Polarimetric Study of Near-Earth Asteroid (1566) Icarus

We conducted a polarimetric observation of the fast-rotating near-Earth asteroid (1566) Icarus at large phase (Sun–asteroid–observer’s) angles α = 57°–141° around the 2015 summer solstice. We found that the maximum values of the linear polarization degree are Pmax = 7.32 ± 0.25% at phase angles of αmax = 124° ± 8° in the V-band and Pmax = 7.04 ± 0.21% at αmax = 124° ± 6° in the RC-band. Applying the polarimetric slope–albedo empirical law, we derived a geometric albedo of pV = 0.25 ± 0.02, which is in agreement with that of Q-type taxonomic asteroids. αmax is unambiguously larger than that of Mercury, the Moon, and another near-Earth S-type asteroid (4179) Toutatis but consistent with laboratory samples with hundreds of microns in size. The combination of the maximum polarization degree and the geometric albedo is in accordance with terrestrial rocks with a diameter of several hundreds of micrometers. The photometric function indicates a large macroscopic roughness. We hypothesize that the unique environment (i.e., the small perihelion distance q = 0.187 au and a short rotational period of Trot = 2.27 hr) may be attributed to the paucity of small grains on the surface, as indicated on (3200) Phaethon.