X-ray fluorescence spectra obtained by the MESSENGER spacecraft orbiting Mercury indicate that the planet's surface differs in composition from those of other terrestrial planets Relatively high Mg/Si and low Al/Si and Ca/Si ratios rule out a lunarlike feldspar-rich crust The sulfur abundance is at least 10 times higher than that of the silicate portion of Earth or the Moon, and this observation, together with a low surface Fe abundance, supports 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

http://science.sciencemag.org/content/sci/333/6051/1847.full.pdf

The Major-Element Composition of Mercury’s Surface from MESSENGER X-ray Spectrometry

[1] We present the analysis of 205 spatially resolved measurements of the surface composition of Mercury from MESSENGER’s X-Ray Spectrometer. The surface footprints of these measurements are categorized according to geological terrain. Northern smooth plains deposits and the plains interior to the Caloris basin differ compositionally from older terrain on Mercury. The older terrain generally has higher Mg/Si, S/Si, and Ca/Si ratios, and a lower Al/Si ratio than the smooth plains. Mercury’s surface mineralogy is likely dominated by high-Mg mafic minerals (e.g., enstatite), plagioclase feldspar, and lesser amounts of Ca, Mg, and/or Fe sulfides (e.g., oldhamite). The compositional difference between the volcanic smooth plains and the older terrain reflects different abundances of these minerals and points to the crystallization of the smooth plains from a more chemically evolved magma source. High-degree partial melts of enstatite chondrite material provide a generally good compositional and mineralogical match for much of the surface of Mercury. An exception is Fe, for which the low surface abundance on Mercury is still higher than that of melts from enstatite chondrites and may indicate an exogenous contribution from meteoroid impacts.

/pdf/chemical-heterogeneity-on-mercury-s-surface-revealed-by-the-1zsbrpa5bx.pdf

Chemical heterogeneity on Mercury's surface revealed by the MESSENGER X-Ray Spectrometer

Evidence for geochemical terranes on Mercury: Global mapping of major elements with MESSENGER's X-Ray Spectrometer

We report major element composition ratios for regions of the asteroid 433 Eros imaged during two solar flares and quiet sun conditions during the period of May to July 2000. Low aluminum abundances for all regions argue against global differentiation of Eros. Magnesium/silicon, aluminum/silicon, calcium/silicon, and iron/silicon ratios are best interpreted as a relatively primitive, chondritic composition. Marked depletions in sulfur and possible aluminum and calcium depletions, relative to ordinary chondrites, may represent signatures of limited partial melting or impact volatilization.

The elemental composition of asteroid 433 Eros: Results of the NEAR-Shoemaker X-ray spectrometer

Remote X-ray spectrometry will play a key role in the geochemical exploration of solar system bodies, provided the methodology for data analysis efficiently detects and removes solar source and flight trajectory-induced geometric variations. In this paper, we discuss the nature of such variations expected for missions to an asteroid, the Moon, and Mercury. An effective means of removing the effects of solar variability from surface measurements, as indicated by the agreement between theoretical models presented here and Apollo X-ray observations, is also discussed. We calculate X-ray spectra anticipated for these targets using probable surface compositions, solar outputs, and flight trajectories. Generally, the spectra show three distinctive regions where line intensities are clearly correlated with surface abundances: a high-energy Fe region, a moderate-energy Ca region, and a low-energy region which contains Mg, Al, and Si lines. In addition, we calculate anticipated integration times required for acceptable levels of certainty and estimate spatial resolutions achievable for those integration times for elements Mg, Al, Si, S, Ca, Ti, and Fe. Required integration times are lower (on the order of minutes or even seconds) and achievable spatial resolutions improved (on the order of kilometers) for the lower energy lines and for periods of higher solar activity. Using the Near Earth Asteroid Rendezvous (NEAR) mission to asteroid 433 Eros as an example, we describe a recommended approach for analysis of X-ray measurements based on our findings. Most importantly, we clearly demonstrate that major scientific goals for future exploration of asteroids, Mercury, and the Moon can be met by obtaining remote orbital X-ray measurements of these bodies.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JE01086

J. I. Trombka

Papers

Remote X-ray spectrometry for NEAR and future missions: Modeling and analyzing X-ray production from source to surface