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).

We report NanoSIMS Si and Mg-Al isotopic data (and C, N, and Ti isotopic data, when available) for 85 submicron- to micron-sized presolar SiC grains from the CM2 Murchison meteorite, including 60 mainstream (MS), 8 AB1, 8 X, 7 AB2, and 2 Y grains. The MS and Y grain data demonstrate that (1) C and N contamination mainly appears as surface contamination, and sufficient presputtering is needed to expose a clean grain surface for obtaining intrinsic C and N signals, and (2) Mg and Al contamination appears as adjacent grains and rims, and high-resolution imaging and the choice of small regions of interest during data reduction together are effective in suppressing the contamination. Our results strongly indicate that previous studies of presolar SiC grains could have sampled differing degrees of contamination for C, N, Mg, and Al. Compared to the literature data, our new MS and Y grains are in better agreement with carbon star observations for both the C and N isotopic ratios. By comparing our new, tighter distributions of C-12/C-13, N-14/N-15, and initial Al-26/Al-27 ratios for MS and Y grains with FRUITY low-mass asymptotic giant branch (AGB) stellar models, we provide more stringent constraints on the occurrence of cool bottom processing and the production of Al-26 in N-type carbon stars, which are classical AGB stars.

The first maps of electron-induced X-ray emission from the dayside of Mercury's surface are presented, generated by the development of a solar X-ray flux filter. This enables the isolation of the X-ray fluorescence of calcium driven by probable electron precipitation. A catalog of such events has been generated and dayside maps of implied electron precipitation zones have been produced. We find that, similar to electron induced emission events on the nightside, these zones are strongly organized by latitude and magnetic local time. The majority of the dayside events appear in the southern hemisphere and there is a strong enhancement observed centered about local dawn (06:00 LT). There is apparent poleward continuation of emission in the north, but very few events were observed on the duskward hemisphere. These results carry implications for Mercury's magnetosphere by constraining zones of electron precipitation, for the exosphere as a potential source of exospheric species, and for surface science as an additional source of X-ray fluorescence.

The Hayabusa2 spacecraft investigated the C-type (carbonaceous) asteroid (162173) Ryugu. The mission performed two landing operations to collect samples of surface and subsurface material, the latter exposed by an artificial impact. We present images of the second touchdown site, finding that ejecta from the impact crater was present at the sample location. Surface pebbles at both landing sites show morphological variations ranging from rugged to smooth, similar to Ryugu's boulders, and shapes from quasi-spherical to flattened. The samples were returned to Earth on 6 December 2020. We describe the morphology of >5 grams of returned pebbles and sand. Their diverse color, shape, and structure are consistent with the observed materials of Ryugu; we conclude that they are a representative sample of the asteroid.

We report the use of several cluster analysis techniques to evaluate the classification of pre-solar silicon carbide (SiC) grains. The stability of clusters and the confidence of individual cluster assignments of grains are assessed using consensus clustering with resampling methods. Our analysis shows that pre-solar SiC grains can be divided into seven groups that are found to be highly stable with most of the grains being assigned to the same cluster for at least 90 percent of the time over multiple aggregated clustering. Among the seven groups, two groups are dominated by AB grains, three groups by MS grains, one group by Z grains, and one group by X grains. The further division of X grains into two groups is highly dependent on the chosen algorithm and is therefore uncertain. Z and Y grains are clustered jointly with MS grains, with one group dominated by Z grains, pointing to their common origins from low-mass asymptotic giant branch stars. The most stable N grain-containing clusters are dominated by N-15-rich AB grains. However, some methods assign N grains with X grains, but in less stable clusters. The suggested genetic relationship among N-15-rich AB, N, and X grains is in line with the recent proposal that all three types of pre-solar SiC grains came from core collapse supernovae. We discuss the results from different clustering techniques based on our assessment of the cluster stabilities and the extent to which the cluster assignments overlap across the different methods.

Carbonaceous meteorites are thought to be fragments of C-type (carbonaceous) asteroids. Samples of the C-type asteroid (162173) Ryugu were retrieved by the Hayabusa2 spacecraft. We measured the mineralogy and bulk chemical and isotopic compositions of Ryugu samples. The samples are mainly composed of materials similar to those of carbonaceous chondrite meteorites, particularly the CI (Ivuna-type) group. The samples consist predominantly of minerals formed in aqueous fluid on a parent planetesimal. The primary minerals were altered by fluids at a temperature of 37° ± 10°C, about [Formula: see text] million (statistical) or [Formula: see text] million (systematic) years after the formation of the first solids in the Solar System. After aqueous alteration, the Ryugu samples were likely never heated above ~100°C. The samples have a chemical composition that more closely resembles that of the Sun's photosphere than other natural samples do.