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

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 × 10⁷ yr for 5 mJ and ∼7.9 × 10⁶ 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 μm band depths decreased by 12% and 50%, respectively. The particle surface in the 5 mJ irradiated area shows melted and vesiculated structures, indicating higherature 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.