Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p21a0216c&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #P21A-0216
Physics
5400 Planetology: Solid Surface Planets, 5410 Composition, 5464 Remote Sensing, 6225 Mars
Scientific paper
The thermal infrared spectral signature of bulk, grey hematite was the chemical "beacon" that focused the selection of Meridiani Planum as the landing site for the Mars Exploration Rover Opportunity, and aqueous processes were favored for its formation. Orbital data suggesting more bound water in accessory minerals at this location also supported this interpretation. After landing January 24, 2004, the Mini-TES instrument rapidly confirmed the thermal spectral signature of bulk hematite in soils on the plains surrounding Eagle crater and unevenly distributed within the crater. Observations within Eagle crater soon uncovered unusual spherical grains in abundance surrounding the outcrop and Microscopic Imager (MI) showed these grains eroding from within these rocks. They were dubbed "blueberries" by the team due to their spherical nature and their grey or blue appearance compared to their surroundings in various color composites of Pancam images. Extensive observations with the Mössbauer, Alpha-Particle X-ray Spectrometer (APXS) and Mini-TES instruments, especially of the "Berry Bowl" (i.e. with berries and adjacent berry-free rock), confirmed that these spherules are dominantly composed of hematite. Pancam spectra of individual spheres also match laboratory spectra of hematite. These spheres are found within and around outcrop rocks, across the plains of Meridiani, and rolling into the interior of Endurance crater. They are ubiquitous and remarkably even in size and tone. Grains are spherical to subspherical typically 2 to 6 mm in diameter. Opportunity has performed numerous operations with the Rock Abrasion Tool (RAT), and has sliced through individual spherules. The appearance after ratting shows limited or no interior structure and they remain uniformly grey in color. They can preserve scratches from the grinding wheel or become dislodged by ratting, indicating they are harder than the host rock. Several lines of evidence suggest the spheres are post-depositional diagenetic products. On Earth, oolitic iron occurs as concretions in Utah and in the Clinton Formation outcropping from New York into Alabama. Ferromanganese nodules occur in abundance on the modern sea floor and are found in the Great Lakes. Although diagenetic in origin, these terrestrial analogs have strong contrasts with what is observed on Mars, including diverse size, shape and composition of terrestrial samples, significant interior lamination and presence of nucleation centers that are lacking in the Martian spheres. The uniformity of composition and size of the Martian examples provide clues to the redox state of fluids at the time of formation and by analogy with seafloor nodules a rough estimate of the time required for growth.
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