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，以及大陆地壳的形成是否需要类似现代板块构造循环的过程。地球上保存最古老的矿物是距今44亿年的锆石，它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了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可以如此早地开始的。在这项工作中，研究人员将探索仅在地球历史早期发生的一系列替代过程，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率的快速变化，也就是一天的长度。这是由关于月球起源的最成功的理论所推动的，这些理论都涉及到一次巨大的撞击，可能是由火星大小的物体造成的，那次撞击使地球熔化并迅速旋转，喷射出的碎片凝结成一个非常近的月球。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，地壳与大陆完全不同）。众所周知，在接下来的1000万年左右的时间里（并且一直以不断下降的速度持续到现在），由如此近的月球引起的巨大潮汐会引起引力相互作用，从而减缓地球的自转，并将月球推到更高的轨道上。没有被详细研究的是，这对地球本身的形状和地球岩石外壳在改变形状时所经历的变形的影响。一颗快速旋转的行星会明显变平，形成一个巨大的赤道凸起，所以在月球形成后不久，地球穿过赤道的半径可能是穿过两极的半径的两倍。随着白昼长度的增加和赤道直径的减小，整个赤道地区处于一种压应力状态，这将使岩石圈变厚，将可能含水的物质（与富含蒸汽的早期大气相互作用的物质）推向深处，在那里它将经历另一阶段的融化，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所经历的熔化和结晶过程。该团队将记录这些代码并将其发布给科学界，以供许多其他研究使用。他们还打算根据我们的星球诞生时所经历的一系列独特和戏剧性的事件，开发教育和外联资源。这项工作将涉及对博士后学者的先进科学和专业培训以及本科生的研究经历。将用于探索早期地球旋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。HERCULES在不假设球对称或小扰动的框架中求解行星结构的流体静力方程。它能够精确地描述一颗行星的压力、密度和重力，该行星的旋转速度任意快，直到其稳定极限（赤道的角加速度正好抵消了重力）。HERCULES将在这个项目中得到改进，以纳入更广泛的状态方程。然后，在HERCULES预测的结构中，使用pmelt校准（在MATLAB或Python（由NSF地理信息支持开发的工具）中实现），计算熔化程度作为纬度和原始地壳组成和厚度的函数。下一阶段将从静力学转向动力学，并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计算变形率和机制。了解了预测的断层类型和运动幅度，研究人员可以用平流-扩散模型描述原地壳物质将经历的压力-温度路径。最后，沿着这些P-T路径，将利用Perplex_X伪剖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具有浮力稳定性，是否有可能存活足够长的时间，以便在表面被侵蚀并产生碎屑锆石。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.