标题： Theia的动态起源，地球上的最后一个巨大撞击体 显示英文标题

标题： Dynamical origin of Theia, the last giant impactor on Earth

摘要： 宇宙化学研究提出，地球大约有5-10%的质量来自于碳质（CC）物质，其中大部分是在最后通过其撞击体Theia（形成月球的撞击体）输送的。 在这里，我们使用地行星形成的动力学模拟来评估这一观点，从一个标准设置开始，即在金星和地球轨道之间的环形区域内分布着行星胚胎和小行星，并且还包括由木星散射向内的CC小行星和行星胚胎群体。 我们发现，这种情景可以满足许多约束条件，包括i）类地行星的质量和轨道；ii）地球的CC质量分数；iii）火星的CC质量分数要低得多，只要火星只吸积了CC小行星（但没有CC胚胎）；iv）最后一次巨大（形成月球的）撞击的时间；以及v）后期吸积阶段主要由非碳质（NC）天体主导。 为了使这种情景成立，散射的CC天体的总质量必须约为0.2 - 0.3 M$_{\oplus}$，其中胚胎与小行星的质量比至少为8，并且CC胚胎的质量范围约为0.01 - 0.05 M$_{\oplus}$。 在这种情况下，我们的模拟显示，地球最后一次巨大撞击体（Theia）是碳质物体的可能性约为50-50——要么是一个纯的CC胚胎，要么是一个之前吸积了CC胚胎的NC胚胎。 因此，我们的模拟为宇宙化学研究提供了动力学验证。 显示英文摘要

摘要： Cosmochemical studies have proposed that Earth accreted roughly 5-10% of its mass from carbonaceous (CC) material, with a large fraction delivered late via its final impactor, Theia (the Moon-forming impactor). Here, we evaluate this idea using dynamical simulations of terrestrial planet formation, starting from a standard setup with a population of planetary embryos and planetesimals laid out in a ring centered between Venus and Earth's orbits, and also including a population of CC planetesimals and planetary embryos scattered inward by Jupiter. We find that this scenario can match a large number of constraints, including i) the terrestrial planets' masses and orbits; ii) the CC mass fraction of Earth; iii) the much lower CC mass fraction of Mars, as long as Mars only accreted CC planetesimals (but no CC embryos); iv) the timing of the last giant (Moon-forming) impact; and v) a late accretion phase dominated by non-carbonaceous (NC) bodies. For this scenario to work, the total mass in scattered CC objects must have been ~ 0.2 - 0.3 M$_{\oplus}$ , with an embryo-to-planetesimal mass ratio of at least 8, and CC embryos in the ~ 0.01 - 0.05 M$_{\oplus}$ mass range. In that case, our simulations show there are roughly 50-50 odds of Earth's last giant impactor (Theia) having been a carbonaceous object - either a pure CC embryo or an NC embryo that previously accreted a CC embryo. Our simulations thus provide dynamical validation of cosmochemical studies.