Corundum, the thermodynamically stable phase of alumina (Al₂O₃), is one of the most refractory dust species to condense around evolved stars. Presolar alumina in primitive chondrites has survived various kinds of processing in circumstellar environments, the interstellar medium (ISM), the Sun’s parent molecular cloud, and the protosolar disk. The morphology and crystal structure of presolar alumina grains may reflect their formation and evolution processes, but the relative importance of these two types of processes is poorly understood. In this study, we performed detailed morphological observations of 185 alumina grains extracted from unequilibrated ordinary chondrites (Semarkona, Bishunpur, and RC075). We also performed electron back-scattered diffraction analyses of 122 grains and oxygen isotopic analyses of 107 grains. Dissolution experiments on corundum and transition alumina phases were carried out to examine the possibility of the alteration of surface structures of alumina grains by the chemical separation procedures of chondrites.
The average size of the alumina grains was 1 μm, and neither whiskers nor extremely flat grains were observed. About one-third of the grains had smooth surfaces, while ~60% of the grains had rough surfaces with 10 to 100 nm-sized fine structures. The rough-surface grains have varieties of morphology and crystallinity, suggesting that the rough surface structures are secondary in origin. Electron back-scattered diffraction patterns from 95% of alumina grains matched with α-Al₂O₃ (corundum), and more than 75% of the alumina grains are single crystals of corundum. Nine presolar alumina grains with anomalous oxygen isotopic compositions were found among 107 alumina grains, and most of them were characterized by rough surface structures. While most of the presolar alumina grains were corundum, the relative abundance of amorphous or low-crystallinity grains is higher in presolar alumina grains than in solar alumina grains. The dissolution experiments showed that all phases except for corundum dissolved during the acid treatments of chondrites. This suggests that smooth surface structures of corundum grains were originally formed in space, and that original surfaces of alumina that had been damaged by energetic particle irradiation in the ISM or the protosolar disk were lost during chemical separations to form the rough surface structures, and that amorphous or low-crystallinity alumina grains in chondrites have acid-resistant structures different from sol-gel-synthesized amorphous alumina. The present results also imply the possible presence of acid-soluble alumina phases, undiscovered by chemical separations, in chondrites.

The fluence and isotopic composition of solar wind xenon have been determined from silicon collector targets flown on the NASA Genesis mission. A protocol was developed to extract gas quantitatively from samples of ~9 – 25 mm², and xenon measured using the RELAX mass spectrometer. The fluence of implanted solar wind xenon is 1.202(87) 10⁶ atoms ¹³²Xe cm^-2, which equates to a flux of 5.14(21) 10⁶ atoms ¹³²Xe cm^-2 year^-1 at the L1 point. This value is in good agreement with those reported in other studies. The isotopic composition of the solar wind is consistent with that extracted from the young lunar regolith and other Genesis collector targets.
The more precise xenon isotopic data derived from the Genesis mission confirm models of relationships among planetary xenon signatures. The underlying composition of Xe-Q is mass fractionated solar wind; small, varying contributions of Xe-HL and ¹²⁹Xe from ¹²⁹I decay are present in reported meteorite analyses. In contrast, an s-process deficit is apparent in Xe-P3, which appears to have been mass fractionated to the same extent as Xe-Q from a precursor composition, suggesting similar trapping mechanisms. Solar wind xenon later evolved by the addition of ~1% (at ¹³²Xe) of s-process xenon to this precursor. As an alternative model to a single source reservoir for Xe-P3, we propose that trapping of xenon onto carbonaceous carriers has been an ongoing process across galactic history, and that preparation of the residues in which Xe-P3 has been identified preferentially preserves longer lived host phases; a higher proportion of these sample xenon isotopic compositions from earlier in galactic chemical evolution, allowing the s-process deficit to become apparent. The relationships among SW-Xe, Xe-Q and Xe-P3 predict that the ¹²⁴Xe/¹³²Xe ratio for the solar wind is 0.00481(6).