Patricio Hernan Figueredo

Geologic mapping of Europa

Galileo data enable the major geological units, structures, and surface features to be identified on Europa. These include five primary units (plains, chaos, band, ridge, and crater materials) and their subunits, along with various tectonic structures such as faults. Plains units are the most widespread. Ridged plains material spans a wide range of geological ages, including the oldest recognizable features on Europa, and appears to represent a style of tectonic resurfacing, rather than cryovolcanism. Smooth plains material 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, although in some areas differences can be discerned in the near infrared which might be related to differences in ice grain size. Chaos material is composed of plains and other preexisting materials that have been severely disrupted by inferred internal activity; chaos is characterized by blocks of icy material set in a hummocky matrix. Band material is arrayed in linear, curvilinear, wedge-shaped, or cuspate zones with contrasting albedo and surface textures with respect to the surrounding terrain. Bilateral symmetry observed in some bands and the relationships with the surrounding units suggest 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, doublet ridges, and ridge complexes. Ridge materials are considered to represent tectonic processes, possibly accompanied by the extrusion or intrusion of subsurface materials, such as diapirs. The tectonic processes might be related to tidal flexing of the icy lithosphere on diurnal or longer timescales. Crater materials include various interior (smooth central, rough inner, and annular massif) and exterior (continuous ejecta) subunits. Structural features and landforms are shown with conventional symbols. Type localities for the units are identified, along with suggestions for portraying the features on geological maps, including colors and letter abbreviations for material units. Implementing these suggestions by the planetary mapping community would facilitate comparisons of maps for different parts of Europa and contribute to an eventual global synthesis of its complex geology. On the basis of initial mapping results, a stratigraphic sequence is suggested in which ridged plains 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 formation of most chaos follows the development of ridged plains; although chaos is among the youngest materials on Europa, some chaos units might have formed contemporaneously with ridged plains. Smooth plains generally embay all other units and are late-stage in the evolution of the surface. C 1 craters are superposed on ridged plains but are crosscut by other materials, including bands and ridges. Most c2 craters postdate all other units, but a few c2 craters are cut by ridge material. C3 craters constitute the youngest recognizable material on Europa.

Geologic mapping of the northern leading hemisphere of Europa from Galileo solid‐state imaging data

The northern leading hemisphere of Europa was imaged at regional mapping resolution (∼230 m/pixel) by the Galileo spacecraft SSI camera. We produced geologic maps from a regional-scale mosaic and a high resolution inset of this region. Twelve geologic units were sufficient to produce correlative geologic maps at both regional and local scales. Stratigraphic relationships indicate four major episodes in the geologic history of this area: The first episode includes background plains formation and modification, followed by a second period of extensive lineament formation. The third episode included extensive chaotic disruption of the surface at low and middle latitudes. The final episode includes the formation of new sets of ridges and bands, especially at high latitudes. The low crater density indicates that all these episodes in Europas geologic history are geologically recent. The latitudinal distribution of chaos areas broadly matches that of areas of crustal thinning from models of tidal heating. We show that stress directions rotated clockwise with time, which is consistent with predictions from global stress models involving tidal deformation and nonsynchronous rotation of Europas crust. On the basis of the change in lineament orientation with time, the reconstructed longitudinal positions of the studied area indicate that Europas crust completed a full rotation relative to the tidally deformed interior.

Locating potential biosignatures on Europa from surface geology observations.

We evaluated the astrobiological potential of the major classes of geologic units on Europa with respect to possible biosignatures preservation on the basis of surface geology observations. These observations are independent of any formational model and therefore provide an objective, though preliminary, evaluation. The assessment criteria include high mobility of material, surface concentration of non-ice components, relative youth, textural roughness, and environmental stability. Our review determined that, as feature classes, low-albedo smooth plains, smooth bands, and chaos hold the highest potential, primarily because of their relative young age, the emplacement of low-viscosity material, and indications of material exchange with the subsurface. Some lineaments and impact craters may be promising sites for closer study despite the comparatively lower astrobiological potential of their classes. This assessment will be expanded by multidisciplinary examination of the potential for habitability of specific features.

Geology and mapping of dark terrain on Ganymede and implications for grooved terrain formation

Geological mapping of regional and high-resolution Galileo images reveals a variety of units and structures within Ganymedes dark terrain. We have made a detailed study of areas within Galileo, Marius, and Nicholson Regiones in order to investigate the style of tectonic deformation experienced by dark terrain adjacent to swaths of grooved terrain. Dark terrain appears to become fractured as a precursor to grooved terrain formation; in places, dark fractured swaths are recognized which have similar characteristics to brighter grooved terrain swaths. Tectonic deformation may be, but is not always, focused through preexisting weaknesses caused by impact craters and furrows. A prominent groove lane, Anshar Sulcus, is inferred to have formed by the process of hanging wall rollover, accompanied by a small amount of right-lateral horizontal shear offset resulting from NE-SW extension.