We report Mo isotopic compositions of 37 presolar SiC grains of types Y (19) and Z (18), rare types commonly argued to have formed in lower-than-solar metallicity asymptotic giant branch (AGB) stars. Direct comparison of the Y and Z grain data with data for mainstream grains from AGB stars of close-to-solar metallicity demonstrates that the three types of grains have indistinguishable Mo isotopic compositions. We show that the Mo isotope data can be used to constrain the maximum stellar temperatures (T-MAX) during thermal pulses in AGB stars. Comparison of FRUITY Torino AGB nucleosynthesis model calculations with the grain data for Mo isotopes points to an origin from low-mass (similar to 1.5-3 M-circle dot) rather than intermediate-mass (> 3-similar to 9 M-circle dot) AGB stars. Because of the low efficiency of Ne-22(alpha, n)Mg-25 at the low T-MAX values attained in low-mass AGB stars, model calculations cannot explain the large Si-30 excesses of Z grains as arising from neutron capture, so these excesses remain a puzzle at the moment.

Here we report the relative degrees of thermal metamorphism for five Antarctic Ornans-like carbonaceous (CO) chondrites, including Dominion Range (DOM) 08006, as determined from the Cr-content of their FeO-rich (ferroan) olivine. These five CO3 chondrites complete the previously poorly-defined CO3.00 to 3.2 chondrite metamorphic trend. DOM 08006 appears to be a highly primitive CO chondrite of petrologic type 3.00. We report the detailed mineralogy and petrography of DOM 08006 using a coordinated, multi-technique approach.

The explosion mechanism of electron-capture supernovae (ECSNe) remains equivocal: it is not completely clear whether these events are implosions in which neutron stars are formed, or incomplete thermonuclear explosions that leave behind bound ONeFe white dwarf remnants. Furthermore, the frequency of occurrence of ECSNe is not known, though it has been estimated to be of the order of a few per cent of all core-collapse supernovae. We attempt to constrain the explosion mechanism (neutron-star-forming implosion or thermonuclear explosion) and the frequency of occurrence of ECSNe using nucleosynthesis simulations of the latter scenario, population synthesis, the solar abundance distribution, pre-solar meteoritic oxide grain isotopic ratio measurements and the white dwarf mass-radius relation. Tracer particles from the 3d hydrodynamic simulations were post-processed with a large nuclear reaction network in order to determine the complete compositional state of the bound ONeFe remnant and the ejecta, and population synthesis simulations were performed in order to estimate the ECSN rate with respect to the CCSN rate. The 3d deflagration simulations drastically overproduce the neutron-rich isotopes Ca-48, Ti-50, Cr-54, Fe-60 and several of the Zn isotopes relative to their solar abundances. Using the solar abundance distribution as our constraint, we place an upper limit on the frequency of thermonuclear ECSNe as 1-3% the frequency at which core-collapse supernovae (FeCCSNe) occur. This is on par with or 1 dex lower than the estimates for ECSNe from single stars. The upper limit from the yields is also in relatively good agreement with the predictions from our population synthesis simulations. The Cr-54/Cr-52 and Ti-50/Ti-48 isotopic ratios in the ejecta are a near-perfect match with recent measurements of extreme pre-solar meteoritc oxide grains, and Cr-53/Cr-52 can also be matched if the ejecta condenses before mixing with the interstellar medium. The composition of the ejecta of our simulations implies that ECSNe, including accretion-induced collapse of oxygen-neon white dwarfs, could actually be partial thermonuclear explosions and not implosions that form neutron stars. There is still much work to do to improve the hydrodynamic simulations of such phenomena, but it is encouraging that our results are consistent with the predictions from stellar evolution modelling and population synthesis simulations, and can explain several key isotopic ratios in a subset of pre-solar oxide meteoritic grains. Theoretical mass-radius relations for the bound ONeFe WD remnants of these explosions are apparently consistent with several observational WD candidates. The composition of the remnants in our simulations can reproduce several, but not all, of the spectroscopically-determined elemental abundances from one such candidate WD.

In this study we investigate the likeliness of the existence of an iron sulfide layer (FeS matte) at the core-mantle boundary (CMB) of Mercury by comparing new chemical surface data obtained by the X-ray Spectrometer onboard the MESSENGER spacecraft with geochemical models supported by high-pressure experiments under reducing conditions. We present a new data set consisting of 233 Ti/Si measurements, which combined with Al/Si data show that Mercury's surface has a slightly subchondritic Ti/Al ratio of 0.035 +/- 0.008. Multiphase equilibria experiments show that at the conditions of Mercury's core formation, Ti is chalcophile but not siderophile, making Ti a useful tracer of sulfide melt formation. We parameterize and use our partitioning data in a model to calculate the relative depletion of Ti in the bulk silicate fraction of Mercury as a function of a putative FeS layer thickness. By comparing the model results and surface elemental data we show that Mercury most likely does not have a FeS layer, and in case it would have one, it would only be a few kilometers thick (<13 km). We also show that Mercury's metallic Fe(Si) core cannot contain more than similar to 1.5 wt.% sulfur and that the formation of this core under reducing conditions is responsible for the slightly subchondritic Ti/Al ratio of Mercury's surface. (C) 2020 Elsevier B.V. All rights reserved.

Meteorites originating from primitive C-type asteroids are composed of materials from the Sun's protoplanetary disk, including up to a few per cent organic carbon. In contrast, some interplanetary dust particles and micrometeorites have much higher carbon contents, up to >90%, and are thought to originate from icy outer Solar System bodies and comets. Here we report an approximately 100-mu m-diameter very carbon-rich clast, with highly primitive characteristics, in the matrix of a CR2 chondrite, LaPaz Icefield 02342. The clast may represent a cometary building block, largely unsampled in meteorite collections, that was captured by a C-type asteroid during the early stages of planet formation. The existence of this cometary microxenolith supports the idea of a radially inward transport of materials from the outer protoplanetary disk into the CR chondrite reservoir during the formation of planetesimals. Moreover, the H-isotopic composition of the clast is suggestive of a temporal evolution of organic isotopic compositions in the comet-forming region of the disk.

We re-examine the Renazzo-like (CR) chondrite metamorphic trend based on Cr2O3 contents of FeO-rich olivine, indicating that it is only appropriate to use such analyses to identify the endmembers of this group (i.e., those that have experienced either no detectable heating or significant heating). As such Miller Range (MIL) 090657 appears to have experienced very minimal (if any) thermal processing and is one of the most pristine CR chondrites analyzed to date, while Graves Nunataks 06100 is the most shock-heated CR chondrite studied.

Interplanetary dust particles (IDPs) and micrometeorites (MMs) were likely major sources of extraterrestrial organics at the surface of the early Earth. However, these particles experience heating to >500 degrees C for up to several seconds during atmospheric entry. In this study, we aim to understand the effects of atmospheric entry heating on the dominant organic component in IDPs and MMs by conducting flash heating experiments (4 s to 400 degrees C, 600 degrees C, 800 degrees C, and 1000 degrees C) on insoluble organic matter (IOM) extracted from the meteorite Cold Bokkeveld (CM2). For each of the experimental charges, the bulk isotopic compositions of H, N, and C were analyzed by IRMS, the H isotopic heterogeneities (occurrence of deuterium hotspots) of the samples were measured by NanoSIMS, and the functional group chemistry and ordering of the IOM was evaluated by using FTIR and Raman spectroscopy, respectively. Organic matter in particles heated to >= 600 degrees C during atmospheric entry experienced significant alteration. Loss of isotopically heavy, labile H and N groups results in decreases in bulk delta D, delta N-15, H/C and, upon heating >= 800 degrees C, in N/C. The H isotopic heterogeneity was not greatly affected by flash heating to <= 600 degrees C, although the hotspots tended to be less isotopically anomalous in the 600 degrees C sample than in the 400 degrees C sample. However, the hotspots all but disappeared in the 800 degrees C sample. Loss of C=O groups occurred at 800 degrees C. Based on the Raman G-band characteristics, the heating resulted in increased ordering of the polyaromatic component of the IOM.

The X-Ray Spectrometer (XRS) on the MESSENGER spacecraft provided measurements of major-element ratios across Mercury's surface. We present global maps of Mg/Si, Al/Si, S/Si, Ca/Si, and Fe/Si derived from XRS data collected throughout MESSENGER's orbital mission. We describe the procedures we used to select and filter data and to combine them to make the final maps, which are archived in NASA's Planetary Data System. Areal coverage is variable for the different element-ratio maps, with 100% coverage for Mg/Si and Al/Si, but only 18% coverage for Fe/Si north of 30 degrees N, where the spatial resolution is highest. The spatial resolution is improved over previous maps by 10-15% because of the inclusion of higher-resolution data from late in the mission when the spacecraft periapsis altitude was low. Unlike typical planetary data maps, however, the spatial resolution of the XRS maps can vary from pixel to pixel, and thus care must be taken in interpreting small-scale features. We provide several examples of how the XRS maps can be used to investigate elemental variations in the context of geological features on Mercury, which range in size from single similar to 100-km-diameter craters to large impact basins. We expect that these maps will provide the basis for and/or contribute to studies of Mercury's origin and geological history for many years to come.

We built a collector to filter interplanetary dust particles (IDPs) larger than 5 mu m from the clean air at the Amundsen Scott South Pole station. Our sampling strategy used long duration, continuous dry filtering of near-surface air in place of short duration, high-speed impact collection on flags flown in the stratosphere. We filtered similar to 10(7) m(3) of clean Antarctic air through 20 cm diameter, 3 mu m filters coupled to a suction blower of modest power consumption (5-6 kW). Our collector ran continuously for 2 years and yielded 41 filters for analyses. Based on stratospheric concentrations, we predicted that each month's collection would provide 300-900 IDPs for analysis. We identified 19 extraterrestrial (ET) particles on the 66 cm(2) of filter examined, which represented similar to 0.5% of the exposed filter surfaces. The 11 ET particles larger than 5 mu m yield about a fifth of the expected flux based on >5 mu m stratospheric ET particle flux. Of the 19 ET particles identified, four were chondritic porous IDPs, seven were FeNiS beads, two were FeNi grains, and six were chondritic material with FeNiS components. Most were <10 mu m in diameter and none were cluster particles. Additionally, a carbon-rich candidate particle was found to have a small N-15 isotopic enrichment, supporting an ET origin. Many other candidate grains, including chondritic glasses and C-rich particles with Mg and Si and FeS grains, require further analysis to determine if they are ET. The vast majority of exposed filter surfaces remain to be examined.