Geologic mapping of Europa

Galileo data enable the major geological units, structures, and surface fea tures to be identified on Europa. These include five primary units (plains, chaos, band, ridge, and crater materials) and their subunits, along with v arious tectonic structures such as faults. Plains units are the most widesp read. Ridged plains material spans a wide range of geological ages, includi ng the oldest recognizable features on Europa, and appears to represent a s tyle of tectonic resurfacing, rather than cryovolcanism. Smooth plains mate rial typically embays other terrains and units, possibly as a type of fluid emplacement, and is among the youngest material units observed. At global scales, plains are typically mapped as undifferentiated plains material, al though in some areas differences can be discerned in the near infrared whic h might be related to differences in ice grain size. Chaos material is comp osed of plains and other preexisting materials that have been severely disr upted by inferred internal activity; chaos is characterized by blocks of ic y material set in a hummocky matrix. Band material is arrayed in linear, cu rvilinear, wedge-shaped, or cuspate zones with contrasting albedo and surfa ce textures with respect to the surrounding terrain. Bilateral symmetry obs erved in some bands and the relationships with the surrounding units sugges t that band material forms by the lithosphere fracturing, spreading apart, and infilling with material derived from the subsurface. Ridge material is mapped as a unit on local and some regional maps but shown with symbols at global scales. Ridge material includes single ridges, doubler ridges, and r idge complexes. Ridge materials are considered to represent tectonic proces ses, possibly accompanied by the extrusion or intrusion of subsurface mater ials, such as diapirs. The tectonic processes might be related to tidal fle xing of the icy lithosphere on diurnal or longer timescales. Crater materia ls include various interior (smooth central, rough inner, and annular massi f) and exterior (continuous ejecta) subunits. Structural features and landf orms are shown with conventional symbols. Type localities for the units are identified, along with suggestions for portraying the features on geologic al maps, including colors and letter abbreviations for material units. Impl ementing these suggestions by the planetary mapping community would facilit ate comparisons of maps for different parts of Europa and contribute to an eventual global synthesis of its complex geology. On the basis of initial m apping results, a stratigraphic sequence is suggested in which ridged plain s form the oldest unit on Europa, followed by development of band material and individual ridges. Band materials tend to be somewhat older than ridges , but in many areas the two units formed simultaneously. Similarly, the for mation of most chaos follows the development of ridged plains; although cha os is among the youngest materials on Europa, some chaos units might have f ormed contemporaneously with ridged plains. Smooth plains generally embay a ll other units and are late-stage in the evolution of the surface. C1 crate rs are superposed on ridged plains but are crosscut by other materials, inc luding bands and ridges. Most c2 craters postdate all other units, but a fe w c2 craters are cut by ridge material. C3 craters constitute the youngest recognizable material on Europa.