A First Look at the Mineralogy and Geochemistry of the Mer-b Landing Site in Meridiani Planum

Introduction: The second MER rover (Opportunity) landed on Meridiani Planum on January 24, 2004 inside a shallow crater. The science rational for the selection of the landing site centered on detection of the mineral hematite from martian orbit by the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) [1,2]. Other smaller occurrences of hematite are in Aram Chaos and several isolated spots in Valles Marineris. Proposed formation pathways for martian hematite include both aqueous (e.g., lowtemperature precipitation of Fe oxides/oxyhydroxides in a lacustrine environment, laterite-style weathering, and precipitation from fluids having a hydrothermal origin) and dry (e.g., oxidation of magnetite rich ash) processes [e.g., 1,2,3]. The crystallographic c-face of martian hematite must be exaggerated to account for the thermal emissions spectra and it must be gray in color so as to account for the absence of the characteristic spectral signature of red hematite at visible wavelengths [e.g., 1,4]. Identification of accessory phases is key to understanding the formation processes for Martian hematite. Detection of goethite and/or silica rich phases would imply aqueous formation processes. Detection of magnetite could be evidence for anhydrous processes. To date, none of these precursor phases has been identified from martian orbit. Opportunity is characterizing the mineralogical and chemical composition of its landing site, in part looking for hematite accessory phases. Its field site is currently a shallow crater which has an outcrop of bedrock. Results are limited to 12 sols on the surface (4 off the lander). Pancam [5, 6]: Pancam is a multispectral stereo camera with 11 geology filters between 0.4 and 1.0 microns. Extensive multispectral imaging has been done while the rover was still on the lander. Typical early results for dusty sky, the outcrop, and soil at Meridiani Planum are shown in Figure 1. For each type of sample, there is a near continuum of similarly shaped spectra between the “brighter” and “darker” spectra for each shown in the figure. The spectra plotted are the average of the five brightest (or darkest) spectra. The y-axis is I/F (incident over flux) so that the spectra have not been corrected for viewing angle. However, this correction will not change the general spectral shape. The spectra of dusty sky, outcrop, and soil are all characterized by a ferric absorption edge extending between 440 and 750 nm. The absence of well-defined features in the absorption edge implies the presence of poorly-crystalline ferric-bearing phases similar to those found in palagonitic tephra, a martian bright regions spectral analogue [e.g., 7]. Spectra for darker outcrop and brighter soil are essentially the same. This results because the two components occur in places as mechanical mixtures of each other. Reflectivity spectra for the rock outcrop and the rock Scooby Doo at the Mars Pathfinder site are similar [e.g., 8]. Because of its soil-like bulk composition, Scooby Doo is considered to be indurated soil [e.g., 9]. All three types of materials also seem to have a very shallow band centered near 900 – 930 nm. The band position is not mineralogically diagnostic and can be associated with both ferric-bearing (e.g., goethite, schwertmannite, and maghemite) and ferrousbearing phases (e.g., orthopyroxene and pigeonite) [7]. The band position for olivine is >1000 nm. Ongoing high spatial resolution imaging of soil and rock patches may provide additional details regarding the spectral properties of thier components.