We report measurements of the flux of fast neutrons at Mercury from 20 degrees S to the north pole. On the basis of neutron transport simulations and remotely sensed elemental compositions, cosmic-ray-induced fast neutrons are shown to provide a measure of average atomic mass, , a result consistent with earlier studies of the Moon and Vesta. The dynamic range of fast neutron flux at Mercury is 3%, which is smaller than the fast-neutron dynamic ranges of 30% and 6% at the Moon and Vesta, respectively. Fast-neutron data delineate compositional terranes on Mercury that are complementary to those identified with Xray, gamma-ray, and slow-neutron data. Fast neutron measurements confirm the presence of a region with high , relative to the mean for the planet, that coincides with the previously identified high Mg region and reveal the existence of at least two additional compositional terranes: a low- region within the northern smooth plains and a high- region near the equator centered near 90 degrees E longitude. Comparison of the fast-neutron map with elemental composition maps show that variations predicted from the combined element maps are not consistent with the measured variations in fast-neutron flux. This lack of consistency could be due to incomplete coverage for some elements or uncertainties in the interpretations of compositional and neutron data. Currently available data and analyses do not provide sufficient constraints to resolve these differences. (C) 2016 The Authors. Published by Elsevier Inc.

Extraterrestrial materials, including meteorites, interplanetary dust, and spacecraft-returned asteroidal and cometary samples, provide a record of the starting materials and early evolution of the Solar System. We review how laboratory analyses of these materials provide unique information, complementary to astronomical observations, about a wide variety of stellar, interstellar and protoplanetary processes. Presolar stardust grains retain the isotopic compositions of their stellar sources, mainly asymptotic giant branch stars and Type II supernovae. They serve as direct probes of nucleosynthetic and dust formation processes in stars, galactic chemical evolution, and interstellar dust processing. Extinct radioactivities suggest that the Sun's birth environment was decoupled from average galactic nucleosynthesis for some tens to hundreds of Myr but was enriched in short-lived isotopes from massive stellar winds or explosions shortly before or during formation of the Solar System. Radiometric dating of meteorite components tells us about the timing and duration over which solar nebula solids were assembled into the building blocks of the planets. Components of the most primitive meteoritical materials provide further detailed constraints on the formation, processing, and transport of material and associated timescales in the Sun's protoplanetary disk as well as in other forming planetary systems.

Orbital data from the MESSENGER mission to Mercury have facilitated a new view of the planet's structure, chemical makeup, and diverse surface, and have confirmed Mercury's status as a geochemical end member among the terrestrial planets. In this work, the most recent results from MESSENGER's X-Ray Spectrometer, Gamma-Ray Spectrometer, and Neutron Spectrometer have been used to identify nine distinct geochemical regions on Mercury. Using a variation on the classical CIPW normative mineralogy calculation, elemental composition data is used to constrain the potential mineralogy of Mercury's surface; the calculated silicate mineralogy is dominated by plagioclase, pyroxene (both orthopyroxene and clinopyroxene), and olivine, with lesser amounts of quartz. The range in surface compositions indicate that the rocks on the surface of Mercury are diverse and vary from komatiitic to boninitic. The high abundance of alkalis on Mercury's surface results in several of the nine regions being classified as alkali rich komatiites and/or boninites. In addition, Mercury's surface terranes span a wide range of SiO2 values that encompass crustal compositions that are more silica-rich than geochemical terranes on the Moon, Mars, and Vesta, but the range is similar to that of Earth. Although the composition of Mercury's surface appears to be chemically evolved, the high SiO2 content is a primitive feature and a direct result of the planet's low oxygen fugacity. (C) 2016 Elsevier Inc. All rights reserved.

We report C, N, and Si isotopic data for 59 highly C-13-enriched presolar submicron- to micron-sized SiC grains from the Murchison meteorite, including eight putative nova grains (PNGs) and 29 N-15-rich (N-14/N-15 <= solar) AB grains, and their Mg-Al, S, and Ca-Ti isotope data when available. These 37 grains are enriched in C-13, N-15, and Al-26 with the PNGs showing more extreme enhancements. The N-15-rich AB grains show systematically higher Al-26 and Si-30 excesses than the N-14-rich AB grains. Thus, we propose to divide the AB grains into groups 1 (N-14/N-15 < solar) and 2 (N-14/N-15 >= solar). For the first time, we have obtained both S and Ti isotopic data for five AB1 grains and one PNG and found S-32 and/or Ti-50 enhancements. Interestingly, one AB1 grain had the largest S-32 and Ti-50 excesses, strongly suggesting a neutron- capture nucleosynthetic origin of the S-32 excess and thus the initial presence of radiogenic Si-32 (t(1/2) = 153 years). More importantly, we found that the N-15 and Al-26 excesses of AB1 grains form a trend that extends to the region in the N-Al isotope plot occupied by C-2 grains, strongly indicating a common stellar origin for both AB1 and C-2 grains. Comparison of supernova models with the AB1 and C-2 grain data indicates that these grains came from supernovae that experienced H ingestion into the He/C zones of their progenitors.

We report Mo isotopic data of 27 new presolar SiC grains, including 12 N-14-rich AB (N-14/N-15 > 440, AB2) and 15 mainstream (MS) grains, and their correlated Sr and Ba isotope ratios when available. Direct comparison of the data for the MS grains, which came from low-mass asymptotic giant branch (AGB) stars with large s-process isotope enhancements, with the AB2 grain data demonstrates that AB2 grains show near-solar isotopic compositions and lack s-process enhancements. The near-normal Sr, Mo, and Ba isotopic compositions of AB2 grains clearly exclude born-again AGB stars, where the intermediate neutron-capture process (i-process) takes place, as their stellar source. On the other hand, low-mass CO novae and early R-and J-type carbon stars show C-13 and N-14 excesses but no s-process enhancements and are thus potential stellar sources of AB2 grains. Because both early R-type carbon stars and CO novae are rare objects, the abundant J-type carbon stars (10%-15% of all carbon stars) are thus likely to be a dominant source of AB2 grains.

Stardust grains recovered from meteorites provide high-precision snapshots of the isotopic composition of the stellar environment in which they formed(1). Attributing their origin to specific types of stars, however, often proves difficult. Intermediate-mass stars of 4-8 solar masses are expected to have contributed a large fraction of meteoritic stardust(2,3). Yet, no grains have been found with the characteristic isotopic compositions expected for such stars(4,5). This is a long-standing puzzle, which points to serious gaps in our understanding of the lifecycle of stars and dust in our Galaxy. Here we show that the increased proton-capture rate of O-17 reported by a recent underground experiment(6) leads to O-17/O-16 isotopic ratios that match those observed in a population of stardust grainsfor proton-burning temperatures of 60-80 MK. These temperatures are achieved at the base of the convective envelope during the late evolution of intermediate-mass stars of 4-8 solar masses(7-9), which reveals them as the most likely site of origin of the grains. This result provides direct evidence that these stars contributed to the dust inventory from which the Solar System formed.

During its four years in orbit around Mercury, the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft's X-ray Spectrometer revealed a large geochemical terrane in the northern hemisphere that hosts the highest Mg/Si, S/Si, Ca/Si, and Fe/Si and lowest Al/Si ratios on the planet. Correlations with low topography, thin crust, and a sharp northern topographic boundary led to the proposal that this high-Mg region is the remnant of an ancient, highly degraded impact basin. Here we use a numerical modeling approach to explore the feasibility of this hypothesis and evaluate the results against multiple mission-wide data sets and resulting maps from MESSENGER. We find that an similar to 3000km diameter impact basin easily exhumes Mg-rich mantle material but that the amount of subsequent modification required to hide basin structure is incompatible with the strength of the geochemical anomaly, which is also present in maps of Gamma Ray and Neutron Spectrometer data. Consequently, the high-Mg region is more likely to be the product of high-temperature volcanism sourced from a chemically heterogeneous mantle than the remains of a large impact event.

Geochemical indicators in meteorites imply that most formed under relatively oxidizing conditions. However, some planetary materials, such as the enstatite chondrites, aubrite achondrites, and Mercury, were produced in reduced nebular environments. Because of large-scale radial nebular mixing, comets and other Kuiper Belt objects likely contain some primitive material related to these reduced planetary bodies. Here, we describe an unusual assemblage in a dust particle from comet 81P/ Wild 2 captured in silica aerogel by the NASA Stardust spacecraft. The bulk of this similar to 20 mu m particle is comprised of an aggregate of nanoparticulate Cr-rich magnetite, containing opaque sub-domains composed of poorly graphitized carbon (PGC). The PGC forms conformal shells around tiny 5-15 nm core grains of Fe carbide. The C, N, and O isotopic compositions of these components are identical within errors to terrestrial standards, indicating a formation inside the solar system. Magnetite compositions are consistent with oxidation of reduced metal, similar to that seen in enstatite chondrites. Similarly, the core-shell structure of the carbide + PGC inclusions suggests a formation via FTT reactions on the surface of metal or carbide grains in warm, reduced regions of the solar nebula. Together, the nanoscale assemblage in the cometary particle is most consistent with the alteration of primary solids condensed from a C-rich, reduced nebular gas. The nanoparticulate components in the cometary particle provide the first direct evidence from comets of reduced, carbon-rich regions that were present in the solar nebula.