On simfitting MER Moessbauer data to characterize Martian hematite

Moessbauer spectra of Eagle Crater outcrop rocks in Meridiani Planum were acquired by the Mars Exploration Rover (MER) Opportunity. Sixty spectra, containing ∼20 to 60% hematite by area, were simultultaneously fit (simfit) in a self-consistent manner to a single chi-squared minimum, where relations among parameters from different spectra were defined for both sol (Martian day) and acquisition temperature (200-280 K). Different spectral models were compared, hematite being modeled optimally with two sextets. Sextet S1 (∼35% of total sextet area) has narrower linewidths, a larger magnetic hyperfine field, and a quadrupole shift that changes smoothly from positive to negative values as the temperature increases through the bulk Morin transition temperature. Sextet S2 has broader linewidths, a likely skewed line shape, a smaller hyperfine field, and a quadrupole shift that remains negative at all temperatures, implying the S2 phase is weakly ferromagnetic at all temperatures.