小惑星およびいん石の起源と進化における有機質星間塵の役割

Abstract

A series of experiments involving interstellar organic materials, formation in interstellar clouds, evaporation in the solar nebula, collision and subsequent sticking, and metamorphism in the parent bodies of carbonaceous chondrite, were performed to investigate the role of organic materials in the formation and evolution of asteroids and meteorites. It was shown that diamond precursor nucleated as a result of UV photolysis of interstellar ice mixtures in molecular clouds, and grew with further UV irradiation in diffuse clouds. It was found that there would be molecular cloud and diffuse cloud organic materials in the solar nebula at heliocentric distances larger than 2.1 and 2.3 AU, respectively. Very reductive gas was evaporated at around 2.1 AU during the evaporation of diffuse cloud organic materials, and this causes very reduced condition in the solar nebula. The organic material was found to be stickiest at 2.3 and 3.0 AU, with a maximum sticking velocity of 5 m/s. This indicated a very rapid coagulation of the very sticky organic grain aggregates and the formation of planetesimals in the asteroid region, covering even the early stage of the turbulent solar nebula. The planetesimals formed in this region appear to represent achondrite parent bodies. In contrast, the formation of planetesimals at < 2.1 and > 3.0 AU begins with the establishment of a passive disk because silicate and ice are not as sticky as organic grains. The planetesimals formed in respective regions appear to represent ordinary and carbonaceous chondrite parent bodies. We found that both C and N contents and infrared spectra of insoluble organic materials in carbonaceous chondrites could be reproduced well by the aqueous alteration and the subsequent thermal metamorphism of organic materials formed in molecular cloud. The resulting sample shows strong evidence of diamond formation. The various characteristics of nano-diamonds in chondrites are well explained by the formation in interstellar clouds and parent bodies of carbonaceous chondrites.