Derivation of Velocity Scales for Mars Mössbauer Data

Introduction: In terrestrial studies of rocks and soils, Mössbauer spectroscopy has not found routine application for mineral identification because other methods such as x-ray diffraction are usually more definitive. On the Mars Exploration Rovers (MERs), however, Mössbauer spectrometers [1] have been called upon to assist in defining the mineralalogy, despite the overlapping Mössbauer parameters of most rock-forming silicates, sulfates, and oxides. Published interpretations of MER Mössbauer data have revealed some very interesting results, including the presence of olivine and magnetite or hematite at Gusev [2] and olivine, hematite, and jarosite at Meridiani [3]. The strength (and uniqueness) of the interpretation of such data lies in the degree to which peak positions can be determined with accuracy and precision, and the data must be understood in the context of the conditions under which they were collected. Recognizing that interpretation of any data set is enriched by scrutiny of multiple research teams, with their different perspectives and approaches, it has been our purpose since release of the first MER data set in August 2004 [4] to enable non-MER team members easy access to MER Mössbauer data in a form that is readily usable by commonly available data analysis routines. The MERView Program: MER data for each sol are contained in an Experimental Data Record (EDR), a single file in binary format containing, among other data, Mössbauer spectra of surface material and an internal reference target, acquired in up to 13 temperature windows, as well as a drive error signal. As reported last year [5], the Windows-based computer program, MERView, was developed in response to the MER data release. With help of menus and interactive dialog boxes, it provides decimal listings and graphical displays of all the EDR spectra and exports spectral data to individual files for use by other programs. Subsequently, MERView was enhanced to compensate for the fairly large velocity non-linearity of the MER Mössbauer drives by automatically adjusting the amount of the drive error signal, ErrAmp, added to a linear (V-shaped) velocity waveform of amplitude, ±Vmax, and phase shifting by an amount, PShift, the modified waveform in channel space [6]. To accomplish this, MERView employs a least-squares criterion based on the requirement that the two mirror halves of a Mössbauer spectrum must overlap perfectly when plotted versus a properly calibrated velocity. The result is a proportional scale, in effect, Vmax = 100%. Velocity Calibration: In this report, we present the most recent enhancement to MERView, the capability to determine Vmax in mm/s, which completes the derivation of the velocity scale. To do this, we refer to the nature of the reference targets, which consist of an α-Fe foil overlaid with undetermined amounts of αFe2O3 and Fe3O4, all enriched in Fe. MERView graphs of reference spectra for MER-A (Spirit) and MER-B (Opportunity) are shown in Figures 1 and 2.