The Mercury Imaging X-ray Spectrometer is a highly novel instrument that is designed to map Mercury's elemental composition from orbit at two angular resolutions. By observing the fluorescence X-rays generated when solar-coronal X-rays and charged particles interact with the surface regolith, MIXS will be able to measure the atomic composition of the upper similar to 10-20 mu m of Mercury's surface on the day-side. Through precipitating particles on the night-side, MIXS will also determine the dynamic interaction of the planet's surface with the surrounding space environment. MIXS is composed of two complementary elements: MIXS-C is a collimated instrument which will achieve global coverage at a similar spatial resolution to that achieved (in the northern hemisphere only - i.e. similar to 50 - 100 km) by MESSENGER; MIXS-T is the first ever X-ray telescope to be sent to another planet and will, during periods of high solar activity (or intense precipitation of charged particles), reveal the X-ray flux from Mercury at better than 10 km resolution. The design, performance, scientific goals and operations plans of the instrument are discussed, including the initial results from commissioning in space.

Presolar silicon carbide (SiC) grains in meteoritic samples can help constrain circumstellar condensation processes and conditions in C-rich stars and core-collapse supernovae (CCSNe). This study presents our findings on eight presolar SiC grains from asymptotic giant branch (AGB) stars (four mainstream and one Y grain) and CCSNe (three X grains), chosen on the basis of mu-Raman spectral features that were indicative of their having unusual non-3C polytypes and/or high degrees of crystal disorder. Analytical transmission electron microscopy (TEM), which provides elemental compositional and structural information, shows evidence for complex histories for the grains. Our TEM results confirm the presence of non-3C,2H crystal domains. Minor-element heterogeneities and/or subgrains were observed in all grains analyzed for their compositions. The C/O ratios inferred for the parent stars varied from 0.98 to >= 1.03. Our data show that SiC condensation can occur under a wide range of conditions, in which environmental factors other than temperature (e.g., pressure, gas composition, heterogeneous nucleation on precondensed phases) play a significant role. Based on previous mu-Raman studies, similar to 10% of SiC grains may have infrared (IR) spectral features that are influenced by crystal defects, porosity, and/or subgrains. Future sub-diffraction-limited IR measurements of complex SiC grains might shed further light on the relative contributions of each of these features to the shape and position of the characteristic IR 11 mu m SiC feature and thus improve the interpretation of IR spectra of AGB stars like those that produced the presolar SiC grains.

A catalog of mercurian craters that retain their central peak or peak-ring structure was created to aid target prioritization for the Mercury Imaging X-ray Spectrometer (MIXS), now on its way to Mercury aboard BepiColombo. Preliminary analysis of the MIXS crater catalog suggested a potential spatial correlation between an abnormally high spatial density of peak-ring basins and a region of Mercury with elevated Mg/Si values (High-Magnesium Terrane [HMT]). Robust statistical analysis of previously published crater catalogs confirmed that the spatial correlation exists, with an overall confidence level of 97.7%, specifically between peak-ring basins and the HMT, delineated by a contour of Mg/Si = mean + 2 sigma = 0.648. Applying empirical impact cratering scaling laws to the 15 basins intersecting the HMT suggested that all have excavated material from similar to 13 to 20 km depth. None of the basins excavated mantle material, predicting instead that deep crustal material contains elevated Mg/Si material. However, five of the basins are predicted to have melted underlying mantle material, which might be a contributing factor in the elevated Mg/Si signature. In the absence of resolvable volcanic features associated with the rise of basaltic melts from the mantle, we favor excavation of deep crustal, high Mg/Si material. MIXS-T is capable of spatially resolving individual features associated with peak-ring basins and it is proposed that the 15 basins within the HMT are prioritized targets for MIXS, to test the hypothesis of exposed deep-crustal material.

We present the results of a coordinated NAUTILUS and NanoSIMS isotopic study of epsilon (epsilon) phase metallic aggregates from the Oklo natural nuclear reactor zone (RZ) 13. We observed that fissionogenic Tc and Cs were heterogeneously sequestered within the aggregates. Isotopes of these elements are relevant for improving the safety of spent nuclear fuel storage and reactor operation on generational timescales. Like the noble metals, nearly all of the Tc was retained within the reactor, though its abundance relative to Ru in the metallic aggregates varied by a factor of 10. The neutron fluence estimated from the production of Ru-100 from neutron capture on Tc-99 was estimated to be up to 1.2 x 10(21) n.cm(-2). In contrast to Tc, nearly all of the fissionogenic Cs in the reactors was lost from the reactor fuel. The metallic aggregates contain the only phases yet identified to have sequestered radiocesium. Fissionogenic Cs isotopes decay over vastly different timescales, but were incorporated and retained within the epsilon-phase in proportions similar to stable Cs-133. This indicates that retention began during criticality and sequestration lasted billions of years, despite local geologic activity and the presence of nearby magmatic dikes. Using fissionogenic Ba isotopes, we estimated that the metallic aggregates continually incorporated their radioactive Cs parents during criticality, though the majority of Cs was flushed out of the reactor on a characteristic timescale of 2.7 +/- 0.6 years. We found that the abundance of Bi was correlated to Rh and Pd, and speculate that this may have been due to primary Np-Rh and Np-Pd alloys forming during or shortly after criticality. Using Pb-Pb data from uraninite and galena grains surrounding the metallic aggregates, we also inferred a final Pb mobility age of 298 Ma for RZ13, which is more recent than most estimates from other RZs.

Water-rock interactions are relevant to planetary habitability, influencing mineralogical diversity and the production of organic molecules. We examine carbonates and silicates in the martian meteorite Allan Hills 84001 (ALH 84001), using colocated nanoscale analyses, to characterize the nature of water-rock reactions on early Mars. We find complex refractory organic material associated with mineral assemblages that formed by mineral carbonation and serpentinization reactions. The organic molecules are colocated with nanophase magnetite; both formed in situ during water-rock interactions on Mars. Two potentially distinct mechanisms of abiotic organic synthesis operated on early Mars during the late Noachian period (3.9 to 4.1 billion years ago).