Implications of High Polarization Degree for the Surface State of RyuguOPEN ACCESS 

Daisuke Kuroda, Jooyeon Geem, Hiroshi Akitaya, Sunho Jin, Jun Takahashi, Koki Takahashi, Hiroyuki Naito, Kana Makino, Tomohiko Sekiguchi, Yoonsoo P. Bach, Jinguk Seo, Shuji Sato, Hiroshi Sasago, Koji S. Kawabata, Aoi Kawakami, Miyako Tozuka, Makoto Watanabe, Seiko Takagi, Kiyoshi Kuramoto, Makoto Yoshikawa, Sunao Hasegawa, Masateru Ishiguro

submitted to ApJL


“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 result indicates 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 millimeter-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. “