Synthesis and characterization of Fe-poor olivine with applications to the surface of Mercury

Abstract Results from X-ray remote sensing aboard NASA’s MErcury Surface Space ENvironment GEochemistry and Ranging (MESSENGER) spacecraft have demonstrated that Mercury has a low, but measurable, concentration of Fe on its surface. However, ultraviolet to near-infrared spectroscopic measurements of the mercurian surface do not show the 1 μm absorption band characteristic of ferromagnesian silicates. This observation is consistent across multiple Fe-bearing terranes with a range of ages, suggesting the Fe present on Mercury’s surface may not be stored within silicate phases. To further constrain the possible mineralogy and composition of Fe-bearing phases on Mercury, we used various spectroscopic techniques to characterize synthetic olivine with minor amounts of Fe (i.e., Fo99.62–Fo99.99) and more Fe-rich natural olivines. Our results indicate that the distinctive 1 μm absorption band of olivine is detectable in reflectance spectra of olivine at a concentration as low as 0.03 wt% FeO and 0.01 wt% in continuum removed data. Additionally, MESSENGER’s lack of a 1 μm absorption, taking into account Mercury Dual Imaging System (MDIS)’s limited spectral resolution and Mercury Atmospheric and Surface Composition Spectrometer (MASCS)’s high signal-to-noise ratio, suggests there is <0.38 wt%, and likely <0.01 wt%, FeO on the surface of Mercury. Because the 1 μm band is not observed in surface spectra, these results indicate that the Fe observed on the surface of Mercury is not bound in an olivine structure. Rather, we posit that Fe is present as nano-phase and macroscopic Fe-rich metal or Fe-sulfide that formed as a result of space weathering and igneous smelting processes. Looking forward to ESA/JAXA’s BepiColombo mission that has a planned Mercury orbit arrival time in December 2025, Mercury Radiometer and Thermal Infrared Imaging Spectrometer (MERTIS) mid-infrared spectra should provide a mineralogical detection or absence of olivine where MIR spectral features are still present even in synthetic olivines with minor amounts of Fe (Fo99.99).