酸素同位体局所精密分析法の開発とそれを用いた太陽系初期の酸素同位体不均一に関する研究

抄録

Oxygen is the third most abundant element in the Universe and the most abundant element of the terrestrial planets. The presence of oxygen in gaseous, ice and dust phases makes oxygen isotopes important tracers of various fractionation processes to form a protoplanetary accretion disk, which are essential for understanding the evolution of building blocks for planet formation. Photodissociation of CO isotopologues in cold interstellar environments forms H₂O ice with depletion of ¹⁶O component relative to the interstellar CO, but with heritage ¹⁷O/¹⁸O ratio from the interstellar CO. Dynamic evolution of protoplanetary disk generates H₂O enrichments inside snowline of the disk to change from ¹⁶O-rich to ¹⁶O-poor gaseous environments. Thermodynamics during heating processes reset oxygen isotopic compositions of dust in the disk to the gaseous oxygen isotope values. Therefore, building blocks of planet show oxygen isotope variations with variable ¹⁶O component, but with similar ¹⁷O/¹⁸O ratio each other. Oxygen isotopic compositions of outer planets would be ¹⁶O-poor in order of increasing distance from the Sun if outer planet formation started from icy planetesimal accretion.