Mathematics – Logic
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p51a1119p&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P51A-1119
Mathematics
Logic
[6280] Planetary Sciences: Solar System Objects / Saturnian Satellites
Scientific paper
Extending for over 120 km, four fairly linear fractures (dubbed tiger stripes) stand out against the background of much shorter, but more numerous, fractures in the tectonically diverse south-polar region of Enceladus. The tiger stripes, and the plumes generated along them, have been the focus of numerous studies, leaving many of the smaller features largely ignored. Using the most recently available Cassini spacecraft images, we have created detailed maps revealing three additional relict, tiger stripe-like fracture sets among the smaller fractures. These old tiger stripe-like features have similar characteristics, but different orientations, to the present-day tiger stripes. The younger fractures have cut and deformed the older fractures, making them much less prominent than the named tiger stripes. However, the older fracture sets do retain some long, over 40 km, linear features. We believe these longer features once looked similar to the present day tiger stripes, before they were rotated away from the primary stress field in which they formed and were overprinted by younger features. By using crosscutting relationships, we were able to establish relative ages of the three old systematic fracture sets and the tiger stripes to determine the sequence of fracturing in the south polar terrain (SPT). The fracture sets show a counterclockwise progression in orientation through time which implies the causal SPT stress field created distinct fracture sets at different points in time, gradually rotating approximately 153° counterclockwise relative to the present day surface and culminating in the stress field that produced the tiger stripes. Older fracture sets clearly influenced the development of younger sets. For example, the tiger stripe Alexandria Sulcus inherited a portion of an older fracture along part of its length, creating a distinct jog along the otherwise nearly linear feature. Similar geometries occur at other locations where younger tiger stripes intersect older and well-defined fractures. The intersections also appear to correlate with the plume source locations, which may represent focused, higher-flow regions even if the whole tiger stripe length is generally eruptive. The systematic nature of the fracture sets, combined with the temporal change in orientations, requires whole-scale reorientation of the SPT relative to the extant stress field. Given the large amount of rotation, it is unlikely that the SPT rotated independently of the rest of the ice shell. We suggest that nonsynchronous rotation (NSR) may be the primary cause of fracturing in the SPT. NSR has been proposed to explain curvilinear fractures on other icy moons, such as Europa. If this is the case for Enceladus, it may hint at the presence of a global liquid ocean beneath the surface that decouples slowly rotating ice shell from the silicate interior. If the shell slowly rotated eastward, distinct fracture sets may have formed at different points in time in the SPT, superimposed on the relatively older, rotated sets. Although NSR may account for a temporal rotation of the stress field, some other process presumably is responsible for the localization of geologic activity in the SPT, such as a thermal plume in the ice beneath the south pole.
Kattenhorn Simon A.
Patthoff Alex D.
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