Sulfur Effect on the Space Weathering of Airless Bodies: Laboratory Simulation

Space weathering is the main process that should control the change of brightness and color of the surface of airless silicate bodies such and the Moon, Mercury and asteroids. S-type asteroids show more overall depletion and reddening of the spectra, and more weakening of absorption bands than ordinary chondrites. Vapor-deposition through at high-velocity dust impacts as well as implantation of intensive solar wind ions may produce the space weathering rims bearing nano-iron particles (npFe0), responsible for spectral change. Simulation experiments using nanosecond pulse laser successfully produced vapor-deposition type npFe0 to change the optical properties [1]. A small (500m) asteroid Itokawa has a weathered surface, although its surface is rocky (rough terrain) or pebble-rich (smooth terrain). In 2011, HAYABUSA returned the particulate samples from the smooth terrain. The most notable discoveries in Itokawa particles are amorphous space-weathering rims containing npFe0. Sulfur and magnesium abundances suggest the presence of nanophase FeS (and MgS) in addition to npFe0 [2]. The presence of npFeS in asteroidal regolith is compatible with the observation of regolith breccia meteorites. On Mercury, MESSENGER revealed a high sulfur abundance (2wt% on average up to 4wt%), which can account for all of Fe by FeS. Both npFeS and npMgS may play an important role also on the surface of Mercury by lowering albedo. In our laboratory simulation using pulsed laser, spectral changes of olivine samples are facilitated when FeS is mixed (5-10wt%) (Fig.1). Nanophase Fe is confirmed by TEM. The darkening feature is reduced by additional heading at 150C, which would suggest the presence of volatile residue. Mixing of pure sulfur particles showed some, but not significant changes after laser irradiation. We acknowledge A. Miyake and A. Tsuchiyama at Kyoto U. for TEM observation. Ref: [1] S. Sasaki et al.: Nature 410 (2001) 555; [2] T. Noguchi et al.: Science 333 (2011) 1121 Fig. 1. Spectral change after pulse laser irradiation. The vertical axis shows normalized reflectance at 2500 nm-infrared darkening, whereas the horizontal axis shows the spectral slope of reflectance ratio of 1600 nm to 560 nm that would show reddening. Size range of fine FeS is smaller than 45 micron.