S. D. Kadel

Does Europa have a subsurface ocean? Evaluation of the geological evidence

It has been proposed that Jupiters satellite Europa currently possesses a global subsurface ocean of liquid water. Galileo gravity data verify that the satellite is differentiated into an outer H2O layer about 100 km thick but cannot determine the current physical state of this layer (liquid or solid). Here we summarize the geological evidence regarding an extant subsurface ocean, concentrating on Galileo imaging data. We describe and assess nine pertinent lines of geological evidence: impact morphologies, lenticulae, cryovolcanic features, pull-apart bands, chaos, ridges, surface frosts, topography, and global tectonics. An internal ocean would be a simple and comprehensive explanation for a broad range of observations; however, we cannot rule out the possibility that all of the surface morphologies could be due to processes in warm, soft ice with only localized or partial melting. Two different models of impact flux imply very different surface ages for Europa; the model favored here indicates an average age of ∼50 Myr. Searches for evidence of current geological activity on Europa, such as plumes or surface changes, have yielded negative results to date. The current existence of a global subsurface ocean, while attractive in explaining the observations, remains inconclusive. Future geophysical measurements are essential to determine conclusively whether or not there is a liquid water ocean within Europa today.

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.

Erosion by flowing lava: Field evidence

Erosion of substrate materials by melting or mechanical means has been suggested in active lava flows on Earth and other planets. Although there are many references to lava erosion on Earth, unambiguous evidence is rare; geological relationships commonly cited as evidence of downcutting by lava can be explained without recourse to erosion. In order to assess possible erosion by flowing lava we carried out field studies of tube-fed basalt flows, sheet flows of the Columbia River Basalt Province (CRB), and Precambrian komatiites. Unequivocal evidence for thermal erosion (melted dacite substrate) was found at the Cave Basalt lava tube, Mount St. Helens, for which fluid dynamic analysis indicates laminar flow, although erosion was enhanced in areas of locally steep slopes, possibly as a result of localized turbulence. Other lava tubes in our study display strong, but inconclusive, evidence for erosion. Komatiite flows display good evidence for erosion of their substrate, possibly in a turbulent regime, but assessment of the extent of erosion is hampered by limited and disrupted exposures. No evidence for thermal erosion was found in the CRB. Our findings suggest that an erosional origin for planetary sinuous rilles and canali would be favored by high Reynolds number flows (high mass flux, low-viscosity lava, steep slopes) and substrates having a lower melting temperature than the lava or low mechanical strength (e.g., regolith).

Geological history of the Tyre region of Europa: A regional perspective on Europan surface features and ice thickness

Galileo images of the Tyre Macula region of Europa at regional (170 m/pixel) and local (∼40 m/pixel) scales allow mapping and understanding of surface processes and landforms. Ridged plains, doublet and complex ridges, shallow pits, domes, “chaos” areas, impact structures, tilted blocks and massifs, and young fracture systems indicate a complex history of surface deformation on Europa. Regional and local morphologies of the Tyre region of Europa suggest that an impactor penetrated through several kilometers of water ice to a mobile layer below. The surface morphology was initially dominated by formation of ridged plains, followed by development of ridge bands and doublet ridges, with chaos and fracture formation dominating the latter part of the geologic history of the Tyre region. Two distinct types of chaos have been identified which, along with upwarped dome materials, appear to represent a continuum of features (domes-platy chaos-knobby chaos) resulting from increasing degrees of surface disruption associated with local lithospheric heating and thinning. Local and regional stratigraphic relationships, block heights, and the morphology of the Tyre impact structure suggest the presence of low-viscosity ice or liquid water beneath a thin (several kilometers) surface ice shell at the time of the impact. The very low impact crater density on the surface of Europa suggests that this thin shell has either formed or been thoroughly resurfaced in the very recent past.

Multispectral studies of western limb and farside maria from Galileo Earth‐Moon Encounter 1

New visible and near-infrared multispectral images of the Moon obtained by the Galileo solid-state imaging system, along with lunar orbiter images (for crater counts), and spectral mixing analyses were used to characterize western limb and eastern farside maria and determine compositional and age relationships in selected regions. Results indicate that (1) western limb mare deposits have less variability in titanium content ( 6 wt% TiO2) mare basalts were observed on the western limb and farside, which may reflect the inability of such denser magmas to penetrate the thicker farside crust.