1. High-temperature processes in the solar nebula
How did our solar system form? This is a very simple but fundamental question. It is difficult to answer precisely. We don't have a time-machine to go back to the world ~4.5 billion years ago. However, there are two major ways to get clues for understanding how our solar system formed. One is astronomical. Astronomers can observe the structures and spectroscopic features of other stellar systems that have masses similar to our solar system. Based on these observations, they can deduce how crystalline materials have formed and evolved in our solar system and other stellar systems. The other is geologic. Earth scientists study chondritic meteorites, the oldest materials in our solar system. They can constrain the physicochemical conditions and processes that have been prevailing in the solar nebula, by studying the minerals in chondritic meteorites and comparing with laboratory simulations and theoretical calculations. In recent years, we study petrography, mineralogy, and rare-earth elements, and oxygen isotope compositions of Ca,Al-rich inclusions (CAIs) and Al-rich chondrules to constrain their origins and implications to related high-temperature processes in the solar nebua.
2. Secondary processes on the Moon and asteroids
It is generally accepted that the planets and asteroids in our solar system have formed within a few million years after the CAI formation. After accretion, the planets and asteroids must have experienced complex thermal, aqueous and impact processes, and space weathering that have changed the petrographic, mineralogical, chemical, and isotope geochemical features of rocks on the surfaces of planets and asteroids to various extents. Constraining how and to what extents the secondary processes affect the surface materials is important to precisely understand the evolution of the planets and asteroids. This is one of the most important aspects of comparative planetology. Recently, we study mineralogical and geochronological records of intense shock metamorphism in lunar and Vestan meteorites and of alteration and metasomatism in both chondrites and achondrites.