2018A&A...611A..31K

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.