Mathematics – Logic
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p71b0462f&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P71B-0462
Mathematics
Logic
5455 Origin And Evolution, 5475 Tectonics (8149), 5480 Volcanism (8450), 6218 Jovian Satellites
Scientific paper
We have completed the geologic mapping and analysis of pole-to-pole transects across the leading and trailing hemispheres of Europa. Our results show that ~50% of the mapped areas has been resurfaced since the period of background ridged plains formation (comprising the last 50-100 Myr.): ~30% by tectonic processes and ~20% by chaotic disruption. Further, the geologic record indicates a transition from tectonic- to cryovolcanic-dominated resurfacing. The style of tectonic processes changed with time, from intensive, closely spaced fracturing and ridge building forming background plains, to infilling of inter-plate gaps forming broad bands, to gradually narrower and farther-spaced ridges and ridge complexes. In both hemispheres, these lineaments rotated with time in senses consistent with nonsynchronous rotation predictions. The lack of lineaments overprinting impact structures (with the exception of Tyre) suggests that the intensity of tectonic resurfacing decreased rapidly after the formation of ridged plains. Units associated with chaotic disruption overprint one another (in areas that broadly match regions where the regional thermal gradient has been raised by tidal dissipation), fragmenting the evidence for early cryovolcanic activity. Old, subdued chaos has been reworked to form younger chaos areas by merging of small patches of disruption; the most recent chaos features appear to be slightly elevated with respect to the surrounding plains. These observations suggest that chaos formed by disruption and emplacement of buoyant material from the subsurface, which became topographically and morphologically subdued with time. One possible interpretation of the mentioned trends and changes is the gradual thickening of Europa's lithosphere throughout the visible geologic history: the degree of fracturing and plate displacements decrease in a thickening shell, while lineaments become narrower and more widely spaced; formation of chaos regions can take place where the thickness threshold for solid-state convection is exceeded, and can be aided by the preferential tidal heating of more ductile ice. In a long-term context, we cannot determine at this point whether this thickening trend would reflect a drastic change in the thermal evolution of the satellite, or more stable cycles of tectonic and cryovolcanic activity. Future global mapping and geophysical measurements of the current state of the lithosphere may provide the information needed to strengthen this emerging scenario and to solve some of the outstanding issues.
Figueredo P.
Greeley Ronald
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