Laboratory Simulation of Space Weathering on Silicate Surfaces in the Water Environment

Silicate nanoparticles occur in various astrophysical environments where they experience substantial processing due to events such as grain-grain collisions and irradiation. However, the structure and chemical evolution together with the origin of these grains are still poorly understood and intensively debated. For this purpose, we performed liquid-phase nanosecond pulsed laser ablation on olivine single crystals to (i) simulate space weathering in a water environment (e.g., hydrous or volatile-rich bodies) and (ii) study the chemical and structural evolution of both the target surface and the ablated material. In particular, optical spectroscopy analyses have been performed on the ablated material and correlated with high-resolution transmission electron microscopy and diffraction, whereas compositional variations of the ablated target surface were determined by X-ray photoelectron spectroscopy. Our results show that the target material is enriched in Fe and depleted in Mg after the ablation process, with the water environment triggering the oxidation of Fe2+ into Fe3+ in a region confined at the solid-liquid interface and thus promoting the formation of magnetite on the sample surface. On the other hand, in the ablated material we find olivine crystalline fragments with shock features together with Mg-rich crystalline nanoparticles. Notably, no metallic iron nanoparticles have been detected in the ablated material. Our simulation of space weathering in the water environment revealed structural and chemical changes which are expected to give rise to distinctive features in the reflectance spectra when compared to those from airless bodies of the inner solar system.