MAJOR ELEMENTS ON MERCURY'S SURFACE FROM MESSENGER X-RAY SPECTROMETRY

Elemental abundances at the surface of a rocky planet reflect the composition of the original materials from which the planet formed, as well as the accretion, differentiation, impact, and geological processes that have shaped the surface over billions of years. Thus, a major scientific objective of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission, in orbit around Mercury since 18 March 2011, is to characterize the chemical composition of the planet’s surface. We report the abundances of major rock-forming elements Mg, Al, Si, S, Ca, Ti, and Fe on the surface of Mercury, based on data acquired with the MESSENGER X-ray Spectrometer (XRS), which detects fluorescent 1 to 10 keV X-ray emissions that are excited by solar X-rays from within the top ~100 μm of the regolith. The high solar X-ray flux during flares allows for detection of elements with atomic numbers up that of Fe. XRS data acquired during 10 solar flares indicate that the planet’s surface differs in composition from those of other terrestrial planets. Mercury’s Mg/Si, Al/Si, and Ca/Si ratios are intermediate between typical basaltic compositions and more ultramafic compositions comparable to terrestrial komatiites. These ratios rule out a lunar-like feldspar-rich crust. The sulfur abundance is at least 10 times higher (up to ~4 wt%) than that of the silicate portion of Earth or the Moon. Fe and Ti abundances are low, less than ~4 wt% and ~0.8 wt%, respectively. The observations support the view that Mercury formed from highly reduced precursor materials, perhaps akin to enstatite chondrite meteorites or anhydrous cometary dust particles. Low Fe and Ti abundances do not support the proposal that opaque oxides of these elements contribute substantially to Mercury’s low and variable surface reflectance and indicate that other elements contribute to the relatively high degree of thermal neutron absorption observed on Mercury. The XRS data indicate compositional heterogeneity across the planet, although the large field of view for many of the observations makes direct correlation with identified geological units difficult. However, the highest Mg/Si, Ca/Si, and S/Si ratios are found in areas that include substantial amounts of low-reflectance material, supporting the possibility that sulfides contribute to the low reflectance.