Opposition Effect of Asteroid (25143) Itokawa

Abstract

On a planetary surface, the reflected brightness is not uniformly risen near zero phase angle (i.e. the angle between Sun – object – observer, α≈0°), which is known as opposition effect. Because most of outer Solar System objects (e.g. asteroids and Kuiper-Belt Objects) have been observed near the zero phase angles, it is of importance to understand the physical mechanism for the opposition effect. However, little is known about the cause of the opposition mechanism, although two possible hypotheses, that is, a shadow hiding effect and a coherent backscattering effect have been suggested mostly based on the theoretical studies (e.g. Hapke 1986, 2002). In 2005, Japanese sample returning interplanetary mission, Hayabusa, succeeded in obtaining regolith sample from S-type asteroid (25143) Itokawa. It is expected that the data may provide a clue for the better understanding of the opposition mechanism on the asteroidal surface, because some of the images were intentionally acquired near the opposition. We analyzed images were taken with the Asteroid Multi-band Imaging Camera (AMICA) at b (429 nm), v (553 nm), w (700 nm), and p (960 nm)-bands, and derived the slope of the reflectance (hereafter, SOE) around the zero phase angles. As a result, we found that (1) SOE changes drastically at α<1.2°-1.5°, whereas becomes constant at α>1.2°-1.5°, (2) this trend does not show wavelength dependence for all four bands, (3) incidence and emission angles do not make a significant influence on SOE. From these results, we conjectured that the coherent backscattering effect dominates on Itokawa under phase angle of 1.2°-1.5° while the shadow hiding effect stands out over the phase angle. Note that the evidence of coherent backscattering effect is hardly confirmed on any Solar System objects because the shadow of the observer itself obscures the signal near the opposition (α<1.4°), making it impossible to recognize the effect. Accordingly, we would insist that the telescopic imaging capability from a distant place (9 km from the surface) as well as the multiband data of AMICA enable to obtain definitive evidence for the mechanisms of the opposition effect.