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

TL;DR

This study reports the highest polarization degree observed in a low-albedo asteroid, revealing unique surface properties and pioneering polarimetric measurements at large phase angles for primitive, low-albedo airless bodies.

Contribution

First polarimetric measurement of a primitive, low-albedo asteroid at large phase angles, indicating highly microporous surface structure with nano-sized carbon grains.

Findings

Polarization degrees of ~44% at 81° phase angle.

Highest polarization observed among airless bodies.

Suggests highly microporous, nano-carbon surface regolith.

Abstract

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.