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 to 104. We found that the polarization degree of Ryugu reached 53% around a phase angle of 100, 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.

小行星探测项目“隼鸟2号”已成功从小行星（162173）龙宫返回样本。在这项研究中，我们在 2020 年 9 月 27 日至 12 月 25 日期间使用四台地面望远镜测量了龙宫的线性偏振度，覆盖了从 28 到 104 的宽相位角（太阳-目标-观测者角）范围。我们发现Ryugu 的偏振度在 100 度左右的相位角达到 53%，是迄今为止报道的所有小行星和彗星中的最高值。Ryugu 的高偏振度可归因于其表层的散射特性，特别是多重散射光的贡献相对较小。我们的极化结果表明 Ryugu 的表面覆盖着大颗粒。在与粉状陨石的偏振测量比较的基础上，我们可以推断出龙宫表层存在亚毫米级颗粒。我们还推测这个尺寸边界代表了构成聚集体的颗粒。很可能在回收的样本中丢失了一个非常脆的结构，尽管它们可能保留了其演化的记录。我们的数据对于未来旨在再现 Ryugu 表面结构的实验将是无价的。