NSF Award
2102571
The evolution of the Hadean Earth (4-4.5 billion years ago) was shaped by large-scale interplanetary collisions, characterized by impactors with diameters ranging from 1,000 to 4,000 km. The defining characteristics of the Earth, such as oceans, continents, life, and plate tectonics are likely to have appeared for the first time on the Hadean eon. It thus seems inevitable that their emergence would have been affected by these early collisions, though to what extent they were affected remains an open question. In particular, little understood are the effects of these collisions on the internal evolution of the Earth. Collisions are thought to have contributed significantly to the abundance of highly iron-loving elements (e.g., Au, Ir, Ru) in the Earth’s mantle, but the details of their delivery through impacts are not fully understood due to complex mixing processes that arise when a projectile collides with the early Earth. In addition, it is not well-understood how the long-term evolution of the Earth’s mantle could have responded to the injection of materials derived from the impactors. This project introduces an innovative computational approach that combines impact simulations with models that explore the long-term evolution of the Earth’s interior to quantify the delivery of highly iron-loving elements, their distribution in the mantle, and the total mass of late accreted materials. These results allow for an increased understanding of the Hadean Earth surface environment. Moreover, the connections between short-term impact dynamics and long-term mantle dynamics may shed light on the origin of anomalies in the mantle (such as the large low-shear-velocity provinces, which are the most significant anomalies on the deep mantle), the dynamics of mantle plumes, and the history of the geomagnetic field. This project also provides support for interdisciplinary training of a graduate student, an undergraduate internship where the Southwest Research Institute will host 1-2 geophysics major from Yale University, and a series of movies that visualize planetesimal impacts and mantle dynamics for education outreach.<br/><br/>The geophysical evolution of Hadean Earth was controlled by large-scale collisions and mantle dynamics, but the interplay of these processes remains largely unexplored. The Earth’s protracted bombardment of leftover planetesimals after the Moon-forming giant impact, called “late accretion”, is supported by the lunar cratering record and is required to explain the abundance of as well as the chondritic proportions of highly siderophile elements (HSEs) in the present-day mantle. Recently, based on impact simulations with smooth-particle hydrodynamics (SPH), the late accreted mass has been suggested to be two to five times higher than previously thought, because the metallic cores of large differentiated planetesimals, where the bulk of HSEs reside, are not efficiently mixed into the mantle. Such an upward revision of late accreted mass could dramatically modify our understanding of the Hadean Earth. This suggestion based on SPH simulations is, however, still provisional because important complications arising from long-term mantle dynamics are not incorporated. Thus, the delivery of HSEs, their distribution in the mantle, and the total mass of the late accretion are still wide-open questions, with important consequences on the Hadean Earth surface environment. This project aims to achieve the following two major objectives: (1) to quantify the fate of differentiated planetesimal cores during impacts, by re-evaluating SPH simulations including the physics of fragmentation, and (2) to understand the long-term fate of fragmented metallic blobs by conducting systematic mantle mixing simulations. The primary goal of this project is to determine the absolute scale of planetesimal impact history, but achieving this goal will also help to address a wide range of important questions, from the habitability of the early Earth to the origin of deep geochemical reservoirs.<br/><br/>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.
