Significantly high polarization degree of the very low-albedo asteroid (152679) 1998 KU₂

¹ Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, Asakuchi, Okayama 719-0232, Japan
e-mail: dikuroda@oao.nao.ac.jp
² Department of Physics and Astronomy, Seoul National University, Gwanak, Seoul 08826, Korea
³ Department of Applied Physics, Okayama University of Science, Ridai-cho, Kita-ku, Okayama, Okayama 700-0005, Japan
⁴ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa 252-5210, Japan
⁵ Asahikawa Campus, Hokkaido University of Education, Hokumon, Asahikawa, Hokkaido 070-8621, Japan
⁶ Nayoro Observatory, Nisshin, Nayoro, Hokkaido 096-0066, Japan
⁷ Center for Planetary Science, Graduate School of Science, Kobe University, Minatojima-Minamimachi, Chuo-Ku, Kobe, Hyogo 650-0047, Japan
⁸ Department of Cosmosciences, Graduate School of Science, Hokkaido University, Kita-ku, Sapporo, Hokkaido 060-0810, Japan

Received: 11 October 2017
Accepted: 15 November 2017

Abstract

We present a unique and significant polarimetric result regarding the near-Earth asteroid (152679) 1998 KU₂, which has a very low geometric albedo. From our observations, we find that the linear polarization degrees of 1998 KU₂ are 44.6 ± 0.5% in the R_C band and 44.0 ± 0.6% in the V band at a solar phase angle of 81.0°. These values are the highest of any known airless body in the solar system (i.e., high-polarization comets, asteroids, and planetary satellites) at similar phase angles. This polarimetric observation is not only the first for primitive asteroids at large phase angles, but also for low-albedo (<0.1) airless bodies. Based on spectroscopic similarities and polarimetric measurements of materials that have been sorted by size in previous studies, we conjecture that 1998 KU₂ has a highly microporous regolith structure comprising nano-sized carbon grains on the surface.