GEOLOGY OF EUROPA AND NEXT STEPS IN ITS EXPLORATION

Europa's surface geology suggests an ice shell ~20 km thick, consisting of a thin brittle lithosphere above a warm, salt-rich ice layer that is at least in part convecting, in turn above a liquid water ocean. Dynamical modeling and visible crater density suggests a surface age of ~50 million years, implying that Europa is probably still active today. Large shallow craters and larger multi-ringed structures imply impact into low-viscosity subsurface material. Bright plains are crossed by narrow troughs and enigmatic double ridges; a morphological sequence exists from troughs to doublet ridges to wider and more complex ridges. Troughs are inferred as widened fractures formed by tensile and shear failure in response to global stressing. Localized shear heating may trigger upwelling of warm ice along fracture zones to form ridges. Wider pull-apart bands represent separation and spreading of the icy lithosphere, in a manner analogous to terrestrial sea-floor spreading. Europa's global lineament pattern implies that nonsynchronous rotation and orbital (diurnal) stressing have worked in tandem to deform the surface. Diurnal stressing explains Europa's extremely enigmatic cycloid ridge and fracture patterns, and may drive rapid strike-slip faulting. Significant tidal amplitude is necessary to produce significant diurnal stressing, arguing strongly for a subsurface liquid layer without constraining its depth. Slow nonsynchronous rotation of the ice shell may drive shear failure in equatorial regions. 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. These landforms suggest vertical deformation and disruption along with localized partial melting. Their formation is likely linked to diapiric upwelling--the expression of solid-state convection of warm subsurface ice--predicted within a floating ice shell >=20 km thick. Europa's astonishing geology and its biological potential makes the satellite a high priority for future orbital and landed exploration. An orbiting Europa Geophysical Explorer, recently recommended by the National Accadamy of Sciences, is the appropriate next step.