Nebular ingassing as a source of volatiles to the Terrestrial planets

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• 作者：Zachary D. Sharp ^{zsharp@unm.edu}
• 关键词：Nebular ingassing ; Volatiles ; Hydrogen isotope ; Chondrites ; Source of volatiles ; Late delivery
• 刊名：Chemical Geology
• 出版年：2017
• 出版时间：5 January 2017
• 年：2017
• 卷：448
• 期：Complete
• 页码：137-150
• 全文大小：1092 K
• 卷排序：448

文摘

The source of volatiles is critical for understanding Terrestrial planet formation and the conditions that lead to habitable planets. The Earth lies inside the so-called ‘snow line’, outside of which H2O condenses to ice and can be incorporated into growing planetary bodies. Inside the snow line, incorporation of volatile elements is more problematic. A number of ideas have been proposed to explain sourcing of volatiles. One of the most common models involves late addition of volatile-rich chondritic material to the Terrestrial planets. This idea satisfies many of the geochemical constraints of the inner planets, but chondrites generally have D/H ratios that are considerably higher than the bulk Earth value. Here it is proposed that ingassing of a nebular atmosphere was an important source of hydrogen and other volatiles to Earth. If the proto-solar nebula was still present by the time the Earth approached its present size, then a dense atmosphere would necessarily develop and ingassing would be inevitable. The low D/H ratio of this component would offset the heavier chondritic and cometary sources and adequately explain the present-day volatile composition of the Earth.Hydrogen ingassing is a two-step process. The first stage involves the formation of a dense nebular atmosphere in the first ~ 10 My after the collapse of the proto-solar nebula. The atmosphere would cause heating of the planetary core to sufficient temperatures so as to cause complete melting. Massive ingassing of H2, H2O and other volatiles would occur. The oxygen fugacity (f(O2)) of the mantle would be lowered and Fe2 + would be reduced to Feo and FeHx and then sequestered into the core. The second stage occurs after the nebula dissipates. Now H2 would degas into the thin atmosphere and be lost to space by hydrodynamic escape. The degassing of H2 would raise the f(O2) of Earth to its present oxidized state and increase the D/H ratio of the remaining hydrogen to a value of ~− 300‰ (vs VSMOW). Evidence for this primitive component is found on samples from Earth, Moon and 4 Vesta. Later addition of chondritic and cometary material brought the D/H and 15N/14N ratios up to their present value. The scenario outlined here eliminates many of the problems inherent in previous models for volatile delivery to Earth.