Shock metamorphism on the surface of the Moon

Abstract

The material characteristics of lunar materials are discussed as indicators of shock metamorphism. As lunar materials are subjected to both impact shock modification due to energetic particles formed at locations of collisionless plasma shocks in cosmic space, it is necessary to identify specific features of each process. All types of lunar glasses formed by various impacts under high pressure (∼ 90 GPa) are found as impact glass, ropy glass, agglutinates and brecciated rocks. The agglutinates and iron particles are identified from X-ray diffraction measurements. Fine-grained lunar materials, including the agglutinates and breccias bonded by crystals and glasses due to impact shocks, contain hydrogen and helium from the energetic particles of collisionless shocks. Lunar shocked minerals of plagioclase and silica show anomalous compositions and densities. The shock metamorphism shows evidence for two major impact processes on evolved and primordial lunar surfaces: (1) shocked silica phases with minor Al contents formed from plagioclase-rich primordial crusts of the Moon, and (2) shocked quartz formed by silica-rich target rocks, especially on evolved parts of the Moon. Silica subjected to both impact and energetic particle modification can grow to coarse-grained normal crystals by long existence in vapor plume under high-temperature or after gradual high-temperature evolution, with the result that original effects of the impact process cannot be distinguished.