2D condensation model for the inner Solar Nebula: an enstatite-rich environment
2D condensation model for the inner Solar Nebula: an enstatite-rich environment
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太阳星云内部的二维凝结模型：富含顽辉石的环境
DOI：
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发表时间：
2016
期刊：
影响因子：
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通讯作者：
G. Brooks
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文献类型：
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作者：
F. Pignatale;K. Liffman;S. Maddison;G. Brooks
红外观测提供了原行星盘表层的尘埃成分，但无法探测行星形成所在中面的尘埃化学。陨石表明，动力学在确定太阳星云中的尘埃分布方面很重要，如果我们要了解圆盘中的全球化学，就需要考虑这一点。一维径向凝聚序列一次只能模拟一个盘状层，不能描述陨石的全球化学或复杂性。为了解决这些限制，我们首次使用热力学平衡模型计算了内太阳星云中凝聚体的二维分布，并导出了尘埃垂直沉降和径向迁移的时间尺度。
我们在距年轻太阳1AU的范围内发现了两个火辉石富集区：在盘内部光学薄层的上部有一条厚约0.1AU的带，厚至0.8AU，而在盘中面的光学厚的带厚达约0.4AU。这两个顽辉石富集带支持最近的证据，即汞和顽辉石球粒陨石共享一种成分相似的块状物质。我们的结果与原行星盘的红外观测结果也是一致的，这表明在盘的内表面有富含顽火辉石的尘埃发射。
化学和动力学结果表明，顽辉石球粒陨石块状物质的形成发生在盘的内表层，在0.4~AU范围内。我们还提出了一种简单的替代方案，在该方案中，凝析油的气体分馏和垂直沉降导致顽火辉石-球粒陨石块状物质。
Infrared observations provide the dust composition in the protoplanetary discs surface layers, but can not probe the dust chemistry in the midplane, where planet formation occurs. Meteorites show that dynamics was important in determining the dust distribution in the Solar Nebula and needs to be considered if we are to understand the global chemistry in discs. 1D radial condensation sequences can only simulate one disc layer at a time and cannot describe the global chemistry or the complexity of meteorites. To address these limitations, we compute for the first time the two dimensional distribution of condensates in the inner Solar Nebula using a thermodynamic equilibrium model, and derive timescales for vertical settling and radial migration of dust.
We find two enstatite-rich zones within 1 AU from the young Sun: a band ~0.1 AU thick in the upper optically-thin layer of the disc interior to 0.8 AU, and in the optically-thick disc midplane out to ~0.4 AU. The two enstatite-rich zones support recent evidence that Mercury and enstatite chondrites shared a bulk material with similar composition. Our results are also consistent with infrared observation of protoplanetary disc which show emission of enstatite-rich dust in the inner surface of discs.
The resulting chemistry and dynamics suggests that the formation of the bulk material of enstatite chondrites occurred in the inner surface layer of the disc, within 0.4~AU. We also propose a simple alternative scenario in which gas fractionation and vertical settling of the condensates lead to an enstatite-chondritic bulk material.