Spectra of extremely reduced assemblages: Implications for Mercury

Abstract

We investigate the possibility that Mercury's crust is very reduced with FeO concentrations of less than ~0.1 wt%. We believe that such a surface could have a composition of enstatite, plagioclase, diopside, and sulfide, similar to the mineral assemblages found in aubritic meteorites. To test this hypothesis, we investigated the spectra of aubrites and their constituent minerals as analogs for the surface of Mercury. We found that some sulfides have distinctive absorption features in their spectra shortwards of ~0.6 μm that may be apparent in the spectrum of such an object. Determination of the surface composition of Mercury using orbital x-ray spectroscopy should easily distinguish between a lunar highlands and enstatite basalt composition since these materials have significant differences in concentrations of Al, Mg, S, and Fe. The strongest argument against Mercury having an enstatite basalt composition is its extreme spectral redness. Significant reddening of the surface of an object (such as Mercury) requires reduction of FeO to nanophase iron, thus requiring a few percent FeO in the material prior to alteration.