Normal faulting on Europa: implications for ice shell properties

Abstract

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.

Planetary Seismology

“…Possible seismic sources on Europa include the opening of tensile cracks due to the Jovian tides, as tide-induced cracks might lead to quakes ranging from 2 to 4 in moment magnitude. Nimmo and Schenk (2006) have also identified regions of normal faults on Europa by analyzing Galileo data, indicating potential quakes up to M w ¼ 5.3 magnitude. The frequency of such proposed events remains unknown.…”

Interpretation and analysis of planetary structures

“…Unlike Mars or Earth, digital elevation models are only available for small areas on the icy satellites, with topographic information being obtained primarily through point photoclinometric and stereographic techniques on these bodies. Nimmo and Schenk (2006) used this technique on Europa to identify a normal fault scarp that was too subtle to easily identify in surface imagery. Digital elevation models created from stereo images have recently been used to investigate graben geometries and to infer properties of lithospheric thicknesses of icy satellites (Giese et al, 2006.…”

Tidal spin-down rates of homogeneous triaxial viscoelastic bodies

“…Young's modulus of ice is estimated to be a few GPa (Nimmo & Schenk 2006;see Collins et al 2010 for a review) and this is about 10 times lower than the Young's modulus for rocky materials. To explore the affect of softer icy ends on the semi-major axis drift rate, we ran a simulation of a body that is not homogeneous.…”