碎石堆NEA（162173）Ryugu的微孔隙和母体
Microporosity and parent body of the rubble-pile NEA (162173) Ryugu
摘要：
对C型近地小行星的观测和对碳质陨石的实验室研究为C型小行星的高孔隙度提供了有力的证据。巨石微孔值来自于现场测量的表面的碎石堆NEA Ryugu的高达55%的显着高于富水CC样品，并可能表明不同的演化路径的母体Ryugu和母体的碳质碳酸盐岩，尽管光谱相似。在本研究中，我们计算的温度和孔隙度的演变早期太阳系的星子，以约束的范围内的参数，导致微孔隙度与Ryugu的高孔隙度的材料和可能的埋藏深度在表面观察到的巨石兼容。通过改变母体的关键属性，如吸积时间t0和半径R，对温度和孔隙度有很强的影响，并通过比较内部孔隙度分布与测得的巨石微孔，水合作用，和部分脱水的材料，我们约束内的字段（R，t0）-空间适合的机构，很可能已经产生这样的材料。计算结果表明，在富Ca-Al包体形成后，其母体的大小仅为几km，其早期吸积时间在<2- 300万年之间。一个渐进的最终孔隙度配置文件的最佳拟合机构表明生产的低密度和高密度巨砾从母体材料。相比之下，通过拟合碳酸盐岩地层数据获得的CI和CM辉长岩的母体性质表明，CAIs后的半径约为20-25 km，增生时间约为375万年。这些结果意味着人口公里大小的早期吸积多孔星子的母体Ryugu（和潜在的，其他NEAs）和人口较大的后期吸积少多孔母体的富水碳质碳酸盐。
英文摘要：
Observations of C-type NEAs and lab investigations of carbonaceous chondritic (CC) meteorites provide strong evidence for a high porosity of C-type asteroids. Boulder microporosity values derived from in-situ measurements at the surface of the rubble-pile NEA Ryugu of up to 55% are substantially higher than for water-rich CC samples and could indicate distinct evolution paths for the parent body of Ryugu and parent bodies of carbonaceous chondrites, despite spectral similarities. In the present study, we calculate the evolution of the temperature and porosity for early solar system's planetesimals in order to constrain the range of parameters that result in microporosities compatible with Ryugu's high-porosity material and likely burial depths for the boulders observed at the surface. By varying key properties of the parent body, such as accretion time t0 and radius R that have strong influence on temperature and porosity and by comparing the interior porosity distribution with the measured boulder microporosity, hydration, and partial dehydration of the material, we constrain a field within the (R,t0)-space appropriate for bodies that are likely to have produced such material. Our calculations indicate a parent body size of only a few km and its early accretion within <~2-3 Myr after the formation of Ca-Al-rich inclusions. A gradual final porosity profile of best-fit bodies indicates production of both low- and high-density boulders from the parent body material. By contrast, parent body properties for CI and CM chondrites obtained by fitting carbonate formation data indicate a radius of ~20-25 km and an accretion time of ~3.75 Myr after CAIs. These results imply a population of km-sized early accreting porous planetesimals as parent bodies of Ryugu (and, potentially, other NEAs) and a population of larger late accreting less porous parent bodies of water-rich carbonaceous chondrites.
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