Characterisation of the magnetic iron phases in Clovis Class rocks in Gusev crater from the MER Spirit Mössbauer spectrometer

Abstract

Mössbauer backscattering spectra of eight Martian rocks, acquired by the MIMOS II spectrometer of Rover Spirit (MER-A) and containing goethite in addition to other iron minerals, have been selected for in-depth numerical analysis. Where feasible, different temperature windows for a given rock were considered. A novel calibration/folding procedure, exclusively based on the fitted positions of the eight prominent absorption lines in the transmission spectra of the reference target and not relying on the error signal of the MIMOS II spectrometer, has been developed. It is demonstrated that this procedure yields reliable and reasonably accurate values for the adjusted Mössbauer parameters of the respective spectra of the rock targets. These spectra are all composed of magnetically split components arising from hematite, magnetite and goethite phases, in addition to a ferrous and a ferric doublet that can be ascribed to the presence of Fe silicates, possibly glasses. It is argued that the Fe 3+ doublet has no significant contribution from superparamagnetic Fe-oxide particles. The hematite components reflect the coexistence of antiferromagnetic and weakly ferromagnetic spin states. In general the magnetite content in the selected rocks is low. The goethite subspectra are very broad and asymmetric and need to be described by a model-independent distribution of hyperfine fields. The as-such obtained parameter values indicate an average particle size of the order of 10 nm. For all examined rock spectra, the adjusted parameter values for the various Fe oxides are completely in line with those known for terrestrial (natural or synthetic) species.