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

▪ Abstract Visible and near-infrared spectra of reflected sunlight from asteroid surfaces exhibit features that hold the promise for identifying surface mineralogy. However, the very surfaces that are observed by remote-sensing are also subject to impingement by micrometeoroids and solar wind particles, which are believed to play the dominant role in space weathering, which is the time-dependent modification of an asteroid's reflectance spectrum. Such space weathering has confused the interpretations of telescopic spectra of asteroids, especially concerning the possible association of common ordinary chondritic meteorites with so-called S-type asteroids. Recent spacecraft studies of asteroids (especially of Eros by NEAR-Shoemaker) have documented aspects of space weathering processes, but we still do not understand the physics of space weathering well enough to confidently assay mineralogy of diverse asteroids by remote-sensing. A review of the intellectual history of this topic reveals the complexity of ...

Space weathering of asteroid surfaces

Extensive collection efforts in Antarctica and the Sahara in the past 10 years have greatly increased the number of known meteorites. Groupings of meteorites according to petrologic, mineralogical, bulkchemical, and isotopic properties suggest the existence of 100–150 distinct parent bodies. Dynamical studies imply that most meteorites have their source bodies in the main belt and not among the near-Earth asteroids. Spectral observations of asteroids are currently the primary way of determining asteroid mineralogies. Linkages between ordinary chondrites and S asteroids, CM chondrites and C-type asteroids, the HEDs and 4 Vesta, and iron meteorites, enstatite chondrites, and M asteroids are discussed. However, it is difficult to conclusively link most asteroids with particular meteorite groups due to the number of asteroids with similar spectral properties and the uncertainties in the optical, chemical, and physical properties of the asteroid regolith.

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Meteoritic parent bodies: Their number and identification

[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 ratios determined for the S-class asteroid 433 Eros using remote- sensing x-ray fluorescence spectroscopy with the near-Earth asteroid rendezvous Shoemaker x-ray spectrometer (XRS). Data analysis techniques and systematic errors are described in detail. Data acquired during five solar flares and during two extended "quiet Sun" periods are presented; these results sample a representative portion of the asteroid's surface. Although systematic uncertainties are potentially large, the most internally consistent and plausible interpretation of the data is that Eros has primitive Mg/Si, Al/Si, Ca/Si and Fe/Si ratios, closely similar to H or R chondrites. Global differentiation of the asteroid is ruled out. The S/Si ratio is much lower than that of chondrites, probably reflecting impact-induced volatilization and/or photo- or ion-induced sputtering of sulfur at the surface of the asteroid. An alternative explanation for the low S/Si ratio is that it reflects a limited degree of melting with loss of an FeS-rich partial melt. Size-sorting processes could lead to segregation of Fe-Ni metal from silicates within the regolith of Eros; this could indicate that the Fe/Si ratios determined by the x-ray spectrometer are not representative of the bulk Eros composition.

Rosemary V. Killen

Papers

X‐ray fluorescence measurements of the surface elemental composition of asteroid 433 Eros