Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..657f&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Mathematics
Logic
Scientific paper
Introduction The miniaturized Mössbauer spectrometers (MIMOS II) on board of the two Mars Exploration rovers Spirit and Opportunity have obtained more than 600 Mössbauer spectra in backscattering geometry on over 300 different rock and soil targets [1-5]. Both instruments have simultaneously collected 6.4 keV Xray and 14.4 keV γ-ray spectra, thus providing depth selective information about the measured samples [6]. Simulation and laboratory results Mössbauer spectra of layered samples are influenced by the composition, the density and the thickness of the layers. The influence of each of these parameters can be studied in a Monte Carlo simulation, which allows the modelling of a sample composed of two homogeneous layers, each containing up to ten different mineral phases. For a given number of γ-rays, the path from the source into the sample and back to the detector is modelled, taking into account the geometry of the experimental setup of MIMOS II. Xrays and γ-rays interact nonresonantly through the photoelectric effect, Compton- and Rayleighscattering, only 14.4-keV γ-rays can be absorbed resonantly via the Mössbauer effect. Each interaction can alter the direction of movement as well as the energy of a photon. For calibration purposes, layered samples with well known composition were measured with a laboratory copy of the MIMOS II sensorhead. These samples were composed of thin sections of pure minerals (olivine, pyroxene, and epidote) or Iron foils with thicknesses between 50 and 100 μm on top of pure mineral samples with flat surfaces (pyrite, hematite, pyroxene, and epidote). These minerals were chosen because their hyperfine parameters render them easy to distinguish in Mössbauer spectra. Subspectral areas from simulated and measured spectra are in good agreement [6]. Mazatzal, Gusev crater On the Adirondack-class rock "Mazatzal", located on the plains of Gusev crater, a distinctive coating interpreted as a thin weathering rind was detected [7]. Figure 1 shows the 6.4 keV and 14.4 keV spectra obtained on the brushed surface of Mazatzal. The weathering product nanophase oxide (npOx) shows a higher subspectral area in the 6.4-keV-spectra (~ 45%) than in the 14.4-keV-spectra (~ 30%). Simulated spectra based on a model with a surface layer composed of mainly npOx and with a thickness of 10 μm compare best to measurements. Meridiani cobbles: Along Opportunity's traverse, several rock fragments with no apparent geological relationship to Meridiani Planum were analyzed. These cobbles appear to have a variety of different origins [8]. BounceRock (Sols 66-70): BounceRock is chemically and mineralogically similar to SNC meteorites [9, 10]. The 6.4 keV spectrum obtained on the undisturbed spot "Fips2" shows an enrichment of npOx in comparison to the 14.4 keV spectrum (see Figure 1). An additional magnetic phase is present, whose Mössbauer parameters are in agreement with pyrrhotite, which is also a typical accessory mineral in SNC meteorites [10]. HeatShieldRock (Sols 348-351) HeatShieldRock was classified as an iron meteorite. Mössbauer spectra show mostly kamacite with little npOx. A coating, probably a remnant of a fusion crust, is visible in Pancam spectra. For Mössbauer measurements, a field of view containing both coated and uncoated portions of the surface was chosen [11]. However, the 6.4 keV spectrum obtained on the brushed spot shows less npOx (2%) than the corresponding 14.4 keV spectrum (4%). This is inconsistent with an oxidized fusion crust on the rock. 14.4 keV and 6.4 keV spectra obtained on other cobbles show differences which can be attributed to minor alteration, surface dust contamination or the inhomogeneous nature of the sample [12]. Soils at both landing sites Generally, soil spectra obtained at both landing sites show great similarities and soil was shown to be a global component [13]. Mössbauer areas obtained from 6.4 keV and 14.4 keV spectra were plotted against each other for olivine, pyroxene, nanophase ferric oxide (npOx), hematite and magnetite (Figure 2). Measurements with the same subspectral areas in 6.4 keV and 14.4 keV spectra should plot on a line through the origin with slope 1. The plots for both landing sites show slightly larger olivine subspectral areas - in combination with slightly smaller pyroxene subspectral areas- in 6.4 keV spectra as compared to 14.4 keV spectra. The same characteristics can be observed for spectra obtained on rock targets at Gusev crater [6]. Hematite spherules at Meridiani Planum With Opportunity's Mössbauer spectrometer, several measurements were performed on hematite-rich spherules (informally also named "berries"). Different kinds of spherule distributions were analyzed (see Figure 3), e.g. four spherules (spherule diameter ~4 mm) on soil between sols 222 and 228 ("Berry Survey"), and a spot of soil covered entirely with small spherules (spherule diameter ~0.5 mm) on sol 368 ("Ripple Crest"). Differences between 14.4 keV spectra and 6.4 keV spectra may result from (1) an inhomogeneous composition of the berries or (2) different penetration depths of 14.4 keV γ-rays and 6.4 keV X-rays. For comparison, we performed laboratory experiments with two different configurations of hematite spherules and basaltic sand. For a first measurement, the spherules were placed on top of coarse basaltic sand (grain size 0.5 - 1 mm). A second measurement was made after covering the spherules with fine basaltic sand (grain size <0.1 mm). Four large (~5 mm diameter) spherules were measured in the first experiment, and enough small spherules (~1.5 mm diameter) to cover the field of view were measured in the second experiment. Fine sand did not adhere to the hematite spherules, but rather accumulated in the gaps between them, comparable to what can be observed for both Meridiani samples (Figure 3). All four spectra obtained on the Meridiani samples are comparable to the laboratory spectra obtained on sand covered spherules (see table 1). These results are in agreement with a homogeneous spherule composition (pure hematite), and a soil signature from substrate and soil accumulated between spherules. The differences between 6.4 keV spectra and 14.4 keV can be attributed to energy-dependent penetration depths, leading to varying amounts of soil visible in the spectra. References [1] Klingelhöfer, G., et al. (2003) JGR 108, E12, 8067. [2] Klingelhöfer, G., et al. (2004), Science, 306. [3] Morris, R.V., et al. (2006a), JGR, 111, E02S13. [4] Morris, R.V., et al. (2006b), JGR, 111, E12S15. [5] Morris, R.V., et al. (2008), JGR, submitted. [6] Fleischer, I., et al. (2008) JGR, in press. [7] Haskin, L., et al (2005) Nature, 436. [8] Jolliff, B., et al. (2006) LPS XXXVII, 2401. [9] Zipfel, J., et al. (2008) MAPS, submitted. [10] Meyer, C., et al. (2003) MMC. [11] Schröder, C., et al. (2008), JGR, 113, E06S22. [12] Fleischer, I., et al. (2008) LPS XXXIX, 1618. [13] Yen, A., et al. (2005), Nature, 436, 49.
Fleischer Iris
Klingelhofer Göstar
Rodionov Daniel
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