Geology of Europa as Revealed by Galileo Imaging

R. T. Pappalardo (Brown Univ.), R. Greeley (ASU), J. W. Head (Brown Univ.), R. J. Sullivan (ASU), M. H. Carr (USGS Menlo Park), C. R. Chapman (Southwest Res. Inst.), R. Greenberg, P. Geissler, B. R. Tufts, C. Phillips, A. McEwen (Univ. of AZ), M.J.S. Belton (NOAO), Galileo Imaging Team

Galileo imaging of Europa reveals a sparsely cratered surface and a complex array of dark and bright bands, ridges, and troughs. "Pull-apart" features are bands that indicate separation and spreading of the satellite's lithosphere, as they can be reconstructed through translation and rotation of lithospheric blocks. Reconstruction of a 5 km wide ridge imaged at high resolution suggests a similar extensional origin for some ridges. At high resolution, Europa's ubiquitous ridges commonly exhibit a crestal depression, creating a "double ridge" morphology. Several models of ridge formation are plausible, including extrusion of mobile material along cracks. Thermal alteration of the surface and mass wasting may play roles in the evolution of triple bands. The morphologies of pits, domes, and spots 7-15 km in diameter suggest upwarping and subsequent collapse of the surface, and local extrusion. Diapirism may account for these features, and solid-state convection of warm subsurface ice is a possible driver. A region of chaotic terrain imaged at high resolution is seen to be comprised of small-scale blocks which have translated and rotated with respect to one another, suggesting a shallow depth to a mobile layer at the time of deformation. Several lines of tectonic evidence, including inferred flexure-induced cracks parallel to ridges, suggest that Europa had a very thin brittle lithosphere (kilometers to only hundreds of meters thick) and high heat flow at the time of deformation. An extremely low crater density suggests that Europa's surface features are relatively young, and that the satellite may be geologically active today.