The Volatile Legacy of the Early Earth

为了响应NERC主题行动（TA），我们提议由七个英国机构的科学家和三个国际合作伙伴组成一个联盟，以“早期地球的不稳定遗产”为中心。地球的可居住性与其挥发物的库存和循环密切相关，今天与板块构造有关，但我们仍然不知道我们的星球是如何发现自己处于这种近乎理想的“金发姑娘”状态的，在这种状态下，挥发物的混合物“刚刚好”。这仅仅是在合适的太阳距离和合适的挥发物供应的问题吗？或者，早期吸积和分化的化学和动力学细节对最终结果至关重要？这些问题对于了解我们自己的行星的发展至关重要，并且考虑到外行星发现领域的蓬勃发展，它们在衡量其他地方生命的可能性方面获得了额外的刺激。在这个提议中，我们研究了地球上挥发物的早期进化如何为宜居性奠定基础。行星在碰撞中成长，这些剧烈的事件可能导致撞击体中携带的挥发物的损失。我们将用数值模拟探讨挥发物在星子碰撞中保留或丢失的条件。我们还将通过研究元素S，尤其是它的地球化学孪生体Se，来评估挥发物“晚”到达地球的可能性，即在大碰撞的大漩涡结束后，地球基本上形成了。我们将把损失过程限制在岩心上，并将此过程所赋予的同位素特征加以限制。我们将进一步使用同位素测量作为现代硒起源的指纹，并将发现它是否与任何已知的陨石类型相对应，或者它是否可能是由彗星带来的。月球为地球的起源提供了进一步的线索，而要探究最近提炼的月球挥发性物质的重要性，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的熔化。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入行星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物循环的一部分，或者永久地被隔离在深部储层中。我们将通过一系列模拟早期岩浆海洋条件的实验来解决这个问题。我们将进一步研究下地幔中相的稳定性，如果在对流岩浆海洋中溶解，这些相可能会将挥发性元素输送到很深的地方。利用地震和建模技术，我们将评估这些储存的挥发物是否有任何残留物目前在最深处的地幔中“可见”。由于岩心的大小，其对挥发性预算的影响可能很大，但对挥发性溶解度知之甚少。我们将研究氢、碳和氮在高压和高温下在液态金属中的溶解度。在这个联盟中，我们还将培养一批博士生和导师，他们将作为一个大团队的一部分，利用钻石中的内含物拼凑出地球挥发性演化的证据。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。本建议提出的问题很复杂，需要广泛的资料才能提供有意义的答案。我们的目标是建立对地球最初如何成为宜居行星的更好理解，并建立一个坚实的基础，使英国的进一步研究能够继续在这个令人兴奋的领域处于领先地位。这将是这个联盟的最终遗产，并通过与其他联盟的联系，整个主题行动。

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 '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.