The initial structure of chondrule dust rims I: Electrically neutral grains
The initial structure of chondrule dust rims I: Electrically neutral grains
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球粒尘埃环的初始结构I：电中性颗粒
DOI：
10.1016/j.icarus.2018.10.014
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发表时间：
2018
期刊：
影响因子：
3.2
通讯作者：
T. Hyde
中科院分区：
文献类型：
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作者：
C. Xiang;A. Carballido;R. Hanna;L. Matthews;T. Hyde
为了描述碳质球粒陨石中通常围绕球粒的细粒尘埃边缘（fgr）的早期生长，我们对球粒表面的尘埃增生进行了数值模拟。我们采用蒙特卡罗算法模拟半径在0.5到10 µm之间的尘埃单体与半径在500到1000 µm之间的球粒以100µm的增量碰撞。碰撞是由布朗运动和太阳星云湍流驱动的。每次碰撞后，碰撞的粒子要么粘在接触点上，要么滚动，要么反弹。为了计算方便，我们将尘埃单体（在某些情况下，尘埃聚集体）的增加限制在球粒表面的一小块区域。我们模拟了尘埃边缘的形态和尘埃粒子的轨迹，这在以前的大多数工作中都没有考虑到。FGR孔隙度径向分布表明，弱湍流条件下形成的涡圈比强湍流条件下形成的涡圈多孔性更强（孔隙度为52-60%），孔隙度为60-74%。每个范围的下端对应较大的球粒，上端对应较小的球粒，说明球粒大小对FGR孔隙度也有影响。与CM球粒陨石的实验室观测结果一致，模拟中获得的fgr厚度与球粒半径呈线性关系。单体的聚集导致尘圈从内层到外层的粒度增大。粉尘聚集体形成的fgr的孔隙率比单体形成的fgr平均高 ~ 20%。总的来说，我们获得的相对较高的孔隙率与先前作者通过数值模拟计算的结果一致，也与从实验室测量的边缘球粒样品和边缘球粒类似物推断的初始FGR孔隙率一致。
In order to characterize the early growth of fine-grained dust rims (FGRs) that commonly surround chondrules in carbonaceous chondrites, we perform numerical simulations of dust accretion onto chondrule surfaces. We employ a Monte Carlo algorithm to simulate the collision of dust monomers having radii between 0.5 and 10 µm with chondrules whose radii are between 500 and 1000 µm, in 100-µm increments. The collisions are driven by Brownian motion and solar nebula turbulence. After each collision, the colliding particles either stick at the point of contact, roll or bounce. We limit accretion of dust monomers (and in some cases, dust aggregates) to a small patch of the chondrule surface, for computational expediency. We model the morphology of the dust rim and the trajectory of the dust particle, which are not considered in most of the previous works. Radial profiles of FGR porosity show that rims formed in weak turbulence are more porous (with a porosity of 60–74%) than rims formed in stronger turbulence (with a porosity of 52–60%). The lower end of each range corresponds to large chondrules and the upper end to small chondrules, meaning that the chondrule size also has an impact on FGR porosity. Consistent with laboratory observations of CM chondrites, the thickness of FGRs obtained in the simulations depends linearly on chondrule radius. The collection of single monomers leads to the increase of grain size from the inner to the outer layers of the dust rim. The porosity of FGRs formed by dust aggregates is ∼ 20% greater on average than that of FGRs formed by single monomers. In general, the relatively high porosities that we obtain are consistent with those calculated by previous authors from numerical simulations, as well as with initial FGR porosities inferred from laboratory measurements of rimmed chondrule samples and rimmed chondrule analogs.