Mathematics – Logic
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.u12b..06p&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #U12B-06
Mathematics
Logic
6218 Jovian Satellites
Scientific paper
Europa's surface geology as viewed by Galileo imaging suggests a thin brittle lithosphere above a warm (potentially salt-rich) ice layer that is at least in part convecting, in turn situated above a liquid water ocean. This configuration is consistent with thermal and geochemical modeling, and with Galileo magnetometer and NIMS results, which suggest that Europa may have a salty global-scale subsurface ocean at relatively shallow depths (~20-30 km). Dynamical modeling and visible crater density suggests a surface age of ~50 million years, implying that Europa is probably still geologically active today. Large shallow craters and even larger multi-ringed structures imply impact into low-viscosity (warm) subsurface material. The satellite's bright plains are criss-crossed by narrow troughs and enigmatic double ridges (paired ridges separated by a medial trough); a morphological sequence (and implied evolutionary sequence) exists from isolated troughs to doublet ridges to wider and more complex ridge morphologies. Troughs are inferred as widened fractures formed though tensile and shear failure in response to global stressing of the ice shell above liquid water. Several models exist to explain ridges, but the most likely is one in which localized shear heating triggers upwelling of warm ice along fracture zones. Triple bands are ridges with diffuse ruddy margins that may have formed through thermal alteration and/or partial melting of briny ice. Wider pull-apart bands represent complete separation and spreading of the icy lithosphere, in a manner broadly analogous to terrestrial sea-floor spreading. Europa's global lineament pattern implies that nonsynchronous rotation and orbital flexing ("diurnal" stressing) have worked in tandem to deform the surface. Diurnal stressing can explain Europa's extremely enigmatic cycloid ridge and fracture patterns, and may drive rapid strike-slip faulting along ridges. Because significant tidal amplitude is necessary to produce significant diurnal stressing, this argues strongly for a subsurface liquid layer, but does not constrain its depth. Extremely slow nonsynchronous rotation of the ice shell may drive shear failure in equatorial regions, and may have opened the satellite's pull-apart bands. Mottled terrain consists of pits, domes, dark spots, patches of smooth plains, and regions of chaos terrain. Chaos is characterized by fragmented blocks of the preexisting surface, some of which have translated a few kilometers from their original positions, in a dark hummocky matrix. Mottled terrain landforms suggest vertical deformation and disruption of the surface along with localized partial melting. Their formation has been interpreted as due to diapiric upwelling--the expression of solid-state convection of warm subsurface ice--predicted to occur within an ice shell tens of kilometers thick above liquid water. Warm ice diapirs can circulate material between Europa's ocean and shallow levels within the ice shell, and can trigger local partial melting of briny ice, potentially creating near-surface biological niches. Europa's astonishing geology and its biological potential makes the satellite a high priority for future orbital and landed exploration.
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