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41st Lunar and Planetary Science Conference (2010) 1974.pdf

Pancam Visible/Near-Infrared Spectra of Large Fe-Ni at , . J.R. Johnson1, K.E. Herkenhoff1, J.F Bell III2, W.H. Farrand3, J. Ashley4, C. Weitz5, S.W. Squyres2, 1U.S. Geological Survey, 2255 N. Gemini Dr., Flagstaff, AZ 86001, [email protected], 2Cornell University, 3Space Science Institute, 4Arizona State Univ., 5Planetary Science Institute.

Introduction: The Panoramic Camera (Pancam) on featureless, positive slope. The “purple” patches are the (MER) darker at all wavelengths than the typical acquired visible/near-infrared (432-1009 nm) images surfaces for all rocks except Heat Shield Rock [1] of three cobble-sized Fe-Ni meteorites informally (HSR), for which the reverse is true for the named “Block Island”, “Shelter Island”, and unbrushed portions, and for the brushed spot at “Mackinac Island” between Sols 1961 (July 30, wavelengths > 600 nm. This may be a consequence 2009) and 2038 (Oct. 17, 2009) (Figure 1). Similar of the dearth of dust contamination compared to the to the Fe-Ni meteorite Heat Shield Rock (Meridiani other meteorites, based on the low 535 nm band Planum) imaged on 352 (Jan. 19, 2005) [2], the depths (Table 1) and lack of a downturn from 934 nm surfaces of these three meteorites demonstrate to 1009 nm that is observed for all other rock variable degrees of dust contamination along with surfaces (Fig. 3) and pockets of airfall dust. discontinuous coatings that exhibit Pancam reflectance spectra consistent with ferric oxides, Table 1. Average 535 nm band depths suggestive of chemical weathering on portions of the Meteorite “Purple” patch Typical surface meteorite surfaces. Heat Shield Rock 0.03 -0.18 Block Island 0.19 0.12 Observations. Block Island was imaged between Shelter Island 0.11 -0.01 Sols 1961-2003 during in situ observations and at six Mackinac Island 0.09 0.00 locations during circumnavigation of the rock. Shelter Island images were acquired during Sols Analysis and Conclusions. The spectral features 2022-2034 from five different locations, and images observed by Opportunity for the “purple” coatings on of Mackinac Island (Sols 2035-2038) were acquired Fe-Ni meteorites are consistent with a more oxidized at three locations, although only one image set surface than typical meteorite surfaces. This is likely included all Pancam geology filters. a mixture of ferric materials, dominated by nanophase (np-) hematite (particles diameters <10 Results. Pancam spectra of the meteorites suggest nm), as evidenced by strong 535 nm absorptions that a coating of variable thickness, ferric-rich combined with lack of the 860 nm band (usually nanophase dust contaminated the rock surfaces. observed in crystalline hematite [3]). This is Nonetheless, distinct color variations were observed consistent with Mössbauer data that suggest minor on each meteorite, including intermittent patches of phases of ferric oxide such as np-hematite in addition smooth materials with “purple” hues in Pancam to [5]. Preliminary APXS data suggest that false-color representations using 432 nm, 535 nm, the purple coatings are enriched in Mg, Br, and Zn 753 nm filters (and in decorrelation stretches using relative to the rest of the meteorite surface. These the same filters; Fig. 1). These materials exhibit observations suggest that the “purple” coatings are an lobate margins in Microscopic Imager (MI) images altered or secondary weathering coating, perhaps that appear to coat the underlying meteorite surface partially eroded by aeolian abrasion, rather than a (Figure 2). These surfaces consistently exhibit remnant fusion crust on the meteorite. greater 535 nm band depths and more negative near- References: [1] Bell, J.F. III et al., Science, 1703-1709, infrared slopes (753 nm to 934 nm) than the typical 306, 2004; Squyres, S.W. et al., Science, 1698-1703, 306, 2004; [2] Schröder, C., et al., JGR 113, E06S22, meteorite surface over the ~40-80 phase angle range ° doi:10.1029/2007JE002990, 2008; Squyres, S., et al., JGR observed. Larger amounts of dust contamination 111, E12S12, doi:10.1029/2006JE002771, 2006; [3] likely cause greater 535 nm band depths, particularly Morris, R.V. and Lauer, H.V., Jr, JGR, vol 95, No. B4, for Block Island (Table 1). However, typical 5101-5109, 1990; [4] Gaffey, M.J., JGR, vol. 81, no. 5, meteorite surfaces exhibit 535 nm band depths near 905-920, 1976; Britt, D.T. and Pieters, C.M., LPSC XVIII, zero (or negative), consistent with Fe-Ni meteorite 131-132, 1987; Johnson, J.R. et al., JGR, 111, E12S16, spectra. Figure 3 shows Pancam spectra of doi:10.1029/2006JE002762, 2006; [5] Fleischer, I., et al., representative surfaces taken from each meteorite In Situ Investigation of Meteorites at Meridiani compared to a laboratory spectrum of the Fe-Ni Planum, Mars, LPSC, this conference., 2010. meteorite Canyon Diablo, which exhibits a 41st Lunar and Planetary Science Conference (2010) 1974.pdf

Figure 1. Pancam images of (a) portion of Block Island (P2549, Sol 1975, image ~35 cm across)), (b) Shelter Island (P2570, Sol 2034; ~50 cm long ), Mackinac Island (P2571, Sol 2037; ~35 cm across), and (d) Heat Shield Rock (Meridiani Planum; P2596, Sol 352, ~35 cm across). From left to right, images are (L257) false-color composite using Pancam 753 nm (L2), 535 nm (L5), and 432 nm filters (L7); (L257DCS) decorrelation stretch of L257 filters; (BD535) absorption band depth at 535 nm; (R2_R6 slope) spectral slope from 754 nm (R2) to 934 nm (R6); (R721DCS) decorrelation stretch using 1009 nm (R7), 754 nm (R2), and 436 (nm) filters. Arrows point toward “purple" patches in L257DCS.

Figure 2. Microscopic Imager mosaic (~7.5 cm across) of Figure 3. Spectra of typical meteorite and “purple” patches for (left) portion of Siahs Swamp and Siahs Swamp-2 targets (Sol Block Island, Shelter Island, and Mackinac Island, and (right) Heat 1976, 1982) on Block Island, colorized using Pancam Shield rock RAT-brushed and unbrushed surfaces. (Canyon Diablo 753nm, 535nm, and 432nm filters. (See Fig. 1a.). spectrum is RELAB MI-CMP-008, spectrum 001.)