The consequences of the Moon-forming impact for the chemistry of Earth

在其成长的最后阶段，地球被另一个行星大小的天体撞击，该天体将物质喷入轨道，月球由此形成。这次碰撞被称为月球形成的巨大撞击，对地球产生了灾难性的影响。它将物质吹到太空中，熔化并蒸发了行星岩石部分的大部分（将地球膨胀到目前大小的许多倍），并使地球快速旋转，一天短至2.3小时。在我们的太阳系中，这样一个灾难性的最后事件被认为是地球所独有的，而月球形成的影响在决定地球的性质和随后的演变中发挥了至关重要的作用。地球如何获得其独特的大气和海洋是一个基本的，没有答案的问题。人们认为，在月球形成之前，地球已经获得了大量的挥发物（如水，氮和碳）。然而，月球形成的影响可能会引发碰撞物体的大气和海洋的大部分。因此，现代地球只继承了撞击前的一小部分挥发性物质，但具体有多大还不确定。撞击后，地球上所有的挥发物都混合在一起，形成了撞击后的地球，岩石液体和蒸汽的混乱状态，没有单独的大气层或海洋层。地球花了几千年的时间才冷却到足以形成大气层，而在海洋开始下雨之前，时间要长得多。在这段时间里，以及之后的数百万年里，地球受到较小的岩石和冰体的轰击。这些天体中有很大一部分是月球形成撞击产生的碎片。这些小天体可能向地球输送挥发物，但它们也可能将更多的大气层吹到太空中。许多这些过程都知之甚少，我们还不知道月球形成的影响以及它引发的一连串事件如何塑造了地球的大气层和海洋。该项目将探索月球形成的影响如何改变地球的化学性质，特别是其挥发性元素的库存。我将专注于确定可用于测试不同拟议场景的关键观测结果，从而让我们更多地了解我们星球的历史。人们已经做了很多尝试来计算在月球形成的撞击中幸存下来的大气和海洋的数量。然而，以前的研究没有包括影响大气和海洋损失的两个关键因素。首先，碰撞的物体可能旋转得很快，这使得大气层更容易流失。其次，当两个天体被吸引到一起时，彼此的引力影响会扭曲另一个天体的海洋和大气层，这可能会使挥发物更难或更容易丢失。我将使用复杂的计算机模拟来计算在撞击过程中大气和海洋损失的比例，包括这两个重要的影响。我还将探索撞击后地球的大气层是如何形成的，这是迄今为止地球演化过程中一个基本上未被探索的阶段。我将计算出地球的结构是如何随着时间的推移而演变的，以及挥发物是如何在整个行星和大气中传播的。我将确定地球历史初期大气层的质量和化学成分，为地球随后的演化以及生命的出现和持续奠定基础。最后，我将探索月球形成过程中被吹走的物质发生了什么，以及它如何影响地球的化学成分。我将确定在撞击过程中喷射出的物质的大小和速度，然后计算这些物质是如何被太阳系中不同的天体扫过的。至关重要的是，我将包括地球冷却时的大小和形状变化，这是以前的研究所没有的。通过找出何时以及有多少物体落回地球，我可以确定撞击碎片是否增加或耗尽了地球的大气层。

At the very end of its growth, Earth was hit by another planet-sized body that spewed material into orbit, out of which the Moon formed. This collision, known as the Moon-forming giant impact, had a catastrophic effect on Earth. It blew material off into space, melted and vaporised substantial fractions of the rocky portion of the planet (inflating Earth to many times its current size), and left Earth spinning rapidly, with a day as short as 2.3 hours. In our solar system, such a catastrophic final event is thought to be unique to Earth, and the Moon-forming impact played a crucial role in dictating Earth's properties and subsequent evolution.How Earth acquired its unique atmosphere and oceans is a fundamental, unanswered question. It is thought that Earth had gained a lot of its volatiles (such as water, nitrogen and carbon) before the Moon-forming impact. However, the Moon-forming impact likely kicked off a substantial fraction of the atmospheres and oceans of the colliding bodies. As a result, the modern Earth only inherited a fraction of the volatiles it had before the impact, but how large a fraction is uncertain. After the impact, all of Earth's volatiles were mixed together in the mess of rock liquid and vapour that was the post-impact Earth, and there were no separate atmosphere or ocean layers. It took thousands of years for Earth to cool enough for the atmosphere to emerge, and much longer before the oceans started to rain out. During this time, and for millions of years afterwards, Earth was bombarded by smaller rocky and icy bodies. A good fraction of these bodies were the debris from the Moon-forming impact. These small bodies could have delivered volatiles to Earth, but they could have also blown more of its atmosphere into space. A lot of these processes are poorly understood, and we do not yet know how the Moon-forming impact, and the chain of events it triggered, shaped Earth's atmosphere and oceans. This project will explore how the Moon-forming impact changed the chemistry of Earth, particularly its inventory of volatile elements. I will focus on identifying key observations that can be used to test different proposed scenarios and so teach us more about the history of our planet. Many attempts have been made to calculate the amount of atmosphere and ocean that survived the Moon-forming impact. However, previous studies do not include two key things that affected how much of the atmosphere and ocean was lost. First, the colliding bodies were likely spinning rapidly, making it easier for atmosphere to be lost. Secondly, as the bodies were drawn together, the gravitational influence of each distorted the oceans and atmospheres of the other which may have made it harder or easier for volatiles to be lost. I will use sophisticated computer simulations to calculate what fraction of the atmosphere and ocean were lost during the impact, including these two important effects. I will also explore how the atmosphere of Earth emerged after the impact, a hitherto largely unexplored stage in Earth's evolution. I will calculate how the structure of Earth evolved over time and how volatiles were spread throughout the planet and atmosphere. I will determine the mass and chemistry of Earth's atmosphere at the very start of its history, setting the stage for Earth's subsequent evolution and the emergence and persistence of life.Finally, I will explore what happened to the material that was blown off during the Moon-forming impact and how it influenced Earth's chemistry. I will determine the size and velocity of the material that was ejected during the impact, and then calculate how that material was swept up by different bodies in the solar system. Crucially, I will include the changing size and shape of Earth as it cooled, something that previous studies did not. By finding out when, and how many, bodies fall back to Earth, I can determine whether impact debris added to or depleted Earth's atmosphere.