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Alan P Boss - One of the best experts on this subject based on the ideXlab platform.
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mixing in the Solar Nebula implications for isotopic heterogeneity and large scale transport of refractory grains
Earth and Planetary Science Letters, 2008Co-Authors: Alan P BossAbstract:Abstract The discovery of refractory grains amongst the particles collected from Comet 81P/Wild 2 by the Stardust spacecraft [Brownlee, D.E. et al., Comet 81P/Wild 2 under a microscope, 2006, Science, 314, 1711–1716.] provides the ground truth for large-scale transport of materials formed in high temperature regions close to the protosun outward to the comet-forming regions of the Solar Nebula. While accretion disk models driven by a generic turbulent viscosity have been invoked as a means to explain such large-scale transport, the detailed physics behind such an “alpha” viscosity remains unclear. We present here an alternative physical mechanism for large-scale transport in the Solar Nebula: gravitational torques associated with the transient spiral arms in a marginally gravitationally unstable disk, of the type that appears to be necessary to form gas giant planets. Three dimensional models are presented of the time evolution of self-gravitating disks, including radiative transfer and detailed equations of state, showing that small dust grains will be transported upstream and downstream (with respect to the mean inward flow of gas and dust being accreted by the central protostar) inside the disk on time scales of less than 1000 yr inside 10 AU. These models furthermore show that any initial spatial heterogeneities present (e.g., in short-lived isotopes such as 26Al) will be homogenized by disk mixing down to a level of ~ 10%, preserving the use of short-lived isotopes as accurate Nebular chronometers, while simultaneously allowing for the spread of stable oxygen isotope ratios. This finite level of Nebular spatial heterogeneity appears to be related to the coarse mixing achieved by spiral arms, with radial widths of order 1 AU, over time scales of ~ 1000 yr.
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evolution of the Solar Nebula viii spatial and temporal heterogeneity of short lived radioisotopes and stable oxygen isotopes
arXiv: Astrophysics, 2007Co-Authors: Alan P BossAbstract:Isotopic abundances of short-lived radionuclides such as 26Al provide the most precise chronometers of events in the early Solar system, provided that they were initially homogeneously distributed. On the other hand, the abundances of the three stable isotopes of oxygen in primitive meteorites show a mass-independent fractionation that survived homogenization in the Solar Nebula. As as result of this and other cosmochemical evidence, the degree of spatial heterogeneity of isotopes in the Solar Nebula has long been a puzzle. We show here that based on hydrodynamical models of the mixing and transport of isotopic anomalies formed at, or injected onto, the surface of the Solar Nebula, initially high levels of isotopic spatial heterogeneity are expected to fall to steady state levels (~10%) low enough to validate the use of 26Al for chronometry, but high enough to preserve the evidence for mass-independent fractionation of oxygen isotopes. The solution to this puzzle relies on the mixing being accomplished by the chaotic fluid motions in a marginally gravitationally unstable disk, as seems to be required for the formation of gas giant planets and by the inability of alternative physical processes to drive large-scale mixing and transport in the planet-forming midplane of the Solar Nebula. Such a disk is also capable of large-scale outward transport of the thermally annealed dust grains found in comets, and of driving the shock fronts that appear to be responsible for much of the thermal processing of the components of primitive meteorites, creating a self-consistent picture of the basic physical processes shaping the early Solar Nebula.
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chondrule forming shock fronts in the Solar Nebula a possible unified scenario for planet and chondrite formation
The Astrophysical Journal, 2005Co-Authors: Alan P Boss, Richard H DurisenAbstract:Chondrules are millimeter-sized spherules found throughout primitive chondritic meteorites. Flash heating by a shock front is the leading explanation of their formation. However, identifying a mechanism for creating shock fronts inside the Solar Nebula has been difficult. In a gaseous disk capable of forming Jupiter, the disk must have been marginally gravitationally unstable at and beyond Jupiter's orbit. We show that this instability can drive inward spiral shock fronts with shock speeds of up to ~10 km s-1 at asteroidal orbits, sufficient to account for chondrule formation. The mixing and transport of solids in such a disk, combined with the planet-forming tendencies of gravitational instabilities, results in a unified scenario linking chondrite production with gas giant planet formation.
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chondrule forming shock fronts in the Solar Nebula a possible unified scenario for planet and chondrite formation
arXiv: Astrophysics, 2005Co-Authors: Alan P Boss, Richard H DurisenAbstract:Chondrules are mm-sized spherules found throughout primitive, chondritic meteorites. Flash heating by a shock front is the leading explanation of their formation. However, identifying a mechanism for creating shock fronts inside the Solar Nebula has been difficult. In a gaseous disk capable of forming Jupiter, the disk must have been marginally gravitationally unstable at and beyond Jupiter's orbit. We show that this instability can drive inward spiral shock fronts with shock speeds of up to about 10 km/s at asteroidal orbits, sufficient to account for chondrule formation. Mixing and transport of solids in such a disk, combined with the planet-forming tendencies of gravitational instabilities, results in a unified scenario linking chondrite production with gas giant planet formation.
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evolution of the Solar Nebula vi mixing and transport of isotopic heterogeneity
The Astrophysical Journal, 2004Co-Authors: Alan P BossAbstract:Meteoritical studies have shown that the Solar Nebula was basically well mixed, as shown by the homogeneity of many isotopes of refractory elements. However, isotopic anomalies in certain elements, such as the short-lived isotope 26Al, imply the presence of either spatial or temporal heterogeneity. Spatial heterogeneity could result from the injection of short-lived isotopes into the Solar Nebula following their production in a supernova or from spraying these isotopes across the Nebula's surface following their production by Solar flares and transport outward by an X-wind. If the Nebula was thoroughly mixed soon after the introduction of spatially heterogeneous 26Al, producing a homogeneous spatial distribution of 26Al, then the measured abundances of 26Al would provide a chronometer for early Solar system processes. For these reasons and others it is important to understand the efficiency of mixing and transport processes in the Solar Nebula. Here we study mixing and transport in fully three-dimensional models of gravitationally unstable disks that are likely to occur during the phase when planetesimal growth is beginning. The three-dimensional models show that isotopes that are sprayed onto an annular region of the surface of a disk around 9 AU remain remarkably concentrated after ~103 yr, in spite of mixing by convection, which transports material between the surface and midplane on timescales of ~30 yr, and radially inward and outward as well. Mixing caused by a generic turbulent viscosity dominates only when ? ? 0.01, implying that the effective ? of the convective motions and gravitational torques is ? ~ 10-3. The three-dimensional models show that spatial heterogeneity can persist in a disk evolving by gravitational torques for significant periods, even in the most dynamically active region. This period, ~103 yr, is similar to the timescale for preSolar dust grains to coagulate while settling to the midplane and growing to centimeter size. If such solids can be preserved and incorporated into chondrites, then it is conceivable that some observed isotopic anomalies could have been derived from a spatially heterogeneous Nebula.
S J Desch - One of the best experts on this subject based on the ideXlab platform.
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thermal histories of chondrules in Solar Nebula shocks
The Astrophysical Journal, 2010Co-Authors: Melissa A Morris, S J DeschAbstract:Chondrules are important early Solar System materials that can provide a wealth of information on conditions in the Solar Nebula, if their formation mechanism can be understood. The theory most consistent with observational constraints, especially thermal histories, is the so-called shock model, in which chondrules were melted in Solar Nebula shocks. However, several problems have been identified with previous shock models. These problems all pertained to the treatment of the radiation field, namely, the input boundary condition to the radiation field, the proper treatment of the opacity of solids, and the proper treatment of molecular line cooling. In this paper, we present the results of our updated shock model, which corrects for the problems listed above. Our new hydrodynamic shock code includes a complete treatment of molecular line cooling due to H{sub 2}O. Previously, shock models including line cooling predicted chondrule cooling rates exceeding 10{sup 5} K hr{sup -1}. Contrary to these expectations, we have found that the effect of line cooling is minimal; after the inclusion of line cooling, the cooling rates of chondrules are 10-1000 K hr{sup -1}. The reduction in the otherwise rapid cooling rates attributable to line cooling is due to a combination of factors, includingmore » buffering due to hydrogen recombination/dissociation, high column densities of water, and backwarming. Our model demonstrates that the shock model for chondrule formation remains consistent with observational constraints.« less
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thermal histories of chondrules in Solar Nebula shocks
arXiv: Earth and Planetary Astrophysics, 2010Co-Authors: Melissa A Morris, S J DeschAbstract:Chondrules are important early Solar System materials that can provide a wealth of information on conditions in the Solar Nebula, if their formation mechanism can be understood. The theory most consistent with observational constraints, especially thermal histories, is the so-called "shock model", in which chondrules were melted in Solar Nebula shocks. However, several problems have been identified with previous shock models. These problems all pertained to the treatment of the radiation field, namely the input boundary condition to the radiation field, the proper treatment of the opacity of solids, and the proper treatment of molecular line cooling. In this paper, we present the results of our updated shock model, which corrects for the problems listed above. Our new hydrodynamic shock code includes a complete treatment of molecular line cooling due to H2O. Previously, shock models including line cooling predicted chondrule cooling rates exceeding 100,000 K/hr. Contrary to these expectations, we have found that the effect of line cooling is minimal; after the inclusion of line cooling, the cooling rates of chondrules are 10-1000 K/hr. The reduction in the otherwise rapid cooling rates attributable to line cooling is due to a combination of factors, including buffering due to hydrogen recombination/dissociation, high column densities of water, and backwarming. Our model demonstrates that the shock model for chondrule formation remains consistent with observational constraints.
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timescales for the evolution of oxygen isotope compositions in the Solar Nebula
Geochimica et Cosmochimica Acta, 2009Co-Authors: J R Lyons, F J Ciesla, S J Desch, Edwin A Bergin, Andrew M Davis, K Hashizume, Jeongeun LeeAbstract:Abstract We review two models for the origin of the calcium-, aluminum-rich inclusion (CAI) oxygen isotope mixing line in the Solar Nebula: (1) CO self-shielding, and (2) chemical mass-independent fractionation (MIF). We consider the timescales associated with formation of an isotopically anomalous water reservoir derived from CO self-shielding, and also the vertical and radial transport timescales of gas and solids in the Nebula. The timescales for chemical MIF are very rapid. CO self-shielding models predict that the Sun has Δ 17 O SMOW ∼ −20‰ (Clayton, 2002), and chemical mass-independent fractionation models predict Δ 17 O SMOW ∼0‰. Preliminary Genesis results have been reported by McKeegan et al. (McKeegan K. D., Coath C. D., Heber, V., Jarzebinski G., Kallio A. P., Kunihiro T., Mao P. H. and Burnett D. S. (2008b) The oxygen isotopic composition of captured Solar wind: first results from the Genesis. EOS Trans. AGU 89(53), Fall Meet . Suppl., P42A-07 (abstr)) and yield a Δ 17 O SMOW of ∼ −25‰, consistent with a CO self-shielding scenario. Assuming that subsequent Genesis analyses support the preliminary results, it then remains to determine the relative contributions of CO self-shielding from the X-point, the surface of the Solar Nebula and the parent molecular cloud. The relative formation ages of chondritic components can be related to several timescales in the self-shielding theories. Most importantly the age difference of ∼1–3 My between CAIs and chondrules is consistent with radial transport from the outer Solar Nebula (>10 AU) to the meteorite-forming region, which supports both the Nebular surface and parent cloud self-shielding scenarios. An elevated radiation field intensity is predicted by the surface shielding model, and yields substantial CO photolysis (∼50%) on timescales of 0.1–1 My. An elevated radiation field is also consistent with the parent cloud model. The elevated radiation intensities may indicate Solar Nebula birth in a medium to large cluster, and may be consistent with the injection of 60 Fe from a nearby supernova and with the photoevaporative truncation of the Solar Nebula at KBO orbital distances (∼47 AU). CO self-shielding is operative at the X-point even when H 2 absorption is included, but it is not yet clear whether the self-shielding signature can be imparted to silicates. A simple analysis of diffusion times shows that oxygen isotope exchange between 16 O-depleted Nebular H 2 O and chondrules during chondrule formation events is rapid (∼minutes), but is also expected to be rapid for most components of CAIs, with the exception of spinel. This is consistent with the observation that spinel grains are often the most 16 O-rich component of CAIs, but is only broadly consistent with the greater degree of exchange in other CAI components. Preliminary disk model calculations of self-shielding by N 2 demonstrate that large δ 15 N enrichments (∼ +800‰) are possible in HCN formed by reaction of N atoms with organic radicals (e.g., CH 2 ), which may account for 15 N-rich hotspots observed in lithic clasts in some carbonaceous chondrites and which lends support to the CO self-shielding model for oxygen isotopes.
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mass distribution and planet formation in the Solar Nebula
LPI, 2008Co-Authors: S J DeschAbstract:The surface density profile Σ(r) of the Solar Nebula protoplanetary disk is a fundamental input to all models of disk processes and evolution. Traditionally it is estimated by spreading out the augmented masses of the planets over the annuli in which the planets orbit today, the so-called minimum-mass Solar Nebula. Doing so implicitly assumes that the planets completely accreted all planetesimals in their feeding zones, but this assumption has not been tested. Indeed, models of the growth of Uranus and Neptune predict that these planets could not have grown to ~10 M⊕ within the lifetime of the disk, even though they must have, to accrete H/He atmospheres. In this paper we adopt the starting positions of the planets in the "Nice" model of planetary dynamics (Tsiganis and coworkers), in which the Solar system started in a much more compact configuration. We derive a surface density profile that is well approximated by the power law Σ(r) = 343(fp/0.5)-1(r/10 AU)-2.168 g cm-2, where fp is the fraction of the solid mass in the form of planetesimals. We show that this profile is inconsistent with a steady state accretion disk but is consistent with a steady state decretion disk that is being photoevaporated. We calculate the growth of planets in the context of this disk model and demonstrate for the first time that all of the giant planets can achieve their isolation masses and begin to accrete H/He atmospheres within the lifetime of the disk. The fit of our inferred Σ(r) to the augmented masses of the planets is excellent (<10%), but only if Uranus and Neptune swtiched places early in the Solar system's evolution, a possibility predicted by the Nice model.
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a model of the thermal processing of particles in Solar Nebula shocks application to the cooling rates of chondrules
Meteoritics & Planetary Science, 2002Co-Authors: S J Desch, Harold C ConnollyAbstract:We present a model for the thermal processing of particles in shock waves typical of the Solar Nebula. This shock model improves on existing models in that the dissociation and recombination of H2 and the evaporation of particles are accounted for in their effects on the mass, momentum and energy fluxes. Also, besides thermal exchange with the gas and gas-drag heating, particles can be heated by absorbing the thermal radiation emitted by other particles. The flow of radiation is calculated using the equations of radiative transfer in a slab geometry. We compute the thermal histories of particles as they encounter and pass through the shock. We apply this shock model to the melting and cooling of chondrules in the Solar Nebula. We constrain the combinations of shock speed and gas density needed for chondrules to reach melting temperatures, and show that these are consistent with shock waves generated by gravitational instabilities in the protoplanetary disk. After their melting, cooling rates of chondrules in the range 10-1000 K h^(-1) are naturally reproduced by the shock model. Chondrules are kept warm by the reservoir of hot shocked gas, which cools only as fast as the dust grains and chondrules themselves can radiate away the gas's energy. We predict a positive correlation between the concentration of chondrules in a region and the cooling rates of chondrules in that region. This correlation is supported by the unusually high frequency of (rapidly cooled) barred chondrules among compound chondrules, which must have collided preferentially in regions of high chondrule density. We discuss these and other compelling consistencies between the meteoritic record and the shock wave model of chondrule formation.
Alexander N Krot - One of the best experts on this subject based on the ideXlab platform.
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machiite al2ti3o9 a new oxide mineral from the murchison carbonaceous chondrite a new ultra refractory phase from the Solar Nebula
American Mineralogist, 2020Co-Authors: Alexander N Krot, K Nagashima, George R RossmanAbstract:Machiite (IMA 2016-067), Al₂Ti₃O₉, is a new mineral that occurs as a single euhedral crystal, 4.4 μm in size, in contact with an euhedral corundum grain, 12 μm in size, in a matrix of the Murchison CM2 carbonaceous chondrite. The mean chemical composition of holotype machiite by electron probe microanalysis is (wt%) TiO₂ 59.75, Al₂O₃ 15.97, Sc₂O₃ 10.29, ZrO₂ 9.18, Y₂O₃ 2.86, FeO 1.09, CaO 0.44, SiO2 0.20, MgO 0.10, total 99.87, giving rise to an empirical formula (based on 9 oxygen atoms pfu) of (Al_(1.17)Sc_(0.56)Y_(0.10)Ti4+0.08Ti0.084+Fe_(0.06)Ca_(0.03)Mg_(0.01))(Ti4+2.71Ti2.714+Zr_(0.28)Si_(0.01))O₉. The general formula is (Al,Sc)₂ (Ti⁴⁺,Zr)₃O₉. The end-member formula is Al₂Ti₃O₉. Machiite has the C2/c schreyerite-type structure with a = 17.10 A, b = 5.03 A, c = 7.06 A, β = 107°, V = 581 A3, and Z = 4, as revealed by electron backscatter diffraction. The calculated density using the measured composition is 4.27 g/cm³. The machiite crystal is highly ¹⁶O-depleted relative to the coexisting corundum grain (Δ¹⁷O = –0.2 ± 2.4‰ and –24.1 ± 2.6‰, respectively; where Δ¹⁷O = δ¹⁷O – 0.52 × δ¹⁸O). Machiite is a new member of the schreyerite (V₂Ti₃O₉) group and a new Sc,Zr-rich ultrarefractory phase formed in the Solar Nebula, either by gas-solid condensation or as a result of crystallization from a Ca,Al-rich melt having Solar-like oxygen isotopic composition (Δ¹⁷O~ –25‰) under high-temperature (~1400–1500 °C) and low-pressure (~10⁻⁴–10⁻⁵ bar) conditions in the CAI-forming region near the protosun. The currently observed disequilibrium oxygen isotopic composition between machiite and corundum may indicate that machiite subsequently experienced oxygen isotopic exchange with a planetary-like ¹⁶O-poor gaseous reservoir either in the Solar Nebula or on the CM chondrite parent body. The name machiite is in honor of Chi Ma, mineralogist at California Institute of Technology, for his contributions to meteorite mineralogy and discovery of many new minerals representing extreme conditions of formation.
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addibischoffite ca2al6al6o20 a new calcium aluminate mineral from the acfer 214 ch carbonaceous chondrite a new refractory phase from the Solar Nebula
American Mineralogist, 2017Co-Authors: Alexander N Krot, Kazuhide NagashimaAbstract:Addibischoffite (IMA 2015-006), Ca_2Al_6Al_6O_(20), is a new calcium aluminate mineral that occurs with hibonite, perovskite, kushiroite, Ti-kushiroite, spinel, melilite, anorthite, and FeNi-metal in the core of a Ca-Al-rich inclusion (CAI) in the Acfer 214 CH3 carbonaceous chondrite. The mean chemical composition of type addibischoffite measured by electron probe microanalysis is (wt%) Al_2O_3 44.63, CaO 15.36, SiO_2 14.62, V_2O_3 10.64, MgO 9.13, Ti_2O_3 4.70, FeO 0.46, total 99.55, giving rise to an empirical formula of (Ca_(2.00))(Al_(2.55)Mg_(1.73)V^(3+)_(1.08)Ti^(3+)_(0.50)Ca_(0.09)Fe^(2+)_(0.05))_(∑6.01)(Al_(4.14)Si_(1.86))O_(20). The general formula is Ca_2(Al,Mg,V,Ti)_6(Al,Si)_6O_(20). The end-member formula is Ca_2Al_6Al_6O_(20). Addibischoffite has the P1 aenigmatite structure with a = 10.367 A, b = 10.756 A, c = 8.895 A, α = 106.0°, β = 96.0°, γ = 124.7°, V = 739.7 A^3, and Z = 2, as revealed by electron backscatter diffraction. The calculated density using the measured composition is 3.41 g/cm^3. Addibischoffite is a new member of the warkite (Ca_2Sc_6Al_6O_(20)) group and a new refractory phase formed in the Solar Nebula, most likely as a result of crystallization from an ^(16)O-rich Ca, Al-rich melt under high-temperature (~1575 °C) and low-pressure (~10^(−4) to 10^(−5) bar) conditions in the CAI-forming region near the protosun, providing a new puzzle piece toward understanding the details of Nebular processes. The name is in honor of Addi Bischoff, cosmochemist at University of Munster, Germany, for his many contributions to research on mineralogy of carbonaceous chondrites, including CAIs in CH chondrites.
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a link between oxygen calcium and titanium isotopes in 26 al poor hibonite rich cais from murchison and implications for the heterogeneity of dust reservoirs in the Solar Nebula
Geochimica et Cosmochimica Acta, 2016Co-Authors: Alexander N Krot, L Koop, Daisuke Nakashima, Changkun Park, Kazuhide Nagashima, T J Tenner, A M Davis, P R HeckAbstract:PLACs (platy hibonite crystals) and related hibonite-rich calcium-, aluminum-rich inclusions (CAIs; hereafter collectively referred to as PLAC-like CAIs) have the largest nucleosynthetic isotope anomalies of all materials believed to have formed in the Solar system. Most PLAC-like CAIs have low inferred initial 26Al/27Al ratios and could have formed prior to injection or widespread distribution of 26Al in the Solar Nebula. In this study, we report 26Al–26Mg systematics combined with oxygen, calcium, and titanium isotopic compositions for a large number of newly separated PLAC-like CAIs from the Murchison CM2 chondrite (32 CAIs studied for oxygen, 26 of these also for 26Al–26Mg, calcium and titanium). Our results confirm (1) the large range of nucleosynthetic anomalies in 50Ti and 48Ca (our data range from −70‰ to +170‰ and −60‰ to +80‰, respectively), (2) the substantial range of Δ17O values (−28‰ to −17‰, with Δ17O = δ17O − 0.52 × δ18O), and (3) general 26Al-depletion in PLAC-like CAIs. The multielement approach reveals a relationship between Δ17O and the degree of variability in 50Ti and 48Ca: PLAC-like CAIs with the highest Δ17O (∼−17‰) show large positive and negative 50Ti and 48Ca anomalies, while those with the lowest Δ17O (∼−28‰) have small to no anomalies in 50Ti and 48Ca. These observations could suggest a physical link between anomalous 48Ca and 50Ti carriers and an 16O-poor reservoir. We suggest that the Solar Nebula was isotopically heterogeneous shortly after collapse of the protoSolar molecular cloud, and that the primordial dust reservoir, in which anomalous carrier phases were heterogeneously distributed, was 16O-poor (Δ17O ⩾ −17‰) relative to the primordial gaseous (CO + H2O) reservoir (Δ17O < −35‰). However, other models such as CO self-shielding in the protoplanetary disk are also considered to explain the link between oxygen and calcium and titanium isotopes in PLAC-like CAIs.
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discovery of dmisteinbergite hexagonal caal2si2o8 in the allende meteorite a new member of refractory silicates formed in the Solar Nebula
American Mineralogist, 2013Co-Authors: Alexander N Krot, Martin BizzarroAbstract:Dmisteinbergite, CaAl_2Si_2O_8 with P6_3/mcm structure, was identified in a rounded coarse-grained igneous Type B2 Ca-,Al-rich inclusion (CAI) STP-1 from the Allende CV3 carbonaceous chondrite. STP-1 belongs to a very rare type of refractory inclusions, Fractionation and Unknown Nuclear effects (FUN) CAIs, which experienced melt evaporation and crystallization at low total gas pressure (P 1200 °C) region, possibly near the proto-Sun and were subsequently radially transported away from region, possibly by a disk wind. The Allende dmisteinbergite occurs as irregular single crystals (100–600 μm in size) in contact with gehlenitic melilite and Al,Ti-diopside, poikilitically enclosing euhedral spinel, and rare anorthite. It is colorless and transparent. The mean chemical composition, determined by electron microprobe analysis, is (wt%) SiO_2 42.6, Al_2O_3 36.9, CaO 20.2, MgO 0.05, sum 99.75, giving rise to an empirical formula of Ca_(1.01)Al_(1.96)Si_(2.02)O_8. Its electron backscatter diffraction patterns are a good match to that of synthetic CaAl_2Si_2O_8 with the P6_3/mcm structure and the unit cell a = 5.10 A, c = 14.72 A, and Z = 2. Dmisteinbergite could have crystallized from a silicate melt at high temperature (~1200–1400 °C) via rapid cooling. Dmisteinbergite in Allende, the first find in a meteorite, is a new member of refractory silicates, among the first solid materials formed in the Solar Nebula.
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evolution of oxygen isotopic composition in the inner Solar Nebula
The Astrophysical Journal, 2005Co-Authors: Alexander N Krot, I D Hutcheon, Hisayoshi Yurimoto, Jeffrey N Cuzzi, K D Mckeegan, E Scott, Guy Libourel, Marc Chaussidon, J AleonAbstract:Changes in the chemical and isotopic composition of the Solar Nebula with time are reflected in the properties of different constituents that are preserved in chondritic meteorites. CR-group carbonaceous chondrites are among the most primitive of all chondrite types and must have preserved Solar Nebula records largely unchanged. We have analyzed the oxygen and magnesium isotopes in a range of the CR constituents of different formation temperatures and ages, including refractory inclusions and chondrules of various types. The results provide new constraints on the time variation of the oxygen isotopic composition of the inner (<5 AU) Solar Nebula—the region where refractory inclusions and chondrules most likely formed. A chronology based on the decay of short-lived 26 Al (t1=2 � 0:73 Myr) indicates that the inner Solar Nebula gas was 16 O-rich when refractory inclusions formed, but less than 0.8 Myr later, gas in the inner Solar Nebula became 16 O-poor, and this state persisted at least until CR chondrules formed � 1‐2 Myr later. We suggest that the inner Solar Nebula became 16 O-poor because meter-sized icy bodies, which were enriched in 17 Oa nd 18 O as a result of isotopic self-shielding during the ultraviolet photodissociation of CO in the protoSolar molecular cloud or protoplanetary disk, agglomerated outside the snow line, drifted rapidly toward the Sun, and evaporated at the snow line. This led to significant enrichment in 16 O-depleted water,whichthenspreadthroughtheinnerSolarsystem.Astronomicalstudiesofthespatialandtemporalvariations of water abundance in protoplanetary disks may clarify these processes. Subject headingg meteors, meteoroids — nuclear reactions, nucleosynthesis, abundances — planetary systems: formation — Solar system: formation
F J Ciesla - One of the best experts on this subject based on the ideXlab platform.
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the phases of water ice in the Solar Nebula
The Astrophysical Journal, 2014Co-Authors: F J CieslaAbstract:Understanding the phases of water ice that were present in the Solar Nebula has implications for understanding cometary and planetary compositions as well as the internal evolution of these bodies. Here we show that amorphous ice formed more readily than previously recognized, with formation at temperatures <70 K being possible under protoplanetary disk conditions. We further argue that photodesorption and freeze-out of water molecules near the surface layers of the Solar Nebula would have provided the conditions needed for amorphous ice to form. This processing would be a natural consequence of ice dynamics and would allow for the trapping of noble gases and other volatiles in water ice in the outer Solar Nebula.
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the phases of water ice in the Solar Nebula
arXiv: Earth and Planetary Astrophysics, 2014Co-Authors: F J CieslaAbstract:Understanding the phases of water ice that were present in the Solar Nebula has implications for understanding cometary and planetary compositions as well as internal evolution of these bodies. Here we show that amorphous ice formed more readily than previously recognized, with formation at temperatures <70 K being possible under protoplanetary disk conditions. We further argue that photodesorption and freeze-out of water molecules near the surface layers of the Solar Nebula would have provided the conditions needed for amorphous ice to form. This processing would be a natural consequence of ice dynamics, and would allow for the trapping of noble gases and other volatiles in water ice in the outer Solar Nebula.
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the distributions and ages of refractory objects in the Solar Nebula
Icarus, 2010Co-Authors: F J CieslaAbstract:Abstract Refractory objects such as Calcium, Aluminum-rich Inclusions, Amoeboid Olivine Aggregates, and crystalline silicates, are found in primitive bodies throughout our Solar System. It is believed that these objects formed in the hot, inner Solar Nebula and were redistributed during the mass and angular momentum transport that took place during its early evolution. The ages of these objects thus offer possible clues about the timing and duration of this transport. Here we study how the dynamics of these refractory objects in the evolving Solar Nebula affected the age distribution of the grains that were available to be incorporated into planetesimals throughout the Solar System. It is found that while the high temperatures and conditions needed to form these refractory objects may have persisted for millions of years, it is those objects that formed in the first 105 years that dominate (make up over 90%) those that survive throughout most of the Nebula. This is due to two effects: (1) the largest numbers of refractory grains are formed at this time period, as the disk is rapidly drained of mass during subsequent evolution and (2) the initially rapid spreading of the disk due to angular momentum transport helps preserve this early generation of grains as opposed to later generations. This implies that most refractory objects found in meteorites and comets formed in the first 105 years after the Nebula formed. As these objects contained live 26Al, this constrains the time when short-lived radionuclides were introduced to the Solar System to no later than 105 years after the Nebula formed. Further, this implies that the t = 0 as defined by meteoritic materials represents at most, the instant when the Solar Nebula finished accreting significant amounts of materials from its parent molecular cloud.
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timescales for the evolution of oxygen isotope compositions in the Solar Nebula
Geochimica et Cosmochimica Acta, 2009Co-Authors: J R Lyons, F J Ciesla, S J Desch, Edwin A Bergin, Andrew M Davis, K Hashizume, Jeongeun LeeAbstract:Abstract We review two models for the origin of the calcium-, aluminum-rich inclusion (CAI) oxygen isotope mixing line in the Solar Nebula: (1) CO self-shielding, and (2) chemical mass-independent fractionation (MIF). We consider the timescales associated with formation of an isotopically anomalous water reservoir derived from CO self-shielding, and also the vertical and radial transport timescales of gas and solids in the Nebula. The timescales for chemical MIF are very rapid. CO self-shielding models predict that the Sun has Δ 17 O SMOW ∼ −20‰ (Clayton, 2002), and chemical mass-independent fractionation models predict Δ 17 O SMOW ∼0‰. Preliminary Genesis results have been reported by McKeegan et al. (McKeegan K. D., Coath C. D., Heber, V., Jarzebinski G., Kallio A. P., Kunihiro T., Mao P. H. and Burnett D. S. (2008b) The oxygen isotopic composition of captured Solar wind: first results from the Genesis. EOS Trans. AGU 89(53), Fall Meet . Suppl., P42A-07 (abstr)) and yield a Δ 17 O SMOW of ∼ −25‰, consistent with a CO self-shielding scenario. Assuming that subsequent Genesis analyses support the preliminary results, it then remains to determine the relative contributions of CO self-shielding from the X-point, the surface of the Solar Nebula and the parent molecular cloud. The relative formation ages of chondritic components can be related to several timescales in the self-shielding theories. Most importantly the age difference of ∼1–3 My between CAIs and chondrules is consistent with radial transport from the outer Solar Nebula (>10 AU) to the meteorite-forming region, which supports both the Nebular surface and parent cloud self-shielding scenarios. An elevated radiation field intensity is predicted by the surface shielding model, and yields substantial CO photolysis (∼50%) on timescales of 0.1–1 My. An elevated radiation field is also consistent with the parent cloud model. The elevated radiation intensities may indicate Solar Nebula birth in a medium to large cluster, and may be consistent with the injection of 60 Fe from a nearby supernova and with the photoevaporative truncation of the Solar Nebula at KBO orbital distances (∼47 AU). CO self-shielding is operative at the X-point even when H 2 absorption is included, but it is not yet clear whether the self-shielding signature can be imparted to silicates. A simple analysis of diffusion times shows that oxygen isotope exchange between 16 O-depleted Nebular H 2 O and chondrules during chondrule formation events is rapid (∼minutes), but is also expected to be rapid for most components of CAIs, with the exception of spinel. This is consistent with the observation that spinel grains are often the most 16 O-rich component of CAIs, but is only broadly consistent with the greater degree of exchange in other CAI components. Preliminary disk model calculations of self-shielding by N 2 demonstrate that large δ 15 N enrichments (∼ +800‰) are possible in HCN formed by reaction of N atoms with organic radicals (e.g., CH 2 ), which may account for 15 N-rich hotspots observed in lithic clasts in some carbonaceous chondrites and which lends support to the CO self-shielding model for oxygen isotopes.
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outward transport of high temperature materials around the midplane of the Solar Nebula
Science, 2007Co-Authors: F J CieslaAbstract:The Stardust samples collected from Comet 81P/Wild 2 indicate that large-scale mixing occurred in the Solar Nebula, carrying materials from the hot inner regions to cooler environments far from the Sun. Similar transport has been inferred from telescopic observations of protoplanetary disks around young stars. Models for protoplanetary disks, however, have difficulty explaining the observed levels of transport. Here I report the results of a new two-dimensional model that shows that outward transport of high-temperature materials in protoplanetary disks is a natural outcome of disk formation and evolution. This outward transport occurs around the midplane of the disk.
Neal J Turner - One of the best experts on this subject based on the ideXlab platform.
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forming chondrites in a Solar Nebula with magnetically induced turbulence
The Astrophysical Journal, 2016Co-Authors: Yasuhiro Hasegawa, Neal J Turner, Joseph R Masiero, Shigeru Wakita, Yuji Matsumoto, Shoichi OshinoAbstract:Chondritic meteorites provide valuable opportunities to investigate the origins of the Solar system. We explore impact jetting as a mechanism of chondrule formation and subsequent pebble accretion as a mechanism of accreting chondrules onto parent bodies of chondrites, and investigate how these two processes can account for the currently available meteoritic data. We find that when the Solar Nebula is ≤5 times more massive than the minimum-mass Solar Nebula at a 2–3 au and parent bodies of chondrites are ≤1024 g (≤500 km in radius) in the Solar Nebula, impact jetting and subsequent pebble accretion can reproduce a number of properties of the meteoritic data. The properties include the present asteroid belt mass, the formation timescale of chondrules, and the magnetic field strength of the Nebula derived from chondrules in Semarkona. Since this scenario requires a first generation of planetesimals that trigger impact jetting and serve as parent bodies to accrete chondrules, the upper limit of parent bodies' masses leads to the following implications: primordial asteroids that were originally ≥1024 g in mass were unlikely to contain chondrules, while less massive primordial asteroids likely had a chondrule-rich surface layer. The scenario developed from impact jetting and pebble accretion can therefore provide new insights into the origins of the Solar system.
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forming chondrites in a Solar Nebula with magnetically induced turbulence
arXiv: Earth and Planetary Astrophysics, 2016Co-Authors: Yasuhiro Hasegawa, Neal J Turner, Joseph R Masiero, Shigeru Wakita, Yuji Matsumoto, Shoichi OshinoAbstract:Chondritic meteorites provide valuable opportunities to investigate the origins of the Solar system. We explore impact jetting as a mechanism of chondrule formation and subsequent pebble accretion as a mechanism of accreting chondrules onto parent bodies of chondrites, and investigate how these two processes can account for the currently available meteoritic data. We find that when the Solar Nebula is $\le 5$ times more massive than the minimum-mass Solar Nebula at $a \simeq 2-3$ AU and parent bodies of chondrites are $\le 10^{24}$ g ($\le$ 500 km in radius) in the Solar Nebula, impact jetting and subsequent pebble accretion can reproduce a number of properties of the meteoritic data. The properties include the present asteroid belt mass, the formation timescale of chondrules, and the magnetic field strength of the Nebula derived from chondrules in Semarkona. Since this scenario requires a first generation of planetesimals that trigger impact jetting and serve as parent bodies to accrete chondrules, the upper limit of parent bodies' masses leads to the following implications: primordial asteroids that were originally $\ge 10^{24}$ g in mass were unlikely to contain chondrules, while less massive primordial asteroids likely had a chondrule-rich surface layer. The scenario developed from impact jetting and pebble accretion can therefore provide new insights into the origins of the Solar system.
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a hot gap around jupiter s orbit in the Solar Nebula
The Astrophysical Journal, 2012Co-Authors: Neal J Turner, Mathieu Choukroun, Julie C Castillorogez, G BrydenAbstract:The Sun was an order of magnitude more luminous during the first few hundred thousand years of its existence, due in part to the gravitational energy released by material accreting from the Solar Nebula. If Jupiter was already near its present mass, the planet's tides opened an optically thin gap in the Nebula. Using Monte Carlo radiative transfer calculations, we show that sunlight absorbed by the Nebula and re-radiated into the gap raised temperatures well above the sublimation threshold for water ice, with potentially drastic consequences for the icy bodies in Jupiter's feeding zone. Bodies up to a meter in size were vaporized within a single orbit if the planet was near its present location during this early epoch. Dust particles lost their ice mantles, and planetesimals were partially to fully devolatilized, depending on their size. Scenarios in which Jupiter formed promptly, such as those involving a gravitational instability of the massive early Nebula, must cope with the high temperatures. Enriching Jupiter in the noble gases through delivery trapped in clathrate hydrates will be more difficult, but might be achieved by either forming the planet much farther from the star or capturing planetesimals at later epochs. The hot gap resulting from an early origin for Jupiter also would affect the surface compositions of any primordial Trojan asteroids.
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ice lines planetesimal composition and solid surface density in the Solar Nebula
Icarus, 2009Co-Authors: Sarah E Dodsonrobinson, Neal J Turner, Karen Willacy, Peter Bodenheimer, Charles A BeichmanAbstract:Abstract To date, there is no core accretion simulation that can successfully account for the formation of Uranus or Neptune within the observed 2–3 Myr lifetimes of protoplanetary disks. Since solid accretion rate is directly proportional to the available planetesimal surface density, one way to speed up planet formation is to take a full accounting of all the planetesimal-forming solids present in the Solar Nebula. By combining a viscously evolving protostellar disk with a kinetic model of ice formation, which includes not just water but methane, ammonia, CO and 54 minor ices, we calculate the solid surface density of a possible giant planet-forming Solar Nebula as a function of heliocentric distance and time. Our results can be used to provide the starting planetesimal surface density and evolving Solar Nebula conditions for core accretion simulations, or to predict the composition of planetesimals as a function of radius. We find three effects that favor giant planet formation by the core accretion mechanism: (1) a decretion flow that brings mass from the inner Solar Nebula to the giant planet-forming region, (2) the fact that the ammonia and water ice lines should coincide, according to recent lab results from Collings et al. [Collings, M.P., Anderson, M.A., Chen, R., Dever, J.W., Viti, S., Williams, D.A., McCoustra, M.R.S., 2004. Mon. Not. R. Astron. Soc. 354, 1133–1140], and (3) the presence of a substantial amount of methane ice in the trans-saturnian region. Our results show higher solid surface densities than assumed in the core accretion models of Pollack et al. [Pollack, J.B., Hubickyj, O., Bodenheimer, P., Lissauer, J.J., Podolak, M., Greenzweig, Y., 1996. Icarus 124, 62–85] by a factor of 3–4 throughout the trans-saturnian region. We also discuss the location of ice lines and their movement through the Solar Nebula, and provide new constraints on the possible initial disk configurations from gravitational stability arguments.
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turbulent mixing in the outer Solar Nebula
The Astrophysical Journal, 2006Co-Authors: Neal J Turner, Karen Willacy, Geoffrey Bryden, Harold W YorkeAbstract:The effects of turbulence on the mixing of gases and dust in the outer Solar Nebula are examined using three-dimensional MHD calculations in the shearing-box approximation with vertical stratification. The turbulence is driven by the magnetorotational instability. The magnetic and hydrodynamic stresses in the turbulence correspond to an accretion time at the midplane about equal to the lifetimes of T Tauri disks, while accretion in the surface layers is 30 times faster. The mixing resulting from the turbulence is also fastest in the surface layers. The mixing rate is similar to the rate of radial exchange of orbital angular momentum, so that the Schmidt number is near unity. The vertical spreading of a trace species is well matched by solutions of a damped wave equation when the flow is horizontally averaged. The damped wave description can be used to inexpensively treat mixing in one-dimensional chemical models. However, even in calculations reaching a statistical steady state, the concentration at any given time varies substantially over horizontal planes, due to fluctuations in the rate and direction of the transport. In addition to mixing species that are formed under widely varying conditions, the turbulence intermittently forces the Nebula away from local chemical equilibrium. The different transport rates in the surface layers and interior may affect estimates of the grain evolution and molecular abundances during the formation of the Solar system.
