Abstract: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 sulfaterich rocks at Meridiani Planum. Main Text:Introduction.

The Spirit landing site in Gusev Crater on Mars contains dark, fine-grained, vesicular rocks interpreted as lavas. Pancam and Mini-Thermal Emission Spectrometer (Mini-TES) spectra suggest that all of these rocks are similar but have variable coatings and dust mantles. Magnified images of brushed and abraded rock surfaces show alteration rinds and veins. Rock interiors contain Յ25% megacrysts. Chemical analyses of rocks by the Alpha Particle X-ray Spectrometer are consistent with picritic basalts, containing normative olivine, pyroxenes, plagioclase, and accessory FeTi oxides. Mössbauer, Pancam, and Mini-TES spectra confirm the presence of olivine, magnetite, and probably pyroxene. These basalts extend the known range of rock compositions composing the martian crust.Rocks at the Spirit landing site are finegrained with irregular vesicles and vugs, suggesting a volcanic origin. The rocks are angular and strewn across the surface, which suggests they were ejected from nearby Bonneville crater (1) by an impact event (2).Pancam and Mini-Thermal Emission Spectrometer (Mini-TES) spectra of rock surfaces indicate two end members, represented by dark-and light-toned rocks, with most rocks falling between these extremes. Three representative rocks, Adirondack, Humphrey, and Mazatzal, located several hundred meters apart, have been analyzed with the Athena instruments (3). Pancam images (4) show that Adirondack and Humphrey are partly coated and that Mazatzal is uniformly coated by light-toned material (Plates 9A, 10A, and 11A). Pancam spectra (Fig. 1) of the dark portions of Adirondack and Humphrey are consistent with the presence of olivine, which has a broad (composite) absorption band near 1000 nm. A weak band center near 930 nm suggests the presence of pyroxene or contamination by ferric oxides.The long-wavelength portion of Mini-TES spectra of dark rock surfaces resembles the Mars Global Surveyor (MGS)-TES spectra of