Pulse-laser irradiation experiments of Murchison CM2 chondrite for reproducing space weathering on C-type asteroids Moe Matsuoka a,⇑ , Tomoki Nakamura a , Yuki Kimura b , Takahiro Hiroi c , Ryosuke Nakamura d , Satoshi Okumura a , Sho Sasaki e a Division of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan b Institute of Low Temperature Science, Hokkaido University, Sapporo, Hokkaido 060-0819, Japan c Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02912, USA d National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan e Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan article info Article history: Received 31 October 2014 Revised 27 February 2015 Accepted 28 February 2015 Available online 14 March 2015 Keywords: Spectroscopy Meteorites Asteroids abstract We performed pulse-laser irradiation experiments of a primitive meteorite to simulate space weathering by micrometeorite bombardments on C-type asteroids. Pellets of powdered Murchison CM2 chondrite were set in vacuum and exposed to pulse laser with a diameter of 0.5 mm and delivered energies of 5, 10, and 15 mJ. We measured reflectance spectra of unirradiated and irradiated surfaces of the pellets. During analysis the pellet was heated to approximately 100 °C and purged in N 2 gas in order to reduce absorption of ambient water. The spectra become darker and bluer with increasing laser energies. Their UV reflectance increases and 0.7- and 3-lm band depths decrease from 0 to 15 mJ. The spectral blu- ing observed in our experiments reproduces the bluing occurred during space weathering of C-type aster- oids. High-resolution observation by a transmission electron microscope showed that the laser heating causes preferential melting and evaporation in FeS-rich fine-grained portions, which results in dispersion and deposition of numerous FeS-rich amorphous silicate particles 20–1000 nm in size on the surface of the pellet. In addition, at the laser-irradiated but unmelted areas, heat-induced amorphization and decomposition of serpentine occur. These mineralogical changes make the reflectance spectra of the Murchison CM chondrite darker and bluer. Ó 2015 Elsevier Inc. All rights reserved. 1. Introduction It is known that space weathering gradually alters optical prop- erties of the surface materials of airless bodies such as asteroids and the Moon. Ordinary chondrites, the most abundant meteorites, show absorption features similar to those of S-type asteroids, but their reflectance spectra do not match each other due to space weathering of S-type asteroids. The processes of space weathering, including the formation of nano-phase metallic iron (npFe 0 ) parti- cles, decrease reflectance in the shorter wavelength range, redden spectral profile, and weaken absorption bands. The optical effects of npFe 0 were described and interpreted in several papers (Keller and McKay, 1997; Pieters et al., 2000; Hapke, 2001; Taylor et al., 2001, 2010; Clark et al., 2002; Noble et al., 2007). Several experiments have been performed for space weathering simulation. Ion irradiation experiments were performed to simulate solar-wind implantations on asteroids and showed a vari- ety of spectral changes (Hapke, 1965; Brunetto and Strazzulla, 2005; Marchi et al., 2005; Strazzulla et al., 2005; Loeffler et al., 2009; Fu et al., 2012; Nakauchi et al., 2014). On the other hand, Yamada et al. (1999) and Sasaki et al. (2001, 2003) used pulse-laser irradiation to simulate micrometeorite bombardments on S-type asteroids and succeeded in reproducing reddening and darkening of reflectance spectra and forming npFe 0 particles. Furthermore, in the Japanese sample return mission, the Hayabusa spacecraft has succeeded in recovering samples from the surface of an S-type asteroid, Itokawa. The analyses revealed that Itokawa dust particles are mineralogically and composition- ally similar to equilibrated LL chondrites and therefore proved that parent bodies of ordinary chondrites are included among S-type asteroids (Nakamura et al., 2011). In addition, npFe 0 and sulfur- bearing Fe-rich nanoparticles were found in the surface layers of Itokawa dust particles and these nano particles appear to have been produced by space weathering (Noguchi et al., 2011). Thus, space weathering effects and products of S-type asteroids and ordinary chondrites are understood relatively well. However, http://dx.doi.org/10.1016/j.icarus.2015.02.029 0019-1035/Ó 2015 Elsevier Inc. All rights reserved. ⇑ Corresponding author. E-mail address: mmatsuoka@dc.tohoku.ac.jp (M. Matsuoka). Icarus 254 (2015) 135–143 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus