Global mapping of the degree of space weathering on asteroid 25143 Itokawa by Hayabusa/AMICA observations

Abstract— We obtained color images of near‐Earth asteroid 25143 Itokawa by the Hayabusa multiband imaging camera to characterize the regional color properties. Images were obtained for the whole disk from the gate position (GP) and home position (HP) at a spatial resolution of 0.8–3.7 m/pixel. Whole‐disk spectra are adjusted to the telescopic data obtained by the University of Hawai'i's 88‐inch telescope using the Eight Color Asteroid Survey (ECAS) system. The disk‐resolved measurements show large variations in the three visible channels. We present a map of an index related to the degree of space weathering, which has been newly developed based on laboratory measurements. We find large variations in the degree of space weathering on Itokawa. Fresh materials are observed in regions of steep slopes and craters, whereas mature materials are ubiquitously distributed. This result suggests that pristine ordinary chondrite‐like materials have been exposed through weathered layers by excavation. By also examining close‐up images obtained during touchdown rehearsal, we find that most rocks in Itokawa's rough terrains are weathered. Instead of a regolith blanket, the surface of this small asteroid is covered with weathered rocks and gravels.     

海底地震学

在海洋地区进行高质量的长期地震观测是全球地震观测的一个重要组成部分，DSDP／ODP是唯一能够钻穿软的沉积物、在坚硬的岩石里安置地震传感器这一目标的科学计划。介绍了ODP航次在井孔中设立地震台站、并获得一些有趣的结果的成功例子。ODP在日本外海布置了两个井中地震台站，与陆上台站一起来观测板块边界的活动性。此外，还介绍了西太平洋井中宽带台站、海上地震信号和噪音等问题。