Physics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p11b0118l&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #P11B-0118
Physics
6221 Europa, 6969 Remote Sensing, 7200 Seismology, 8010 Fractures And Faults
Scientific paper
Among Europa's surface features, cycloidal cracks are probably the most important for proving the existence of a subsurface liquid ocean. This is because (1) there is strong evidence that they are caused by tidally induced stress [1], and (2) this stress likely only approaches the ice failure strength if an ocean is present. There are a number of outstanding issues, however, in quantitatively explaining cycloidal cracks. First, current estimates of the pure diurnal tidal stress necessary to cause cycloidal cracks even in the presence of an ocean [1,2] is well below the typical stress known to cause tensile failure in natural terrestrial ice [3]. Second, models of ridge formation suggest that cycloidal cracks penetrate through the entire brittle-ice layer [1,4], but current models limit the depth of tidally induced surface cracks to be less than 100 m even in the presence of an ocean [1,5]. Third, the 3-km/h crack propagation speed determined by [1] is three orders of magnitude lower than the roughly 2-km/s speed at which cracks are known to propagate in ice. Our goal is to quantitatively address these issues in a unified manner. To do this, a fracture mechanics model is developed for the initiation and propagation of a crack through an ice layer of finite thickness in the presence of gravitational overburden and porosity. It is shown that Europa's ice shell may be highly porous and salt-rich. This implies that the strength of Europa's outer ice shell may be sufficiently low to make the crack initiation strengths arrived at by current kinematic models [1,2] highly plausible, even though they are much lower than those typically measured for terrestrial ice. A model is developed for the stress intensity factor at a crack tip in an ice shell with finite thickness, gravitational overburden, and depth-dependent porosity. This leads to the conclusion that cycloids are generated as a sequence of discrete and near instantaneous fracture events, each of which penetrates through the entire brittle layer with horizontal length on the order of the brittle layer thickness. This mechanism yields an apparent propagation speed that is consistent with the 3 km/h crack propagation speed necessary to generate cycloids in current kinematic models [1,2]. An implication of this model is that the level of seismic activity should be higher by orders of magnitude in the presence of an ocean. High correlation is then expected between the level of seismic activity and the tidal period in the presence but not in the absence of an ocean. The cracks associated with cycloids that fully penetrate the brittle layer should be at least 106 times more energetic than the shallow, roughly 100-m deep, surface cracks. We show that this greatly improves the signal-to-noise ratio for the type of seismic profiling discussed in [6] if fully penetrating cracks are used as sources of opportunity. Although Europa's ice is likely highly porous, the size of vacuous pores is likely on the order of a millimeter. Since the pore size is at least three orders of magnitude smaller than the ice-penetrating radar wavelength, our calculations show that porosity-induced scattering should not be significant. [1] Hoppa et al. 1999, Science 285. [2] Crawford et al. 2005, LPSC XXXVI #2042. [3] Weeks and Cox 1984, Ocean Sci. Eng. 9. [4] Pappalardo et al. 1999, J. Geophys. Res. 97. [5] Crawford and Stevenson 1988, Icarus 73. [6] Lee et al. 2003, Icarus 165.
Lee Seongsu
Makris Nicholas C.
Pappalardo Robert T.
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