Komatiites as Mercury Surface Analogues: Spectral Measurements at PEL

Abstract

X-ray fluorescence and gamma ray measurements by the NASA MESSENGER spacecraft orbiting Mercury show a surface composition strongly different from other terrestrial planets [1, 2]. The X-Ray Spectrometer (XRS) measures elemental composi-tion for the topmost 0.1 mm of Mercury’s regolith, while the Gamma Ray and Neutron Spectrometer (GRNS) is sensitive to depths of tens of centimeters. High Mg/Si and low Al/Si and Ca/Si ratios exclude a feldspathic surface composition like that of the lunar highlands. Mercury's surface mineralogy is likely dom-inated by magnesian orthopyroxene, sodium-rich pla-gioclase feldspar, and lesser amounts of Ca, Mg, and/or Fe sulfides. Enstatite chondrite melts and cer-tain enstatite achondrites provide a good compositional and mineralogical match for much of the surface of Mercury [3]. Elemental results from the GRNS [4] are consistent with those previously obtained by the X-Ray Spectrometer, including the high sulfur and low iron abundances. The elemental results from gamma-ray and X-ray spectrometry are most consistent with petro-logic models suggesting that Mercury's surface is dom-inated by magnesian silicates [5]. Mercury’s magne-sian silicates and high Mg/Si ratio (in excess of the Mg/Si ratio of terrestrial and lunar basalts [1]) invites comparison to the terrestrial ultramafic lavas known as komatiites. To assess komatiites as analogs for the surface of Mercury, at the Planetary Emissivity Laboratory (PEL) at the German Aerospace Center (DLR) we have measured visible and infrared spectra for a suite of komatiite samples under a range of environmental conditions.