Chemostratigraphy and Fe Mineralogy of the Victoria Crater Duck Bay Section: Opportunity Apxs and Mössbauer Results

Introduction: Meridiani Planum is a vast plain of approximately horizontally bedded sedimentary rocks composed of mixed and reworked basaltic and evaporitic sands containing secondary, diagenetic minerals [e.g., 1-5]. Because bedding planes are subparallel to topography, investigation of contiguous stratigraphy requires examining exposures in impact craters. Early in the mission (sols 130-317), Opportunity was commanded to do detailed study of exposed outcrops in Endurance crater, including the contiguous Karatepe section at the point of ingress. Just over 1000 sols later and roughly 7 km to the south, the rover is being commanded to do a similar study of the Duck Bay section of Victoria crater. Here we report on the preliminary results from the Alpha Particle Xray Spectrometer (APXS) and Mössbauer instruments. Stratigraphy: The stratigraphy of Duck Bay was defined based on decorrelation-stretched false-color Panoramic Camera images that showed variation within a bright layer that forms a more-or-less continuous “bathtub ring” below an ejecta layer at the crater rim. The “bathtub ring” can be followed along the whole crater rim. At the Duck Bay ingress location, three stratigraphic units have been defined; Steno, Smith and Lyell (Figure 1). Steno is light-toned and has topographic relief expressed locally. Below Steno is the light-toned Smith unit which exhibits fine-scale lamina. In areas where relief is absent in Steno, its contact with Smith is indistinct. Lyell is a darker unit with coarser laminae. The contact with overlying Smith is distinct. APXS and Mössbauer analyses have been done on all three units, including natural and brushed surfaces, and rock interiors exposed by the Rock Abrasion Tool (RAT). Figure 1. A portion of the sol 1382 Navigation Camera mosaic showing the upper stratigraphy of the Duck Bay section, Victoria crater. APXS Results: Meridiani bedrock possibly differentially weathers, may have chemical coatings, and invariably has some surface contamination of air-fall dust and/or saltating sands [3]. Because of this, natural and brushed surfaces may not faithfully record the compositions of the underlying rock. Here, we will concentrate on APXS measurements made on holes ground by the RAT. One RAT hole each has been done on the three units the targets Steno, Smith2 and Lyell1. The compositions of rocks in Duck Bay fall within the range of data for Meridiani Planum rocks in general with one exception; Zn in Smith2 is 24% higher than in any other rock. There are variations in major element chemistry with depth in the section (Fig. 2). Compared to Steno, Smith2 has a higher silicate component, expressed by higher Al and Si, a concomitant decrease in sulfate component, a 23% increase in Cl and a 20% decrease in Ca. Lyell1 is also enriched in a silicate component and depleted in a sulfate component compared to Steno, but less so than is Smith2. Calcium is depleted in Lyell1 compared to Steno. The major distinction of Lyell1 is the more than a factorof-two increase in Cl. In this, Lyell1 is similar to outcrops encountered lower in the section in Endurance crater [3].