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A geologic traverse along the rim of the 22-km-diameter Endeavour Crater by the Opportunity Mars rover has provided the first field geologic observations of outcrop-scale structure and stratigraphy at a complex impact crater, characteristics that were previously undocumented due to erosion of similar-size craters on Earth. Two findings of note are (1) the attitude of sheets, foliations, and contacts between rim impact breccias and pre-impact substrate is antiformal, the limbs dipping inward toward the center of the crater inside the crater rim and outward exterior to the crater rim; and (2) coherent blocks of crust segment the rim topographically and structurally into a series of right- and left-stepping elongate rises of variable size and orientation. These segments experienced differing magnitudes of uplift during crater formation along identified vertical scissors faults. Brecciation along the faults bounding rim segments created zones of enhanced subsurface fluid transport through the crater rim, potentially responsible for localized areas of aqueous alteration identified in outcrops near segment boundaries. [ABSTRACT FROM AUTHOR]

The Spirit rover completed a 2.5 km traverse across gently sloping plains on the floor of Gusev crater from its location on the outer rim of Bonneville crater to the lower slopes of the Columbia Hills, Mars. Using the Athena suite of instruments in a transect approach, a systematic series of overlapping panoramic mosaics, remote sensing observations, sure face analyses, and trenching operations documented the lateral variations in landforms, geologic materials, and chemistry of the surface throughout the traverse, demonstrating the ability to apply the techniques of field geology by remote rover operations. Textures and shapes of rocks within the plains are consistent with derivation from impact excavation and mixing of the upper few meters of basaltic lavas. The contact between sure rounding plains and crater ejecta is generally abrupt and marked by increases in clast abundance and decimeter-scale steps in relief. Basaltic materials of the plains overlie less indurated and more altered rock types at a time-stratigraphic contact between the plains and Columbia Hills that occurs over a distance of one to two meters. This implies that regional geologic contacts are well preserved and that Earth-like field geologic mapping will be possible on Mars despite eons of overturn by small impacts. [ABSTRACT FROM AUTHOR]

Detailed examination of the topography and morphology of western Aphrodite Terra reveals numerous features that are similar to terrestrial divergent plate-boundary characteristics. Individual domains between fracturezone- like discontinuities contain a variety of bilaterally symmetrical topographic elements that suggest that topographic features have been created at rise crests, rifted and separated, and moved laterally to their present symmetrical positions. The topographic and morphologic similarities, together with strikingly similar mirror-image map patterns on both sides of the rise axis, suggest that western Aphrodite Terra shares the characteristics of oceanic divergent plate boundaries, and is the site of crustal spreading on Venus. Topographic profiles are consistent with spreading rates of the order of several centimeters per year. [ABSTRACT FROM AUTHOR]

Opportunity has investigated in detail rocks on the rim of the Noachian age Endeavour crater, where orbital spectral reflectance signatures indicate the presence of Fe+3-rich smectites. The signatures are associated with fine-grained, layered rocks containing spherules of diagenetic or impact origin. The layered rocks are overlain by breccias, and both units are cut by calcium sulfate veins precipitated from fluids that circulated after the Endeavour impact. Compositional data for fractures in the layered rocks suggest formation of Al-rich smectites by aqueous leaching. Evidence is thus preserved for water-rock interactions before and after the impact, with aqueous environments of slightly acidic to circum-neutral pH that would have been more favorable for prebiotic chemistry and microorganisms than those recorded by younger sulfate-rich rocks at Meridiani Planum. [ABSTRACT FROM AUTHOR]

The article discusses observations of the rim of a large ancient impact crater on the planet Mars, called Endeavour Crater, by the Mars exploration rover Opportunity. Data analyses showed the presence of basaltic breccias, which were produced by the impact to form the rim deposits, and stratigraphy similar to that observed at similar-sized craters on Earth. Also observed were highly localized zinc enrichments in some breccia materials, which suggests hydrothermal alteration of rim deposits, and gypsum-rich veins cut into sedimentary rocks adjacent to the crater rim, which is thought to be caused by the result of gypsum precipitated from low-temperature aqueous fluids flowing upward from the ancient materials of the rim.

Home Plate is a layered plateau in Gusev crater on Mars. It is composed of clastic rocks of moderately altered alkali basalt composition, enriched in some highly volatile elements. A coarse- grained lower unit lies under a finer-grained upper unit. Textural observations indicate that the lower strata were emplaced in an explosive event, and geochemical considerations favor an explosive volcanic origin over an impact origin. The lower unit likely represents accumulation of pyroclastic materials, whereas the upper unit may represent eolian reworking of the same pyroclastic materials. [ABSTRACT FROM AUTHOR]

Comprehensive analyses of remote sensing data during the three-year effort to select the Mars Exploration Rover landing sites at Gusev crater and at Meridiani Planum correctly predicted the atmospheric density profile during entry and descent and the safe and trafficable surfaces explored by the two rovers. The Gusev crater site was correctly predicted to be a low-relief surface that was less rocky than the Viking landing sites but comparably dusty. A dark, low-albedo, flat plain composed of basaltic sand and haematite with very few rocks was expected and found at Meridiani Planum. These results argue that future efforts to select safe landing sites based on existing and acquired remote sensing data will be successful. In contrast, geological interpretations of the sites based on remote sensing data were less certain and less successful, which emphasizes the inherent ambiguities in understanding surface geology from remotely sensed data and the uncertainty in predicting exactly what materials will be available for study at a landing site. [ABSTRACT FROM AUTHOR]

The Mars Exploration Rover Opportunity has investigated the Landing site in Eagle crater and the nearby plains within Meridiani Planum. The soils consist of fine-grained basaltic sand and a surface lag of hematite-rich spherules, spherule fragments, and other granules. Wind ripples are common. Underlying the thin soil layer, and exposed within small impact craters and troughs, are flat-lying sedimentary rocks. These rocks are finely laminated, are rich in sulfur, and contain abundant sulfate salts. Small-scale cross-lamination in some locations provides evidence for deposition in flowing liquid water. We interpret the rocks to be a mixture of chemical and siliciclastic sediments formed by episodic inundation by shallow surface water, followed by evaporation, exposure, and desiccation. Hematite-rich spherules are embedded in the rock and eroding from them. We interpret these spherules to be concretions formed by postdepositional diagenesis, again involving liquid water. [ABSTRACT FROM AUTHOR]