Marchi S, Canup R M, Walker R J
Southwest Research Institute, Boulder, CO, USA.
Deptartment of Geology, University of MD, College Park, MD, USA.
Nat Geosci. 2018;11:77-81. doi: 10.1038/s41561-017-0022-3. Epub 2017 Dec 4.
After the Moon's formation, Earth experienced a protracted bombardment by leftover planetesimals. The mass delivered during this stage of late accretion has been estimated to be approximately 0.5% of Earth's present mass, based on highly siderophile element concentrations in the Earth's mantle and the assumption that all highly siderophile elements delivered by impacts were retained in the mantle. However, late accretion may have involved mostly large (≥ 1,500 km in diameter)-and therefore differentiated-projectiles in which highly siderophile elements were sequestered primarily in metallic cores. Here we present smoothed-particle hydrodynamics impact simulations that show that substantial portions of a large planetesimal's core may descend to the Earth's core or escape accretion entirely. Both outcomes reduce the delivery of highly siderophile elements to the Earth's mantle and imply a late accretion mass that may be two to five times greater than previously thought. Further, we demonstrate that projectile material can be concentrated within localized domains of Earth's mantle, producing both positive and negative W isotopic anomalies of the order of 10 to 100 ppm. In this scenario, some isotopic anomalies observed in terrestrial rocks can be explained as products of collisions after Moon formation.
月球形成后，地球经历了来自残余小行星的长期轰击。根据地球地幔中高度亲铁元素的浓度以及撞击所带来的所有高度亲铁元素都保留在地幔中的假设，在这个晚期吸积阶段所输送的物质质量估计约为地球当前质量的0.5%。然而，晚期吸积可能主要涉及大型（直径≥1500千米）——因此已经分异的——抛射体，其中高度亲铁元素主要被隔离在金属核中。在此，我们展示了光滑粒子流体动力学撞击模拟，结果表明大型小行星的相当一部分核心可能会沉入地球核心或完全逃离吸积过程。这两种结果都会减少高度亲铁元素向地球地幔的输送，并意味着晚期吸积质量可能比之前认为的大两到五倍。此外，我们证明抛射体物质可以集中在地球地幔的局部区域内，产生10至100 ppm量级的正负钨同位素异常。在这种情况下，在地球岩石中观察到的一些同位素异常可以解释为月球形成后碰撞的产物。
