggests that the free period of the internal oscillations might provide a new procedure to constrain the internal structure of icy bodies with a subsurface ocean.

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Probing Europa's interior with natural sound sources

Cited by 59 publications

References 45 publications

Normal faulting on Europa: implications for ice shell properties

Normal faulting on Europa: implications for ice shell properties

The librations, shape, and icy shell of Europa

Free Translational Oscillations of Icy Bodies With a Subsurface Ocean Using a Variational Approach

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