Late veneer and late accretion to the terrestrial planets
Late veneer and late accretion to the terrestrial planets
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DOI：
10.1016/j.epsl.2016.09.013
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发表时间：
2016-12-01
影响因子：
5.3
通讯作者：
Ida, S.
中科院分区：
文献类型：
--
作者：
Brasser, R.;Mojzsis, S. J.;Ida, S.
一般认为，硅酸盐金属（roft）行星的形成依赖于亚火星大小的行星胚胎和（较小的）星子的混合物的凝结，这些星子在我们太阳系的最初几百万年中动态地从不断发展的环日盘中出现。一旦行星在大部分情况下在巨大的撞击阶段之后聚集起来，它们就会继续受到吸积留下的大量微行星的轰击。在这里，我们将限制这些星子的质量和演化的基础上，从高度亲铁元素（HSE）预算的月球的约束。结合N体和行星形成的蒙特卡罗模拟结果，我们得出了关于这个早期时代性质的四个关键结论。首先，地球与月球的HSE比率相匹配，需要地球上的晚期单板在4.45 Ga之前由一个单一的月球大小的撞击物撞击地球组成，或者它起源于创造月球的撞击。另一个复杂的问题是，对月球样本的分析表明，月球并没有像地球那样保存有令人信服的晚期饰面证据。第二，火星上预期的南极单板成分是0.06%重量。第三，陆地撞击物的通量必须很低（小于或接近10（-6）M-圈加上Myr（-1）），以避免4.4 Ga后地壳的大规模熔化，同时与观测到的月球盆地数量相匹配。这一结论导致了一个冥古宙这是更多的比假设先前。最后，在类地行星完全形成后，残留微行星的质量近似于10（-3）M圈以上，比大多数先前模型所认为的至少低一个数量级。我们的动力学和地球化学自洽的情况下，要求未来的N体模拟岩石行星的形成，要么直接将碰撞研磨或依赖于卵石吸积。(C)2016爱思唯尔B. V.保留所有权利。
It is generally accepted that silicate-metal ('roft') planet formation relies on coagulation from a mixture of sub-Mars sized planetary embryos and (smaller) planetesimals that dynamically emerge from the evolving circum-solar disc in the first few million years of our Solar System. Once the planets have, for the most part, assembled after a giant impact phase, they continue to be bombarded by a multitude of planetesimals left over from accretion. Here we place limits on the mass and evolution of these planetesimals based on constraints from the highly siderophile element (HSE) budget of the Moon. Outcomes from a combination of N-body and Monte Carlo simulations of planet formation lead us to four key conclusions about the nature of this early epoch. First, matching the terrestrial to lunar HSE ratio requires either that the late veneer on Earth consisted of a single lunar-size impactor striking the Earth before 4.45 Ga, or that it originated from the impact that created the Moon. An added complication is that analysis of lunar samples indicates the Moon does not preserve convincing evidence for a late veneer like Earth. Second, the expected chondritic veneer component on Mars is 0.06 weight percent. Third, the flux of terrestrial impactors must have been low (less than or similar to 10(-6) M-circle plus Myr(-1)) to avoid wholesale melting of Earth's crust after 4.4 Ga, and to simultaneously match the number of observed lunar basins. This conclusion leads to an Hadean eon which is more clement than assumed previously. Last, after the terrestrial planets had fully formed, the mass in remnant planetesimals was similar to 10(-3) M-circle plus, lower by at least an order of magnitude than most previous models suggest. Our dynamically and geochemically self-consistent scenario requires that future N-body simulations of rocky planet formation either directly incorporate collisional grinding or rely on pebble accretion. (C) 2016 Elsevier B.V. All rights reserved.