2017 年 51 巻 2 号 p. 49-59
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.