10Kの氷から光脱離する水の核スピン異性体比と宇宙の水の起源について

摘要

宇宙で最も多く存在する元素は水素であり，次いでへリウム(水素との存在比でおよそ10~(-1))，酸素(同様に10~(-4))と続く.超高真空環境である宇宙では水素とヘリウムは固体としては安定に存在できないが，水素結合ネットワークを形成する水分子(H_2O)は，固体(氷)として宇宙のさまざまな領域に存在している.宇宙の元素存在度から，宇宙で最も豊富に存在する固体物質は氷である.%Chemistry of interstellar H_2O is essential for understanding the formation of stars and planetary systems because of the ubiquity of H_2O in space. The abundance ratio of nuclear spin isomers (the ortho-to-para ratio, OPR) can be a key for interstellar water chemistry, when assuming that the OPR desorbed from ice is closely related to the ice formation temperature. However, the above assumption has not been experimentally validated. Here, we report that H_2O photodesorbed from ice at 10 K shows a statistical OPR of 3, even when the ice is produced in situ by hydrogenation of O_2, a known formation process of interstellar H_2O. This invalidates the hypothesis for relation between OPR and temperature. Reinterpretation of previous observations is necessary to improve our understanding of interstellar chemistry and the formation of the solar system and comets.