2013-11-12

第３６８回 †

日時　平成２５年１１月１２日　火曜日　午後５時より
場所　東京工業大学　石川台２号館　３１８号室　

＜講師＞　圦本 尚義 先生 (北海道大学 理学研究科)
＜題名＞「太陽系の惑星の水の成因と水素、窒素、酸素同位体組成との関連性」

＜要旨＞

Hydrogen, Nitrogen and Oxygen are among the most abundant elements of the universe. Isotopic compositions of these elements between molecules are highly variable in molecular clouds. Due to highly volatile nature of these elements, the chemical forms are easily changed between vapor and solid (ice) by environmental temperature and pressure. Thus, the standard planetary formation model of the solar system suggests that inner planets deplete these elements, but outer planets enrich them as major elements. Isotopic compositions for planets of these three elements should be determined spontaneously according to the standard planetary formation processes. Therefore, the isotopic variation between planets would be an important key to clarify how to form planets in the solar system. In this report, we propose new systematic approach to infer isotopic compositions for H, N and O of outer and inner planets.

We have proposed a model for oxygen isotopic evolution in proto-planetary disk [1], and infered O isotopic compositions of outer planets [2]. The augmented model based on [2] in this study assumes the initial condition of isotopic compositions of molecules observed in molecular clouds and in chondrites, and includes two key points, i.e., 1) temporal preservation of chemical species fractionated in mass and 2) astronomical space separation by dynamic coupling/decoupling due to the chemical form changes for H, N and O in the disk.

The model infer systematic increase of heavier isotope components of H, N and O for outer planets towards increasing redial distance from the sun, whereas relatively uniform isotopic composition for inner planets. Inferred H isotope variations between outer planets are quantitatively consistent with observation data by planet explorations [e.g. 3]. We infer enrichments of 15N in the order of Jupiter, Saturn, and Uranus/Neptune. The 15N/14N ratio of Uranus/Neptune would be larger than the Earth’s value. Oxygen isotope systematics between outer planets would be mass independent and 16O component would be depleted in the order from Jupiter towards Neptune. The isotopic compositions of inner planets suggest significant accretions of ices from outer solar system during planetary growth and as late veneer. This mechanism can be traced in asteroidal water as results of water-rock interaction.

[1] Yurimoto & Kuramoto (2004) Science 305, 1763-1766. [2] Kuramoto & Yurimoto (2005) In Chondrites and the Protoplanetary Disk 341, pp. 181-192. [3] Hartogh et al. (2011) Nature 478, 218–220.