Major-element composition of Mercury's surface from MESSENGER X-ray Spectrometry

The MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission has been in orbit around Mercury since 18 March 2011. The X-Ray Spectrometer (XRS) onboard is revealing the abundances of major elements in the planet's surface for the first time and is thus providing fresh information on Mercury's accretional, differentiation, impact, and geological history. X-rays that emanate from the solar corona during flares excite fluorescent X-rays with characteristic energies from the top ~100 μm of Mercury's surface that are measured by XRS. Forward modelling of the resulting spectra allows elemental abundances for major-elements (Mg, Al, Si, S, Ca, Ti and Fe) to be determined. The data so far show that Mercury's bulk surface composition is distinct from those of the other terrestrial planets, with relatively high Mg/Si and low Al/Si and Ca/Si ratios that lie between those typical of basaltic and ultramafic compositions and are comparable to those for terrestrial komatiites. Derived sulfur abundances are at least ten times higher (up to ~4 wt %) than for the silicate portions of the Earth or Moon, whereas Ti and Fe abundances are low (~0.8 wt % and ~4 wt % respectively). Although the individual footprints for much of the flare data are large, there is compositional heterogeneity across the planet that can be correlated with surface reflectance and geological units. These results, together with the low Fe abundance, indicate that Mercury does not possess a feldspathic flotation crust such as that of the Moon. Rather, it suggests the surface is more likely dominated by lava flows that are products of high-degree partial melting of the mantle. The observations also support the idea that Mercury accreted from highly reduced, but not strongly volatile depleted, precursor materials similar to enstatite chondrites or anhydrous cometary dust particles. The low Fe and Ti abundances seen by XRS do not support the proposal that opaque oxides of these elements are the source of the low and variable surface reflectance of Mercury, nor can they account fully for the high neutron absorption detected during the MESSENGER flybys.