The effect of rotational evolution on the surface and interior of the early Earth

在地球历史的未解之谜中，包括第一个大陆地壳的年龄，以及类似现代板块构造旋回的过程是否需要形成大陆地壳。地球上发现的保存最古老的矿物是有44亿年历史的锆石，这只比地球本身年轻几千万年。一些地球化学家将这些锆石解释为当时已经有类似大陆的地壳的证据，但目前还不清楚板块构造是如何以及是否这么早开始的。在这项工作中，研究人员将探索一系列仅在地球历史早期运行的替代过程，并确定这些过程是否可能导致大陆地壳的形成，并解释锆石证据。要研究的过程是地球自转速度或日长的快速变化。这是由最成功的月球起源理论推动的，这些理论都涉及一次巨大的撞击，可能是与一个火星大小的物体相撞，导致地球熔化，快速旋转和抛射出碎片，凝聚形成一个非常接近的月球。地球的外壳只需要大约一百万年的时间就会冻结并变得坚硬，形成地壳和岩石圈(在这个阶段，地壳与大陆截然不同)。众所周知，在接下来的1000万年左右(并一直以不断下降的速度一直持续到现在)，如此近的月球引发的巨大潮汐引起了引力相互作用，减缓了地球的自转速度，并将月球推向更高的轨道。没有详细研究的是这对地球本身形状的影响，以及地球岩石外壳在改变形状时所经历的变形。一颗快速自转的行星会显著地变平，形成一个具有巨大赤道凸起的形状，因此就在月球形成后，地球通过赤道的半径可能是通过两极的半径的两倍。随着白天长度的增加和赤道直径的减小，这将使整个赤道地区进入一种压应力状态，这将使岩石圈增厚，可能会将含水物质(与富含蒸汽的早期大气相互作用)推向深处，在那里它经历另一个阶段的融化，其产物应该类似于大陆地壳。为了详细研究这些现象，研究人员将创建一系列必要的计算机代码来描述快速旋转的行星及其经历的融化和结晶过程。该团队将记录这些代码，并将其发布给科学界，以用于许多其他研究。他们还打算根据我们星球诞生时所经历的独特和戏剧性的一系列事件，开发教育和宣传资源。这项工作将包括对博士后学者的高级科学和专业培训以及本科生的研究经验。将用于探索早期地球自转演化后果的数值模型包括Hercules、AlphaMELTS、Perple_X和自定义平流-扩散代码。大力神在一个不假定球对称或小扰动的框架内求解行星结构的流体静力学方程。它能够准确地描述行星中的压力、密度和重力，行星的自转速度可以达到其稳定极限(赤道角加速度正好抵消了重力)。大力神将为这个项目进行改进，以纳入更广泛的状态方程。融化程度作为纬度的函数以及原壳的成分和厚度，然后将在Hercules预测的构造中使用pMELTS校准计算，该校准是在AlphaMELTS for matlab或Python(在NSF地理信息学支持下开发的工具)中实现的。下一阶段是从静力学转向动力学，使用包含弹塑性流变学、脆性破坏和粘性流动的构造物理学代码来计算变形速率和机制。知道了预测的断层类型和运动幅度，研究人员可以使用平流-扩散模型描述原地壳物质将经历的压力-温度路径。最后，在这些P-T路径上，将利用Perplex_X伪截面来预测长英质熔体将在哪里形成，以及由此产生的地壳是否会浮力稳定并有可能存活足够长的时间以被表面侵蚀并产生碎屑锆石。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Among the unsolved mysteries of Earth’s history are the age of the first continental crust and whether processes akin to the modern plate tectonic cycle are required to form continental crust. The oldest preserved minerals found on Earth are zircons up to 4.4 billion years old, which is only a few tens of million years younger than the Earth itself. Some geochemists interpret these zircons as evidence that there was already, at that time, crust similar to continents, but it is very unclear how and whether plate tectonics could have started so early. In this work the investigators will explore an alternative series of processes that operated only in this early era of Earth history and determine whether these processes could have led to the formation of continental crust and could explain the zircon evidence. The process to be studied is rapid changes in the Earth’s rotation rate, or length of day. This is motivated by the most successful theories of the origin of the Moon, which all involve a giant impact, perhaps with a Mars-sized object, that left the Earth molten and rapidly spinning and ejected debris that condensed to form a very close-in Moon. It only takes about a million years for the outer shell of the Earth to freeze and become rigid, forming a crust and lithosphere (a crust that would be, at this stage, nothing like continents). It is well-understood that, over the next ten million years or so (and continuing at an ever-decreasing rate all the way to the present), the enormous tides raised by such a close moon cause a gravitational interaction that slows the Earth’s rotation and boosts the Moon to a higher orbit. What has not been studied in detail is the consequence of this for the shape of the Earth itself and for the deformation experienced by the rocky outer shell of the Earth as it changes shape. A rapidly spinning planet flattens significantly into a shape with a large equatorial bulge so that just after the Moon formed, the radius of the Earth through the equator may have been twice as large as that through the poles. As the length of day increases and the equatorial diameter decreases, this puts the entire equatorial region into a state of compressive stress that will thicken the lithosphere, driving possibly water-bearing material (that has interacted with the steam-rich early atmosphere) to depths where it undergoes another stage of melting, the products of which should resemble continental crust. In order to study these phenomena in detail, investigators will create a series of computer codes necessary to describe rapidly rotating planets and the melting and crystallization processes they experience. The team will document and release these codes to the scientific community for use in numerous other studies. They also intend to develop educational and outreach resources based on the unique and dramatic series of events that our planet experienced at its birth. This work will involve advanced scientific and professional training for a postdoctoral scholar as well as undergraduate research experiences.The numerical models that will be used to explore the consequences of rotational evolution on the early Earth include HERCULES, alphaMELTS, Perple_X, and custom advection-diffusion codes. HERCULES solves the hydrostatic equations of planetary structure in a frame that does not assume spherical symmetry or small perturbations thereto. It is able to accurately describe pressure, density, and gravity in a planet with arbitrarily fast rotation rate up to its stability limit (where angular acceleration at the equator exactly cancels gravity). HERCULES will be improved for this project to incorporate a wider range of equations of state. Extents of melting as a function of latitude and the composition and thickness of proto-crust will then be computed in the structures predicted by HERCULES using the pMELTS calibration, implemented in alphaMELTS for MATLAB or Python (tools developed with NSF geoinformatics support). The next stage is to move from statics to dynamics and calculate deformation rates and mechanisms using tectonophysics codes that incorporate elastic-plastic rheologies, brittle failure and viscous flow. Knowing the type of faults that are predicted and the amplitude of motion, the investigators can describe the pressure-temperature paths that protocrustal material will experience using advection-diffusion models. Finally, along those P-T paths Perplex_X pseudosections will be utilized to predict where felsic melts will form and whether the resulting crust will be buoyantly stable and likely to survive long enough to be eroded at the surface and create detrital zircons.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.