Elucidation of origin of water on rocky planets using isotopic records in meteorites

2021 Fiscal Year Final Research Report

• Project/Area Number: 19K14790
• Research Category: Grant-in-Aid for Early-Career Scientists
• Allocation Type: Multi-year Fund
• Review Section: Basic Section 17010:Space and planetary sciences-related
• Research Institution: Hiroshima University (2020-2021); Japan Aerospace EXploration Agency (2019)
• Principal Investigator: Koike Mizuho 広島大学, 先進理工系科学研究科(理), 助教 (60836154)
• Project Period (FY): 2019-04-01 – 2022-03-31
• Keywords: 分化隕石 / 小惑星 / 放射年代測定 / 二次イオン質量分析計

Outline of Final Research Achievements

This study focused on geochemical records in the ancient differentiated meteorites, which are believed to have originated from remnant planetesimals or protoplanets. Rocky meteorite groups named howardite, eucrite, and diogenite (called HED) are considered as initial crustal materials of asteroid 4-Vesta, that recorded various processes among the rocky planets region. To understand the origin of volatiles and evolution process of rocky planets, I have conducted in-situ radiometric dating by a secondary ion mass spectrometer (NanoSIMS) along with petrological observations and stable isotopic analysis of several differentiated meteorites, including HED and other samples. Based on this study, it is suggested that the collisional rate of the rocky planets region was much higher at early stage and decreased monotonically. Major collisional events should have ceased by 4.15 billion years ago, inferring that supply of volatiles ended before that time.

Free Research Field

宇宙地球化学

Academic Significance and Societal Importance of the Research Achievements

小惑星起源の試料には、地球や月以前の記録が残されている。従来、月試料などの同位体記録から、地球を含む岩石惑星領域での天体衝突は約39億年前（形成から６－７億年後）に急激に増加したと予想されてきた。本研究から、小惑星帯における主要な天体衝突は約45億―41.5億年前という比較的早い段階で完了していたことが示され、岩石惑星領域における物質移動と供給時期について新たな知見が得られた。太古の天体衝突と物質供給は、地球や火星における原始海洋の形成や最古の生命誕生にも関わる問題である。本研究の成果は、地球を始めとする惑星古環境進化の解明に貢献すると期待される。