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Shock-wave heating model is one of the plausible models for chondrule formation, though the generation of shock waves has not been understood clearly. In this study, we have carried out MHD (MagnetoHydroDynamic) simulations of X-ray flares and subsequent expansion of magnetic bubbles and we examine whether the conditions of the shock waves inside of the solar nebula are suitable for chondrule formation. We found that shock waves of about 15 km/s with velocity could be generated at the upper region of the solar nebula at 3 AU from the central star. Such shock conditions at the upper region of the nebula at 3 AU seem to be appropriate for chondrule formation., 資料番号: AA0063350009

Chondrules are considered to be formed by flash heating events in the protoplanetary disk. In order to evaluate some basic factors in the heating mechanism, we examined the rate of heating events that explains the abundance and the observed age distribution of chondrules in unequilibrated ordinary chondrites. First, we compiled the literature data of ^(26)Al ages of chondrules from the least equilibrated ordinary chondrites (LL3.0-3.1), ranging from 1Myr to 3Myr with a peak at about 1.8Myr relative to the formation time of Ca, Al-rich inclusions (CAIs) in carbonaceous chondrites, the oldest solid materials formed in the solar system. Next, we made a simple phenomenological chondrule formation model assuming that each event heated only a small fraction of existing dust at one time and numerous heating events produced chondrules. Results indicate that (1) an average number of heating events experienced by a dust particle should be 1.2 or higher, (2) more than a half of the present chondrules were reheated, (3) chondrule formation started sometime between 0.4-1.5Myr and ended at 2.2-2.3Myr after the CAI formation, and (4) the rate of heating events has a peak at 0.1-0.8Myr earlier than the peak of the observed chondrule age distribution and should decrease monotonically with time after the peak.

Shock-wave heating model is one of the plausible models for chondrule formation, though the generation of shock waves has not been understood clearly. In this study, examined are whether the shock waves in the solar nebula are generated by X-ray flares and whether the shock conditions are suitable for chondrule formation. MHD simulations of X-ray flares and subsequent expansion of magnetic bubbles have been conducted, and it was found that shock waves of several tens km/s with velocity could be generated at the upper region of the solar nebula where the gas density is low. Such shock conditions at the upper region of the nebula seem to be appropriate for chondrule formation., 資料番号: AA0047898013

Chondrules are major components in primitive meteorites and consist of 70 percent by volume in ordinary chondrites. They are considered to be formed by a flash heating of dusts in the protoplanetary disk, though the nature of the heating mechanism is not yet well understood. In order to evaluate some basic factors in the heating mechanism of chondrule formation, examined were the rate of chondrule precursor heating and its time variation that explain the abundance and the observed age distribution of chondrules. The literature data of Al-26 ages of chondrules from the least equilibrated ordinary chondrites (LL3.0 to 3.1) was compiled, that range from 1 Myr to 3 Myr with a peak at about 1.8 Myr relative to the formation time of the oldest CAIs. Making a simple phenomenological model for chondrule formation, the total amount of heating, the fraction of reheated chondrules, and the evolution of the rate of chondrule precursor heating were evaluated. The model considers that the each event heated only a small fraction of existed dust material in the chondrule forming region at one time and numerous heating events produced a huge amount of chondrules. The result indicates that an average number of heating events experienced by a dust particle should be 1.2 times or higher and more than a half of the present chondrules were reheated. To explain observed chondrule age distributions, the chondrules formation started sometime between 0.6 to 1.3 Myr after the CAI formation and ended at 2.2 to 2.3 Myr. The rate of chondrule precursor heating has a peak at 0.3 to 0.6 Myr earlier than that of the observed age distribution of chondrules. The present calculation indicates that the starting time of the chondrule formation epoch is not zero. Thus, it is proposed that the heating events responsible for the chondrule formation could be different from that for CAI formation., 資料番号: AA0046650012

Primitive clasts in the polymict ureilite Dar al Gani (DaG) 319 include dark clasts, sulfide- or metal-rich clasts, and unusual chondritic fragments. The dark clasts consist mainly of phyllosilicates, sulfides and magnetite with or without fayalitic olivine. The sulfide-rich clasts consist of a silicate-rich matrix and heterogeneously distributed sulfide. The metal-rich clasts consist of a silicate-rich matrix with variable amounts of metal. The unusual chondritic fragments are chondrule and equilibrated chondrite fragments. Oxygen isotopic compositions of the silicate-rich matrices in the sulfide-rich or metal-rich clasts plot on the carbonaceous chondrite anhydrous mineral (CCAM) mixing line between Allende matrix and a dark clast in the Nilpena polymict ureilite. Their oxygen isotopic compositions are similar to those of the monomict ureilites. Considering its chondritic composition and oxygen isotopic composition, the silicate-rich matrix of the sulfide-rich clasts is the best candidate for the ureilite precursors. However, the matrix has an Mg/(Mg+Fe) ratio (mg ratio) of 0.56 and is too ferroan to produce the monomict ureilites with mg ratios of 0.74-0.95. Therefore, it may have experienced various degrees of reduction to produce precursors with the mg ratios, needed to form the monomict ureilites as residues during fractional melting. Oxygen isotopic compositions of the unusual chondritic fragments plot near the ordinary chondrites on a 3-isotope diagram, suggesting that they have no direct genetic relationship to the monomict ureilites. They were projectiles that collided with the ureilite parent body (UPB).

We have developed a technique for in-situ Ni isotopic analysis using the ion microprobe, in order to detect ^Ni excess from the decay of the short lived nuclide ^Fe (half life=1.5Ma) in ureilite samples. The silicate minerals from MET-78008 ureilite with an old U-Pb age of 4.563±0.006 Ga were analyzed. The ^Fe/^Ni ratios of olivine and orthopyroxene are between 2700 and 5400. In spite of the high Fe/Ni ratios, we could not observe any detectable ^Ni excess. From the mean value of olivine core data, we obtain an upper limit of the ^Fe/^Fe ratio at the time of ureilite formation of 1.8×10^. The time difference between CAI formation and ureilite formation was estimated to be more than 4 million years, which is consistent with the UPb data from the same meteorite. We concluded that the impact event for the disruption of the ureilite parent body happened more than 4 million years after CAI formation. However, a large uncertainty in the initial ^Fe/^Fe ratio is introduced by the possibility that the ^Ni excess observed in CAIs is of nucleosynthetic origin. Our conclusion may change if the initial ^Fe/^Fe ratio of the solar system using CAI data is too high.

第46回月・惑星シンポジウム (2013年8月5日-7日. 宇宙航空研究開発機構宇宙科学研究所), 相模原市, 神奈川県, 46th ISAS Lunar and Planetary Symposium (August 5-7, 2013. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan, 形態: カラー図版あり, 形態: CD-ROM1枚, 資料番号: SA6000015009