Space Weathering Simulation with Low-energy Laser Irradiation of Murchison CM Chondrite for Reproducing Micrometeoroid Bombardments on C-type Asteroids

Abstract

Micrometeoroid bombardments are one of the causes of space weathering on airless bodies. We have simulated micrometeoroid bombardments on the surfaces of C-type asteroids by pulse-laser irradiation experiments on Murchison CM2 chondrite samples. In this Letter, we focus in particular on the effect of lower-energy irradiation compared to our previous study, where the laser energy range was set to 5–15 mJ, causing spectral flattening and water absorption band suppression. Murchison powder samples were irradiated with pulse lasers of various laser intensities (0.7, 1, 2, and 5 mJ). The irradiation energies are equivalent to micrometeoroid bombardments on the main-belt asteroids for ∼5.7 × 10 7 yr for 5 mJ and ∼7.9 × 10 6 yr for 0.7 mJ, respectively. We measured reflectance spectra and analyzed chemical compositions and microstructures of the surface of the laser-irradiated Murchison samples. Laser-irradiated Murchison spectra show flattening and darkening in the ultraviolet (UV)–visible (Vis)–infrared (IR) range. As the laser energy was increased up to 5 mJ, the 3 and 0.7 μ m band depths decreased by 12% and 50%, respectively. The particle surface in the 5 mJ irradiated area shows melted and vesiculated structures, indicating high-temperature heating by laser irradiation followed by rapid cooling. The chemical composition of the melted and bubbled portions is similar to FeS-rich amorphous silicate particles observed in the high-energy laser irradiation case. Each mineralogical change of Murchison due to short-duration heating would cause spectral bluing, darkening, and absorption band suppression.