Seeing the Soils of Meridiani Planum Through the Eyes of Pancam and Microscopic Imager

We are using data from the Pancam and Microscopic Imager (MI) on the Opportunity rover to characterize the soil grains at Meridiani Planum. We have traced individual grains in all MI images of the soils using the software application ImageJ distributed by NIH, and subsequently derived size and shape properties about the grains. The resolution of the MI is 31 microns per pixel [1] so we limit our measurements to those grains larger than about 0.3 mm in size. In cases where the grain is partially or substantially buried by other grains or finer soil particles, we do not make a measurement. False-color composites from Pancam images that cover the same location imaged by MI are made from the Left 2,5,6 (753, 535, 482 nm) filters or Right 2,7,1 (753, 1009, 430 nm) filters [2] in the Red, Green, and Blue channels, respectively. These color images are then merged with the MI images to illustrate color properties of particular grains. Pancam spectra are also extracted from grains when there is sufficient spatial coverage. in diameter. Figure 2 illustrates the dominance of these small grains at this particular location, which happens to be on the southern wall of Eagle crater. The Pancam color merge with this MI image suggests that the small spherules are more consistent with the basalt grains than the blueberries (spherulitic concretions derived from outcrop rocks [7]). The resolution of Pancam images of this location is on the order of 0.5 mm so the grains are only barely resolved. A Mossbauer measurement taken on an adjacent soil (Sol 53 Vanilla) that is composed solely of these smaller spherules (Fig 1) is consistent with a basaltic composition for the grains. Their concentration at this particular location in a brighter, elongate patch along the southeastern wall compared to elsewhere inside Eagle crater suggests wind activity favored their transport and subsequent deposition here. Their spherical shape is also possibly the result of wind action rounding them during transport, though water action cannot be ruled out.