Growth and Modifications of Crystals on Airless Surfaces

Understanding the modification and synthesis of crystals in the space environment, which is very different from the terrestrial laboratory, can be useful in exploring new possibilities for synthesis methods of nano-engineered materials. This chapter reviews the analysis of extraterrestrial samples from asteroids and the Moon, focusing on crystal growthCrystal growth and modifications induced by ion irradiation and micrometeoroid impacts that occur on airless surfaces in our solar system. Solid materials exposed to the space environment are irradiated by high-energy ions originating from the Sun, known as solar wind and solar flares. Airless surfaces are also impacted by nanometer- to micrometer-sized dust particles (called micrometeoroids) ejected from asteroids and comets. The solar wind irradiation and micrometeoroid impacts are ongoing on many small bodies in the solar system, such as the Moon and asteroids, because these small bodies, unlike Earth, cannot maintain a dense atmosphere by their own gravity and are directly exposed to the space environment. Several types of crystal growthCrystal growth and nanostructure modifications of space-exposed materials have been reported, such as amorphization, blistering, nanoparticleNanoparticles synthesis, the vapor–liquid-solid (VLS) growth and whiskerWhisker formation. Evidence of interaction between solid materials and the space environment was first recognized by the analysis of lunar samples brought back by NASA’s Apollo program. In addition, the asteroid sample return missions of the Japanese spacecraft Hayabusa and Hayabusa-2 have provided opportunities for the analysis of space-exposed samples from asteroids Itokawa and Ryugu. In the field of planetary materials science, modifications in the space environments on airless bodies are referred to as space weatheringSpace weathering. Space weathering has attracted attention in planetary science and space exploration because it alters optical properties of the surface reflection spectrum, which is important for the interpretation of remote-sensing data, and may promote re-distribution of surface materials. The structure of space weatheringSpace weathering is often less than a micrometer in size and has been observed mainly using scanning electron microscopy (SEM) and transmission electron microscopyTransmission electron microscopy (TEM). Experimental study to understand modification processes in crystals due to space weatheringSpace weathering are performed by ion irradiation experiments simulating solar wind irradiation and femtosecond laser pulse irradiation simulating energy injection during micrometeoroid impacts.