Ejection of iron-bearing giant-impact fragments and the dynamical and geochemical influence of the fragment re-accretion
Ejection of iron-bearing giant-impact fragments and the dynamical and geochemical influence of the fragment re-accretion
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DOI：
10.1016/j.epsl.2017.04.035
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发表时间：
2017-07-15
影响因子：
5.3
通讯作者：
Ikoma, Masahiro
中科院分区：
文献类型：
--
作者：
Genda, Hidenori;Iizuka, Tsuyoshi;Ikoma, Masahiro
地球诞生于暴力之中。许多原行星的巨大碰撞被认为是在类地行星形成过程中发生的。在这里，我们使用了一种混合代码来研究巨型撞击阶段，该代码一致地处理原行星围绕太阳的轨道演化，以及两个原行星之间巨大碰撞过程的细节。相当数量的物质(高达原行星总质量的几十%)是由巨大的撞击抛出的。我们称这些喷出的碎片为巨型撞击碎片(GIF)。在一些侵蚀性的肇事逃逸和高速碰撞中，金属铁也被抛出，金属铁来自相撞的原行星的核心。通过对10个大碰撞阶段的数值模拟，我们发现金属铁在GIF中的质量分数在1wt%到25wt%之间。我们还讨论了GIF对形成的类地行星的动力学和地球化学特征的影响。我们发现，GIF有可能解决以下动力学和地球化学冲突：(1)目前处于近圆形轨道的地球很可能在大碰撞阶段有一个高度偏心的轨道。GIF的总质量足够大，可以通过它们的动力摩擦将地球的偏心率降低到目前的值。(2)地幔中高度亲铁元素(HSE)的浓度高于实验预测。含铁的GIF重新沉积到地球上可能会导致HSE的过剩。此外，含铁的GIF提供了重要的还原剂，可以将原始的CO2-H2O大气和海洋转化为更还原的含H-2大气。因此，GIF对于地球生命的起源及其早期演化是重要的。(C)2017爱思唯尔B.V.保留所有权利。
The Earth was born in violence. Many giant collisions of protoplanets are thought to have occurred during the terrestrial planet formation. Here we investigated the giant impact stage by using a hybrid code that consistently deals with the orbital evolution of protoplanets around the Sun and the details of processes during giant impacts between two protoplanets. A significant amount of materials (up to several tens of percent of the total mass of the protoplanets) is ejected by giant impacts. We call these ejected fragments the giant-impact fragments (GIFs). In some of the erosive hit-and-run and high-velocity collisions, metallic iron is also ejected, which comes from the colliding protoplanets' cores. From ten numerical simulations for the giant impact stage, we found that the mass fraction of metallic iron in GIFs ranges from 1 wt% to 25 wt%. We also discussed the effects of the GIFs on the dynamical and geochemical characteristics of formed terrestrial planets. We found that the GIFs have the potential to solve the following dynamical and geochemical conflicts: (1) The Earth, currently in a near circular orbit, is likely to have had a highly eccentric orbit during the giant impact stage. The GIFs are large enough in total mass to lower the eccentricity of the Earth to its current value via their dynamical friction. (2) The concentrations of highly siderophile elements (HSEs) in the Earth's mantle are greater than what was predicted experimentally. Re-accretion of the iron-bearing GIFs onto the Earth can contribute to the excess of HSEs. In addition, Iron-bearing GIFs provide significant reducing agent that could transform primitive CO2-H2O atmosphere and ocean into more reducing H-2-bearing atmosphere. Thus, GIFs are important for the origin of Earth's life and its early evolution. (C) 2017 Elsevier B.V. All rights reserved.