Significantly high polarization degree of the very low-albedo asteroid (152679) 1998 KU$_\mathrm{2}$ [EPA]

We present a unique and significant polarimetric result regarding the near-Earth asteroid (152679) 1998 KU$\mathrm{2}$ , which has a very low geometric albedo. From our observations, we find that the linear polarization degrees of 1998 KU$\mathrm{2}$ are 44.6 $\pm$ 0.5\% in the R$\mathrm{C}$ band and 44.0 $\pm$ 0.6\% in the V band at a solar phase angle of 81.0\degr. 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 KU$
\mathrm{2}$ has a highly microporous regolith structure comprising nano-sized carbon grains on the surface.

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D. Kuroda, M. Ishiguro, M. Watanabe, et. al.
Wed, 6 Dec 17

Comments: 9 pages, 5 figures, and 3 tables, accepted for publication in A&A