¹Takashi Yoshizaki, ¹Daisuke Nakashima, ¹Tomoki Nakamura,²Changkun Park, ³Naoya Sakamoto, ¹Hatsumi Ishida, ⁴Shoichi Itoh
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.02.034]
¹Department of Earth Science, Graduate School of Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan
²Division of Earth-System Sciences, Korea Polar Research Institute, Incheon 21990, South Korea
³Creative Research Institution Sousei, Hokkaido University, Kita, Sapporo 001-0021, Japan
⁴Department of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan
Copyright Elsevier

Ultrarefractory (UR) phases in calcium-aluminum-rich inclusions (CAIs) could have formed at higher temperature compared to common CAI minerals and thus they potentially provide constraints on very high temperature processes in the solar nebula. We report a detailed characterization of the mineralogy, petrology and oxygen isotopic composition of an UR phase davisite (CaScAlSiO₆) bearing CAI from a reduced type CV chondrite. The CAI is an irregular-shaped, compound inclusion that consists of five units that are composed of melilite + spinel + Al,Ti-rich pyroxene ±perovskite with various modal abundances of minerals and lithologies, and surrounded by the Wark-Lovering (WL) rim. Davisite occurs only in one lithological unit that consists of three chemically and isotopically distinct parts: i) 16O-poor (-20‰⩽δ18O ⩽0‰) regions with reversely-zoned melilite and davisite; ii) 16O-rich (-50‰⩽δ18O ⩽-40‰) regions consisting of unzoned, gehlenitic melilite, Al,Ti-rich diopside and spinel; and iii) spinel framboids composed of 16O-rich spinel and 16O-poor melilite. Absence of secondary iron- and/or alkali-rich phases, occurrence of low-iron, manganese-enriched (LIME) olivine, and random distribution of the oxygen isotopic heterogeneity indicate that primitive chemical and isotopic compositions are preserved in the inclusion. The occurrence of chemical and isotopic heterogeneities with sharp boundaries in the CAI indicates formation of the inclusion by an aggregation of mineral assemblages formed and processed separately at different time and/or space in the solar nebula. Although isotope exchange between 16O-rich solids and 16O-poor gases prior to the final agglomeration of the CAI cannot be ruled out, we suggest that modification of chemical and isotopic composition of porous CAI precursors or aggregation of isotopically distinct mineral assemblages are alternative scenarios for the origin of oxygen isotopic heterogeneity in CAIs. In either case, coexistence of spatially and/or temporally distinct 16O-rich and 16O-poor gaseous reservoirs at the earliest stage of the solar system formation is required. The grain-scale oxygen isotopic disequilibrium in the CAI indicate that post-formation heating of the inclusion (i.e., the WL rim formation event) was short (e.g., ≲103hours at 1400 K; ≲105hours at 1100 K), which can be achieved by rapid outward transport of the CAI. High Ti³⁺/Ti^tot ratios of pyroxene from CAI interior and the rim and LIME composition of the olivine rim document that the entire CAI formation process took place under highly reducing conditions.

Liisa-Ida Sorsa¹, Mika Takala¹, Patrick Bambach², Jakob Deller², Esa Vilenius², and Sampsa Pursiainen¹
Astrophysical Journal 872, 44 Link to Article [DOI: 10.3847/1538-4357/aafba2 ]
¹Laboratory of Mathematics, Tampere University, P.O. Box 553, FI-33101 Tampere, Finland
²Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, D-37077 Göttingen, Germany

In this paper, we investigate full-wave computed radar tomography (CRT) using a rubble-pile asteroid model in which a realistic shape (Itokawa) is coupled with a synthetic material composition and structure model. The aim is to show that sparse bistatic radar measurements can distinguish details inside a complex-structured rubble-pile asteroid. The results obtained suggest that distinct local permittivity distribution changes such as surface layers, voids, low-permittivity anomalies, high-permittivity boulders, and cracks can be detected with bistatic CRT, when the total noise level in the data is around −10 dB with respect to the signal amplitude. Moreover, the bistatic measurement setup improves the robustness of the inversion compared to the monostatic case. Reconstructing the smooth Gaussian background distribution was found to be difficult with the present approach, suggesting that complementary techniques, such as gravimetry, might be needed to improve the reliability of the inference in practice.

Shoji Mori¹, Xue-Ning Bai², and Satoshi Okuzumi¹
Astrophysical Journal 872, 98 Link to Article [DOI: 10.3847/1538-4357/ab0022 ]
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro-ku, Tokyo, 152-8551, Japan
²Institute for Advanced Study and Tsinghua Center for Astrophysics, Tsinghua University, Beijing 100084, People’s Republic of China

The gas temperature in protoplanetary disks (PPDs) is determined by a combination of irradiation heating and accretion heating, with the latter conventionally attributed to turbulent dissipation. However, recent studies have suggested that the inner disk (a few au) is largely laminar, with accretion primarily driven by magnetized disk winds, as a result of nonideal magnetohydrodynamic (MHD) effects from weakly ionized gas, suggesting an alternative heating mechanism by Joule dissipation. We perform local stratified MHD simulations including all three nonideal MHD effects (ohmic, Hall, and ambipolar diffusion) and investigate the role of Joule heating and the resulting disk vertical temperature profiles. We find that in the inner disk, as ohmic and ambipolar diffusion strongly suppress electrical current around the midplane, Joule heating primarily occurs at several scale heights above the midplane, making the midplane temperature much lower than that with the conventional viscous heating model. Including the Hall effect, Joule heating is enhanced/reduced when the magnetic fields threading the disks are aligned/anti-aligned with the disk rotation, but it is overall ineffective. Our results further suggest that the midplane temperature in the inner PPDs is almost entirely determined by irradiation heating, unless viscous heating can trigger thermal ionization in the disk innermost region to self-sustain magnetorotational instability turbulence.