Abstract. It has been proposed that Jupiter's satellite Europa currently possesses a global subsurface ocean of liquid water. Galileo gravity data verify that the satellite is differentiated into an outer H20 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

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Abstract. 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.

Abstract. 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).

Dark mantle deposit compositions derived from SSI data are consistent with Earth-based observations of similar near-side deposits and are interpreted to be pyroclastic materials. However, the modernitc albedo and 1 gin absorption of the dark mantle deposit on the southwest margin of the Orientale basin suggest it is a local pyroclastic deposit contaminated with underlying highland materials from the Orientale impact.