Mössbauer Spectroscopy on the Martian Surface: Constraints on Interpretation of Mer Data

Introduction: Mössbauer spectrometers will be used on martian landers and rovers to identify and quantify relative amounts of Fe-bearing minerals, as well as to determine their Fe/Fe ratios, allowing more realistic modeling of martian mineralogy and evolution. However, derivation of mineral modes, Fe/Fe ratios, and phase identification via Mössbauer spectroscopy (MS) does have limitations. We discuss here the exciting potential of MS for remote planetary exploration, as well as constraints on interpretation of remote Mössbauer data. Summary of the capabilities of the technique: The Mössbauer technique is sensitive only to the Fe atoms in the materials being studied. A Mössbauer spectrometer cannot determine the amount of total Fe in a material; it only determines relative amounts of Fe atoms in various types of sites and valence states. Two types of spectra can arise from each Fe atom in a distinctive site or valence state: a doublet for sites experiencing electric monopole and electric quadrupole interactions (paramagnetism), and a sextet when the electric monopole and magnetic dipole interactions combine (magnetically ordered). In both cases, the doublet/sextet can be defined by its isomer shift (arising from the difference in s-electron density between the source and the absorber). When the magnetic moment is zero, a doublet is formed, and the difference in energies of the two peaks is its quadrupole splitting (arising from the electric field gradient at the nucleus). If the magnetic moment is non-zero, the sextet is further characterized by a hyperfine field. The isomer shift, quadrupole splitting, and hyperfine field are characteristic of various coordination environments and valence states for various materials; these parameters are tabulated in [1]. Mineral Identification from Mössbauer Data: From the above, it is apparent that Fe atoms in a distictive site geometry and valence state will have distinctive parameters that might be used to identify phases. However, the number of mineral species is large and the range of hyperfine parameters is small, so in many cases they do not uniquely identify a phase. This is particularly true for Fe-rich phases, because the range of Mössbauer parameters known for Fe in any coordination is small. For Fe, there is a better chance of identifying phases based upon their Mössbauer parameters, although there are MANY mineral species known to have Fe in octahedral coordination. Unless unusual site distortion is present, minerals with Fe atoms in the same type of site will have roughly the same spectra. Only serendipitous combinations of minerals will allow resolution of doublets arising from distinct species [2]. Consider, for example, a rock containing a mixture of feldspar, clinopyroxene and orthopyroxene. Both Fe-bearing phases have essentially the same structure, so the doublets will overly each other; the Mössbauer spectrum of the mixture will not allow the relative contributions of each type of pyroxene to be determined (and, of course, the feldspar is invisible to the Mössbauer spectrometer). As a different example, the whole rock Mössbauer spectrum of the martian meteorite Nahkla is shown in Figure 1. Its spectrum is the superposition of two phases containing ~97% octahedral Fe. The distortion of the coordination polyhedra surrounding the Fe atoms in each of these minerals is different, so the Mössbauer doublets have distinctly different parameters, and the relative Fe contributions of olivine and pyroxene can be discerned. However, it is impossible to determine from this spectrum which of the two phases hosts the 3% Fe.