The Volatile Legacy of the Early Earth

作为对NERC主题行动(TA)的响应，我们提议成立一个由七个英国机构的科学家和三个国际合作伙伴组成的联盟，重点关注“不稳定的早期地球遗产”。地球的宜居性与其挥发物的库存和循环密切相关，今天这与板块构造有关，但我们仍然几乎不知道我们的星球是如何发现自己处于这种近乎理想的“金发碧眼”状态的，在这种状态下，挥发性混合是“恰到好处的”。这仅仅是因为处于合适的太阳距离和合适的挥发物供应吗？或者，早期增殖和分化的化学和动力学的细节对最终结果至关重要吗？这些问题对于理解我们自己的行星发展至关重要，鉴于系外行星发现领域的蓬勃发展，它们在测量其他地方存在生命的可能性方面获得了额外的吸引力。在这个方案中，我们研究了地球上挥发物的早期演化是如何为宜居奠定基础的。行星通过碰撞而生长，这些剧烈的事件可能会导致在撞击的天体中携带的挥发物的损失。我们将通过数值模拟来探索在小行星碰撞中挥发分被保留或丢失的条件。我们还将通过研究S元素，特别是它的地球化学孪生元素Se，来评估挥发物被“晚”送到地球的可能性，也就是在大碰撞的大漩涡完成之后，地球基本上被建造了。我们将限制核心的损失过程和这个过程所赋予的同位素特征。我们将进一步利用同位素测量作为现代Se起源的指纹，并找出它是否对应于任何已知的陨石类型，或者它是否可能是由彗星输送的。月球为地球的起源提供了进一步的线索，而质疑最近精炼的月球挥发性清单的意义需要在适当的条件下进行新的实验。行星碰撞产生的能量不可避免地导致大规模融化。岩浆海洋中挥发物的溶解度和化学性质决定了气体是被带到地球内部还是留在大气中。保留在岩浆海洋中的挥发物可能成为深部地幔挥发循环的一部分，或者永久地隔离在深部储集层中。我们将通过一系列模拟早期岩浆海洋条件的实验来纠正这个问题。我们将进一步研究下地幔中的相的稳定性，这些相可能含有挥发性元素，如果通过在对流的岩浆海洋中的溶解作用被输送到很深的地方。利用地震和模拟技术，我们将评估目前在地幔最深处是否有任何储存的挥发物残留物是可见的。核心对不稳定预算的影响可能很大，因为它的规模，但不稳定的溶解性鲜为人知。我们将研究高压和高温下氢、碳和氮在液态金属中的溶解度。在这个联盟中，我们还将创建一个博士生和导师队列，他们作为一个大型团队的一部分，利用钻石中的夹杂物拼凑出地球不稳定演化的证据。这些可能是关键的“时空”胶囊，可以将早期地球演化的实验和理论工作与当今地球深处不稳定的预算和通量联系起来。这项提案中提出的问题很复杂，需要广泛的信息才能提供有意义的答案。我们的目标是对地球最初是如何成为一个宜居星球建立一个更好的理解，并建立一个坚实的基础，在此基础上，英国的进一步研究可以继续在这一令人兴奋的领域领先。这将是该联盟的最终遗产，并通过与其他联盟的链接，成为整个主题行动的最终遗产。

In response to the NERC Theme Action (TA) we propose a consortium among scientists at seven UK institutions and with three international partners centred on 'The Volatile Legacy of the Early Earth'. Earth's habitability is strongly linked to its inventory and cycling of volatiles, which today are coupled to plate tectonics, but we still have little notion as to how our planet found itself in this near-ideal 'Goldilocks' state where the volatile mix is 'just right'. Was it simply a matter of being at the right solar distance with the right supply of volatiles? Or were the details of the chemistry and dynamics of early accretion and differentiation crucial to the eventual outcome? Such questions are of critical importance for understanding our own planets development, and given the burgeoning field of exo-planet discovery, they gain extra piquancy for gauging the probability of life elsewhere. In this proposal we investigate how the early evolution of volatiles on Earth set the stage for habitability.Planets grow by collisions and these violent events may lead to loss of the volatiles carried within the impacting bodies. We will explore with numerical modeling the conditions under which the volatiles are retained or lost in planetesimal collisions. We will also assess the likelihood that volatiles were delivered to Earth 'late', namely after the maelstrom of major collisions was finished and the planet was largely constructed, by studying the element S and notably its geochemical twin, Se. We will constrain the process of loss to the core and the isotopic signature imparted by this process. We will further use isotopic measurements as finger-prints of the origin of modern Se, and will find out whether it corresponds to any known meteorite type, or if it was possibly delivered by comets. The Moon provides further clues to the origin of the Earth, and Interrogating the significance of the recently refined volatile inventory of the Moon requires new experiments under appropriate conditions.The energy generated by planetary collisions inevitably results in large-scale melting. The solubility and chemical nature of volatiles within a magma ocean controls whether or not gases are carried into the interior of the planet or left in the atmosphere. Volatiles retained in the magma ocean may become part of a deep mantle volatile cycle or become permanently sequestered in deep reservoirs. We will redress this issue with a series of experiments that simulate conditions of the early magma ocean. We will further investigate the stability of phases in the lower mantle that can potentially hold volatile elements if delivered to great depths by solubility in a convecting magma ocean. Using seismic and modeling techniques, we will assess if any remnants of such stored volatiles are currently 'visible' in the deepest mantle. The influence of the core on volatile budgets is potentially great because of its size, but volatile solubility is poorly known. We will examine the solubility of hydrogen, carbon and nitrogen in liquid metal at high pressures and temperatures.In this consortium we will also create a cohort of PhD students and supervisors who work as part of a large team to piece together the evidence for Earth's volatile evolution using inclusions trapped in diamonds. These may be the key 'space-time' capsules that can link experimental and theoretical work on early Earth evolution to present-day volatile budgets and fluxes in the deep Earth. The questions raised in this proposal are complex and require a wide range of information in order to provide meaningful answers. It is our goal to establish a much-improved understanding of how Earth initially became a habitable planet, and to build a solid foundation on which further UK research can continue to lead the way in this exciting field. This will be the ultimate legacy of this consortium, and through links to other consortia, of the entire Theme Action.