LABORATORY SIMULATION OF THE EFFECT OF FESON SPACE WEATHERINGM. Okazaki1, S. Sasaki1, A. Tsuchiyama2, A. Miyake2, T.
Matsumoto2, T. Hirata2 and T. Hiroi3. 1Department of Earth andSpaceScience,Osaka
University,Japan.
E-mail:okazaki@astroboy-jp.com. 2Division of Earth and Planetary
Sciences, Kyoto University, Japan, 3Department of GeologicalSciences, Brown University, USA.
Introduction: In the standard model of space weathering,changes of surface optical properties of airless silicate bodies reddening, darkening, and attenuation of absorption bands - areexplained by nanophase metallic iron (nanoFe) particles, which
are formed on regolith particles by high velocity dust impacts aswell as irradiation of the solar wind ions [1]. Those nanoFe
particles were discovered in lunar soils, Kapoeta meteorite, andregolith grains from the surface of S-type asteroid Itokawa.
Experimental studies using nano-second pulse laser confirmed thatnanoFe should control the spectral darkening and reddening.
Observed reddening of S-type asteroid families is correlated withdynamical asteroid ages after family-forming disruption [2].
However, weathering degree should depends on mineralcompositions such as olivine/pyroxene ratio [3] and ironabundance.
Itokawa: In ordinary chondrites, iron sulfides is the main
sulfur-bearing mineral. TEM observation of dust grains of Itokawa
revealed the presence of not only iron, but also nanophase FeS
particles, which are embedded within a surface vapor-deposited
thin layer (<10 to 15 nm) [4]. One Itokawa's grain is composed
mainly of FeS (about 40 micron) with smaller olivine andpyroxene particles [5], and some other Itokawa particles also
contain FeS. Simulation using a pulsed laser on the Ehole H5chondrite produced a vapor-deposited coating, which consists ofamorphous Mg-rich silicate glass and abundant nanophase (2-5nm) FeS particles [6]. On the other hand, the surface sulfur
depletion of S-type asteroid Eros was explained by the samemechanism (high velocity dust and solar wind particle impacts) as
space weathering [7].
Effect of FeS: To examine the effect of FeS on the surface
optical properties of silicate bodies , we conducted pulse laserirradiation experiments on mixture of olivine (and pyroxene) andFeS particles with sizes typically 45-75 micron, under various FeS
fraction (0-20 wt %). We find that addition of FeS should
promote the change of optical properties in accordance with space
weathering, especially darkening. Compared with the cases whereiron particles are mixed, darkening of infrared region ischaracterized. Observations by FESEM and HR microscope,surfaces of olivine particles after laser irradiation are likely coatedby vapor-deposited material. TEM observation of FIB section ofa coated particle show feature of layered amorphous rim
containing nanophase iron particles. The search of nanophase
FeS is ongoing.
References: [1] Hapke, B. et al.,2001. J. Geophys. Res.106,10039, [2] Jedicke, R., et al., 2004. Nature 429, 275, [3] Hiroi, T.
and Sasaki, S., 2001. Meteoritics and Planetary Science 36, 1587,[4] Noguchi, T. et al. 2011. Science 333,1121., [5] Yada, T. et al.
2014. LPSC 45th, 1759., [6] Keller, L. P. et al. 2013. 45th LPSC
#2404 [7] Loeffler, M. J. et al. 2008. Icarus 195, 622.