We compare updated Torino postprocessing asymptotic giant branch (AGB) nucleosynthesis model calculations with isotopic compositions of mainstream SiC dust grains from low-mass AGB stars. Based on the data-model comparison, we provide new constraints on the major neutron source, C-13(alpha, n)O-16 in the He-intershell, for the s-process. We show that the literature Ni, Sr, and Ba grain data can only be consistently explained by the Torino model calculations that adopt the recently proposed magnetic-buoyancy-induced C-13-pocket. This observation provides strong support to the suggestion of deep mixing of H into the He-intershell at low C-13 concentrations as a result of efficient transport of H through magnetic tubes.

Asteroid (162173) Ryugu is the target object of Hayabusa2, an asteroid exploration and sample return mission led by Japan Aerospace Exploration Agency (JAXA). Ground-based observations indicate that Ryugu is a C-type near-Earth asteroid with a diameter of less than 1km, but the knowledge of its detailed properties is very limited prior to Hayabusa2 observation. This paper summarizes our best understanding of the physical and dynamical properties of Ryugu based on ground-based remote sensing and theoretical modeling before the Hayabusa2's arrival at the asteroid. This information is used to construct a design reference model of the asteroid that is used for the formulation of mission operation plans in advance of asteroid arrival. Particular attention is given to the surface properties of Ryugu that are relevant to sample acquisition. This reference model helps readers to appropriately interpret the data that will be directly obtained by Hayabusa2 and promotes scientific studies not only for Ryugu itself and other small bodies but also for the solar system evolution that small bodies shed light on.

We report the identification of 19 presolar oxide grains from the Orgueil CI meteorite with substantial enrichments in Cr-54, with Cr-54/Cr-52 ratios ranging from 1.2 to 56 times the solar value. The most enriched grains also exhibit enrichments at mass-50, most likely due in part to Ti-50, but close-to-normal or depleted Cr-53/Cr-52 ratios. There is a strong inverse relationship between 54Cr enrichment and grain size; the most extreme grains are all <80 nm in diameter. Comparison of the isotopic data with predictions of nucleosynthesis calculations indicate that these grains most likely originated in either rare, high-density Type Ia supernovae (SN Ia), or in electron-capture supernovae (ECSN), which may occur as the end stage of evolution for stars of mass 8-10 M-circle dot. This is the first evidence for preserved presolar grains from either type of supernova. An ECSN origin is attractive, as these likely occur much more frequently than high-density SN Ia, and their evolutionary timescales (similar to 20 Myr) are comparable to those of molecular clouds. Self-pollution of the Sun's parent cloud from an ECSN may explain the heterogeneous distribution of n-rich isotopic anomalies in planetary materials, including a recently reported dichotomy in Mo isotopes in the solar system. The stellar origins of three grains with solar Cr-54/Cr-52, but anomalies in Cr-50 or Cr-53, as well as of a grain enriched in Fe-57, are unclear.

Mercury is surrounded by a tenuous, collisionless exosphere where the surface of the planet is directly exposed to the space environment. As a consequence, impacts and space weathering processes are expected to eject atoms and molecules from the surface into the exosphere, implying a direct link between the exospheric composition and the planet's regolith material. However, observational evidence demonstrating this link has been elusive. Here we report that exospheric magnesium, a species recently discovered and systematically measured by the Mercury Surface, Space ENvironment, GEochemistry, and Ranging mission, is enhanced when observed over a portion of the planet's surface regolith rich in magnesium. These observations confirm a direct link between Mercury's magnesium exosphere and the underlying crustal surface composition, providing strong evidence supporting theoretical arguments that impact vaporization can directly supply material to the exosphere from the regolith of a rocky, airless body.

Geochemical data from MESSENGER have revealed details of Mercury's surface composition, showing that it differs from the other rocky planets in the inner solar system. For example, the planet's surface is enriched in S and C, and depleted in Fe, indicating that Mercury formed under much more reducing conditions than other planets. The surface is also enriched in Mg and depleted in Al and Ca. Observed elemental heterogeneities and percent levels of graphite suggest that Mercury underwent a magma ocean phase early in its history. These findings have important implications for understanding Mercury's origin and evolution.

We have found that individual presolar silicon carbide (SiC) dust grains from supernovae show a positive correlation between Ti-49 and Si-28 excesses, which is attributed to the radioactive decay of the short-lived (t(1/2) = 330 days) 49V to Ti-49 in the inner highly Si-28- rich Si/S zone. The V-49-Ti-49 chronometer shows that these supernova SiC dust grains formed at least 2 years after their parent stars exploded. This result supports recent dust condensation calculations that predict a delayed formation of carbonaceous and SiC grains in supernovae. The astronomical observation of continuous buildup of dust in supernovae over several years can, therefore, be interpreted as a growing addition of C-rich dust to the dust reservoir in supernovae.