高极化度对Ryugu表面态的影响
Implications of High Polarization Degree for the Surface State of Ryugu
摘要：
小行星探测项目“隼鸟2号”成功地从小行星（162173）Ryugu返回了样本。在这项研究中，我们从2020年9月27日至12月25日使用四台地面望远镜测量了Ryugu的线偏振度，覆盖了从28 $^\circ$到104 $^\circ$的宽相位角（太阳-目标-观测者的角度）范围。我们发现，Ryugu的偏振度在相位角为100 $^\circ$时达到53 $^\circ$，是迄今为止报道的所有小行星和彗星中的最高值。Ryugu的高偏振度可以归因于其表面层的散射特性，特别是多重散射光的贡献相对较小。我们的偏振结果表明，Ryugu的表面覆盖着大颗粒。在与粉碎陨石的偏振测量比较的基础上，我们可以推断在Ryugu的表面层上存在亚毫米大小的颗粒。我们还推测，这个尺寸边界代表了组成聚集体的颗粒。很可能在回收的样品中丢失了一种非常脆弱的结构，尽管它们可能保留了其演变的记录。我们的数据将是非常宝贵的，为未来的实验，旨在复制表面结构的龙宫。
英文摘要：
The asteroid exploration project "Hayabusa2" has successfully returned samples from the asteroid (162173) Ryugu. In this study, we measured the linear polarization degrees of Ryugu using four ground-based telescopes from 2020 September 27 to December 25, covering a wide-phase angle (Sun-target-observer's angle) range from 28$^\circ$ to 104$^\circ$. We found that the polarization degree of Ryugu reached 53$\%$ around a phase angle of 100$^\circ$, the highest value among all asteroids and comets thus far reported. The high polarization degree of Ryugu can be attributed to the scattering properties of its surface layers, in particular the relatively small contribution of multiply-scattered light. Our polarimetric results indicate that Ryugu's surface is covered with large grains. On the basis of a comparison with polarimetric measurements of pulverized meteorites, we can infer the presence of submillimeter-sized grains on the surface layer of Ryugu. We also conjecture that this size boundary represents the grains that compose the aggregate. It is likely that a very brittle structure has been lost in the recovered samples, although they may hold a record of its evolution. Our data will be invaluable for future experiments aimed at reproducing the surface structure of Ryugu.
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