IMPACTS ON AIRLESS OBJECTS UNDER MICROGRAVITY AND POSSIBLE RELATED STRUCTURES IN ITOKAWA ASTEROID SAMPLE

摘要

Impact processes might produce unusual craters on asteroids relatively to larger objects like the Earth or Moon because of two reasons: porous interior and low gravity environment. Based on observations made by spacecrafts during close flybys at asteroids, strange craters and signs of special processes were observed including collapse crates without ejecta, regolith disturbance produced ejecta-like halos around craters, boulders related to impact processes etc. Laboratory analysis completed these observations on the behavior of granular regolith under low gravity that pointed out to special structures connected to impact processes. For the first direct analysis of impact influenced asteroid surface material in laboratory is possible using the Hayabusa mission returned material of Itokawa asteroid. The small particles and their Micro-Raman as well as Cathodoluminescence properties are expected to evaluate the shock wave history and degree of the space weathering process of asteroid Itokawa. This can aid to understand more about the giant impact-related formation of the rubble-piled asteroids. Furthermore, there are some new insights to contribute the spectral changes of asteroidal bodies due the increasing shock pressure. The expected outcome of the shock determination of the fine-grained astromaterials obtained by the Hayabusa-mission may be applied to the future sample-return missions such as Hayabusa-2, Marco Polo, etc. Cathodoluminescence spectral properties of the Hayabusa-plagioclase particle obtained from asteroid Itokawa that show defect-related broad band centered at around 450 nm with a shoulder peak at 425 nm in the blue region, but there is no Mn- or Fe-related emission peaks in this sample. The absence of these crystal field-related activators indicates that the plagioclase was formed during thermal metarmophism at subsolidus temperature with an extreme low oxygen fugacity.