Comparison of Mössbauer Spectra of Soils from Gusev Crater and Meridiani Planum

Introduction: One of the strengths of Mössbauer spectroscopy is its ability to distinguish between the different oxidation states of iron in minerals. Such information is very useful in the determination of the degree of weathering of a planetary surface. Comparison between the mineralogy and oxidation state of the soils at Gusev and those at Meridiani lends insight into the processes that shaped the Martian surface. Methods: Data on selected soils from the PDS site were processed using MERView [1] and folded, in keeping with general conventions in this discipline. To improve statistics, data from all acquisition temperatures were added together for each measurement. As noted in [2], fits to such data should be compared with parameters corresponding to Mössbauer spectra of candidate phases that were acquired at the temperature midpoint of the data set. In addition, data from sols where the same area was studied were also added together. Data were fit using an implementation of the software described in [3]. Results: A summary of the Mössbauer parameters of selected soils is shown in Tables 1 and 2. All spectra are consistent with the presence of Fe and Fe in octahedral coordination, and in addition display one or more magnetically-split sextets due to some iron oxide. Only the paramagnetic doublets are shown here. The results are somewhat surprising. Although there is much evidence for the well-mixed nature of the Martian regolith [4, 5], there is not only a clear difference between the two sites, but a marked variability within a site. In particular, the Gusev soils have 55%-68% Fe (before f-factor correction), and the Meridiani soils show a much larger range of 47%79%. Note that this variation would not be detected by the APXS, which “sees” only the total Fe contents. The variation in Fe may be due in part to the effect of the hematitic spherules, but the soils from Meridiani studied here were specifically chosen to have relatively low amounts of iron oxides (most were chosen from the “basaltic soils” of [6]), and the Fe proportion does not appear correlated with the percentage of iron in the sextet(s). More likely, this is due to differences in regional geology of the two sites, which might be expected to produce variations in rock type, mineralogy, and bulk composition. Table 1. Mössbauer results for Gusev data sets Sol Site δ Δ Area Fe 1.17 2.89 25.9 Fe 1.16 2.16 40.9 14 (bright) Fe 0.35 0.98 27.0 Fe 1.15 2.89 25.9 Fe 1.13 2.15 37.3 47