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The prospect of a future soft landing on the surface of Europa is enticing, as it would create science opportunities that could not be achieved through flyby or orbital remote sensing, with direct relevance to Europa's potential habitability. Here, we summarize the science of a Europa lander concept, as developed by our NASA-commissioned Science Definition Team. The science concept concentrates on observations that can best be achieved by in situ examination of Europa from its surface. We discuss the suggested science objectives and investigations for a Europa lander mission, along with a model planning payload of instruments that could address these objectives. The highest priority is active sampling of Europa's non-ice material from at least two different depths (0.5-2 cm and 5-10 cm) to understand its detailed composition and chemistry and the specific nature of salts, any organic materials, and other contaminants. A secondary focus is geophysical prospecting of Europa, through seismology and magnetometry, to probe the satellite's ice shell and ocean. Finally, the surface geology can be characterized in situ at a human scale. A Europa lander could take advantage of the complex radiation environment of the satellite, landing where modeling suggests that radiation is about an order of magnitude less intense than in other regions. However, to choose a landing site that is safe and would yield the maximum science return, thorough reconnaissance of Europa would be required prior to selecting a scientifically optimized landing site.

The transection and superposition relationships among channels, chaos, surface materials units, and other features in the circum-Chryse region of Mars were used to evaluate relative age relationships and evolution of flood events. Channels and chaos in contact (with one another) were treated as single discrete flood-carved systems. Some outflow channel systems form networks and are inferred to have been created by multiple flood events. Within some outflow channel networks, several separate individual channel systems can be traced to a specific chaos which acted as flood-source area to that specific flood channel. Individual flood-carved systems were related to widespread materials units or other surface features that served as stratigraphic horizons. Chryse outflow channels are inferred to have formed over most of the perceivable history of Mars. Outflow channels are inferred to become younger with increasing proximity to the Chryse basin. In addition, outflow channels closer to the basin show a greater diversity in age. The relationship of subsequent outflow channel sources to the sources of earlier floods is inferred to disfavor episodic flooding due to the progressive tapping of a juvenile near-surface water supply. Instead, we propose the circum-Chryse region as a candidate site of past hydrological recycling. The discharge rates necessary to carve the circum-Chryse outflow channels would have inevitably formed temporary standing bodies of H2O on the Martian surface where the flood-waters stagnated and pooled (the Chryse basin is topographically enclosed). These observations and inferences have led us to formulate and evaluate two hypotheses: (1) large amounts of the sublimated H2O off the Chryse basin flood lakes precipitated (snowed) onto the flood-source highlands and this H2O was incorporated into the near surface, recharging the H2O sources, making possible subsequent deluges; and (2) ponded flood-water in Chryse basin drained back down an anti basinward dipping subsurface layer accessed long the southern edge of the lake, recharging the flood-source aquifers. H2O not redeposited in the flood-source region was largely lost to the hydrologic cycle. This loss progressively lowered the vitality of the cycle, probably by now killing it. Our numerical evaluations indicate that of the two hypotheses we formulated, the groundwater seep cycle seems by far the more viable. Optimally, approximately 3/4 of the original mass of an ice-covered cylindrical lake (albedo 0.5, 1 km deep, 100-km radius, draining along its rim for one quarter of its circumference into substrata with a permeability of 3000 darcies) can be modeled to have moved underground (on timescales of the order of 10(3) years) before the competing mechanisms of sublimation and freeze down choked off further water removal. Once underground, this water can travel distances equal to the separation between Chryse basin and flood-source sites in geologically short (approximately 10(6) year-scale) times. Conversely, we calculate that optimally only approximately 40% of the H2O carried from Chryse can condense at the highlands, and most of the precipitate would either collect at the base of the highlands/lowlands scarp or sublimate at rates greater than it would accumulate over the flood-source sites. Further observations from forthcoming missions may permit the determination of which mechanisms may have operated to recycle the Chryse flood-waters.

This paper provides a synopsis of our latest understanding of the regional geological setting, mineralisation and ore genesis of the Neoproterozoic Rosh Pinah Zn-Pb deposit in southwestern Namibia. During continental rifting at ∼740 Ma, an intracratonic rift (Rosh Pinah Graben) became detached from the main Gariep depository by a basement high, The rift subsequently fared and was filled mainly by a volcanoclastic rift-fill ,sequence, containing predominantly siliciclastic and subordinate calcareous rocks, and was accompanied by contemporaneous bimodal but dominantly felsic volcanism. Volcanism may have provided the heat engine for contemporaneous sedimentary-exhalative and hydrothermal replacement base metal deposits, which formed within anoxic, carbonaceous mudstone horizons in the deep rift. Mineralisation consists mainly of sphalerite, pyrite, galena and chalcopyrite within mudstone, with interbedded, dolomitic and Ba-rich carbonate lenses, and lesser bornite, tennantite-tetrahedrite, stromeyerite, and native gold. Pervasive hydrothermal alteration has affected the surrounding host rocks, and whole-rock lithogeochemical trends indicate that as the ore body is approached, increases in the elements Mn, Fe, Cu, Zn, Ba and Pb are apparent, with barium being the most extensively dispersed, followed by copper and zinc. Evidence for the presence and locality of a potential hydrothermal vent zone is provided by preliminary metal-zoning studies, which is conformed by the spatial distribution of dominant brecciation and silicification, and the position of peripheral barium-rich carbonate ore bodies. Vertical zoning is indicated by elevated copper contents in the footwall breccia zone, and increases in lead and zinc towards the massive sulphide zones occurring near or at the hanging wall contact. Fluid inclusion studies reveal the presence of two chemically distinct fluid populations -- a pre-orogenic, rift-related Rosh Pinah ore-forming fluid dominated by MgCl2- and CaCl2-rich fluids, probably representing hypersaline brines induced by rift-related magmatism, and a general orogenic fluid dominated by a NaCl-CaCl2-H2O (with minor MgCl2) system. The association of the Rosh Pinah deposit with basin-margin rifting, second-order basins, anoxic sediments, and the finely handed/laminated nature of the ore are indicators of possible sedimentary exhalative (SEDEX) origins for the mineralisation. The close association of the sulphides with silicification and brecciation may also indicate significant post-depositional replacement and/or impregnation. [ABSTRACT FROM AUTHOR]