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

‘Space weathering’ is the term applied to the darkening and reddening of planetary surface materials with time, along with the changes to the depths of absorption bands in their optical spectra. It has been invoked to explain the mismatched spectra of lunar rocks and regolith, and between those of asteroids and meteorites1,2,3,4,5,6. The formation of nanophase iron particles on regolith grains as a result of micrometeorite impacts or irradiation by the solar wind has been proposed as the main cause of the change in the optical properties7,8. But laboratory simulations9,10,11,12,13,14 have not revealed the presence of these particles, although nano-second-pulse laser irradiation did reproduce the optical changes12. Here we report observations by transmission electron microscopy of olivine samples subjected to pulse laser irradiation. We find within the amorphous vapour-deposited rims of olivine grains nanophase iron particles similar to those observed in the rims of space-weathered lunar regolith grains15,16. Reduction by hydrogen atoms implanted by the solar wind is therefore not necessary to form the particles. Moreover, the results support the idea that ordinary chondrites came from S-type asteroids5, and thereby provides some constraints on the surface exposure ages of those asteroids.

Production of iron nanoparticles by laser irradiation in a simulation of lunar-like space weathering

https://www.mtholyoke.edu/~tburbine/binzel.icarus.2004.pdf

Observed spectral properties of near-Earth objects: results for population distribution, source regions, and space weathering processes

The Mars Odyssey Gamma-Ray Spectrometer is a suite of three different instruments, a gamma subsystem (GS), a neutron spectrometer, and a high-energy neutron detector, working together to collect data that will permit the mapping of elemental concentrations on the surface of Mars. The instruments are complimentary in that the neutron instruments have greater sensitivity to low amounts of hydrogen, but their signals saturate as the hydrogen content gets high. The hydrogen signal in the GS, on the other hand, does not saturate at high hydrogen contents and is sensitive to small differences in hydrogen content even when hydrogen is very abundant. The hydrogen signal in the neutron instruments and the GS have a different dependence on depth, and thus by combining both data sets we can infer not only the amount of hydrogen, but constrain its distribution with depth. In addition to hydrogen, the GS determines the abundances of several other elements. The instruments, the basis of the technique, and the data processing requirements are described as are some expected applications of the data to scientific problems.

The Mars Odyssey Gamma-Ray Spectrometer Instrument Suite

[1] We report maps of the concentrations of H, Si, Cl, K, Fe, and Th as determined by the Gamma Ray Spectrometer (GRS) on board the 2001 Mars Odyssey Mission for ±∼45° latitudes. The procedures by which the spectra are processed to yield quantitative concentrations are described in detail. The concentrations of elements determined over the locations of the various Mars landers generally agree well with the lander values except for Fe, although the mean of the GRS Fe data agrees well with that of Martian meteorites. The water-equivalent concentration of hydrogen by mass varies from about 1.5% to 7.5% (by mass) with the most enriched areas being near Apollinaris Patera and Arabia Terra. Cl shows a distribution similar to H over the surface except that the Cl content over Medusae Fossae is much greater than elsewhere. The map of Fe shows enrichment in the northern lowlands versus the southern highlands. Silicon shows only very modest variation over the surface with mass fractions ranging from 19% to 22% over most of the planet, though a significant depletion in Si is noted in a region west of Tharsis Montes and Olympus Mons where the Si content is as low as 18%. K and Th show a very similar pattern with depletions associated with young volcanic deposits and enrichments associated with the TES Surface Type-2 material. It is noted that there appears to be no evidence of significant globally distributed thick dust deposits of uniform composition.

Concentration of H, Si, Cl, K, Fe, and Th in the low- and mid-latitude regions of Mars

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

Status of the NEAR X-ray/gamma-ray spectrometer, end of mission status. Additional information is contained in the original extended abstract.

M. Petaev

Papers

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

The Chemical Composition of Asteroid 433 Eros: End of Mission Results of the NEAR Shoemaker X-Ray/Gamma-Ray Spectrometer