Developing space weathering on the asteroid 25143 Itokawa

There has been a long-standing problem that the parent bodies of ordinary chondrites, the most abundant type of meteorites, do not seem abundant among asteroids. One explanation is that surfaces of such ordinary chondrite parent bodies become optically altered to become the S-type asteroids which are abundant in the main asteroid belt. The process is called 'space weathering' which makes the visible and near-infrared reflectance spectrum of a body darker and redder1. The result of recent survey of small, near-Earth asteroids suggests that the surfaces of small S asteroids may present developing stages of space weathering2. Here we report a discovery that a dark region on a small (550-meter) asteroid 25143 Itokawa is significantly more space-weathered than a nearby bright region. Spectra of both regions show the 1-m absorption band shape consistent with those of LL5-6 chondrites after continuum removal3. A simple calculation based on Hapke's space weathering model4 suggests that the dark area has a shorter mean optical path length and about 0.04 volume % more nanophase reduced iron (npFe0) particles than the bright area. This discovery clearly shows a process of accumulating space-weathered materials on small asteroids, which is likely to be an immediate parent body of LL chondrites. Because LL chondrites are the smallest in abundance among ordinary (H, L, and LL) chondrites, the discovery of an S asteroid having an LL-chondrite composition strongly suggests that there are many S asteroids having ordinary-chondrite compositions in the near-Earth orbits.