Space weathering and the surface evolution of small asteroids: Observation of Itokawa and Laboratory Simulation

Abstract

In 2005, HAYABUSA observed an S-type asteroid Itokawa (with size of 550m) by onboard camera AMICA (Saito et al., 2006). Almost 80% of the surface is rough and boulder-rich but it has weathered spectrum on average. Optically, the surface of Itokawa is divided into brighter (and bluer) areas and darker (and redder) areas. In rough zones, dark boulder-rich surfaces usually superpose on bright materials. We can interpret impact-induced seismic shaking or shaking at planetary encounters should remove dark weathered boulder-rich surface to expose underlying relatively fresh bright area. High resolution images indicate that these bouldered surface are optically weathered. To check the possibility that the rock surface could be weathered, we irradiate pulse laser on meteorite fragments with cut flat surface under vacuum. For comparison, we also irradiate pellet samples where particle size is smaller than 125 microns. Fresh meteorites NWA1794 and Bensour were chosen because spectral observation of Itokawa suggested its similarity with LL5 and LL6 chondrites. As expected, significant darkening and reddening are observed at irradiating pellet samples.