Simulation of space weathering of planet-forming materials: Nanosecond pulse laser irradiation and proton implantation on olivine and pyroxene samples

Abstract

For the purpose of simulating the surface alteration process called "space weathering", experiments of pulse laser irradiation, proton implantation, and laser irradiation to proton implanted samples were performed and reflectance spectra of altered materials were measured. To simulate the impact heating by micrometeorite bombardments, we made a new apparatus using a pulse laser whose pulse duration is 6-8 nanoseconds, comparable with a timescale of micrometeorite impacts. We find that the degree of space weathering, i.e., change of reflectance spectrum should depend on mineral composition. Laser irradiation onto olivine produces the largest reduction of albedo and the highest reddening of reflectance spectrum. In general, variation of olivine spectra is much larger than that of pyroxenes. Depths of absorption bands do not change in the scaled spectra. The olivine spectrum after the laser irradiation can match spectra of some olivine asteroids within a subtype of S-type asteroids. Comparison of Vesta spectrum with altered pyroxene spectra suggests that Vesta surface would be relatively older than olivine asteroids. We also investigate the influence of solar wind proton and pyroxene FeO content. The proton implantation causes small changes in olivine and enstatite spectra. Implanted protons do not influence spectral change by the laser irradiation: the laser irradiation and the proton implantation do not produce multiplicative but additive changes on the reflectance spectra. FeO content of pyroxenes does not relate to the degree of reflectance change.

Surface Composition of Vesta: Issues and Integrated Approach

“…Meteorite regolith breccias, as proxies for asteroidal regolith, were searched for similar nanophase particles, with the howardite Kapoeta being one of the few minor, but successful detections (Noble et al 2010). Laboratory experiments have various degrees of success in simulating these particular space weathering products along with their optical effects (e.g., Cassidy and Hapke 1975;Moroz et al 1996;Yamada et al 1999;. A model describing the optical effects of nanophase metallic iron was successfully developed by Hapke (2001).…”

Space weathering trends among carbonaceous asteroids

“…These moderate albedo (p v ∼0.22; Mainzer et al 2011) asteroids are known to spectrally darken, redden and have increasingly suppressed absorption bands as a function of time (Belton et al 1992(Belton et al , 1994Binzel et al 1996;Chapman 1996). These spectral changes are attributed to the vapor deposition of sub-micron metallic iron (SMFe) particles onto grains during micrometeorite impacts and solar wind irradiation (Yamada et al 1999;Hapke 2001;Sasaki et al 2001;Brunetto & Strazzulla 2005). The intrinsically dark (p v ∼0.06; Mainzer et al 2011) nature of carbonaceous material and the lack of prominent absorption features at visible and near-IR wavelengths implied space weathering trends would be difficult to identify for C-complex asteroids (Hapke 2001;Moroz et al 1996).…”

Dawn: A journey in space and time

“…These variations, inferred to reflect impact-excavated plutonic rocks in the east and lava flows in the west, have not been petrologically homogenized by regolith formations or spectrally obscured by space weathering. The lack of space weathering has been attributed by Hiroi et al, [1994] to a scarcity of olivine, which may be the principal mineral altered by this process [Yamada et al, 1999]. However, the surface of the moon is weathered and it lacks olivine also.…”