IMPACT BOMBARDMENT DURING THE HADEAN: A TUMULTUOUS BEGINNING FOR THE GEOLOGIC EVOLUTION OF THE EARTH

It is a challenge to evaluate the Hadean Earth without a substantial geologic record. We do not have any primordial terranes or even significant scraps of rock. They have been consumed, leaving only a few zircon relicts and isotopic traces of a differentiating planet. The Earth, however, was not alone. It was accompanied by the Moon, which was once a much closer neighbor than it is today. The Moon, we have discovered, is the best place to study the first billion years of Solar System history, including the Hadean Earth. From its impact-cratered surface, we have extracted the size distribution of asteroids that dominated Hadean bombardment of the Earth and begun to constrain the timing of those impacts. The largest impact on the Moon, the 2,500-km-diameter South Pole-Aitken basin, and its smaller siblings, imply the Earth was hit with tens of thousands of objects, some of which produced craters with diameters that exceeded the Earth’s radius. Those impacts repeatedly shattered the earliest crust and blanketed the world with ejecta. The Earth was resurfaced, perhaps multiple times. The crust was mixed in the process and often melted. Impact bombardment had a profound effect on the evolution of the primordial crust and the development of a crust-mantle tectonic regime. Recent scaling of the lunar cratering record is generating quantifiable estimates of the magnitude of those geologic processes and may explain the observed spectrum of Hadean zircon ages. Impact-generated environmental conditions were tumultuous – swinging wildly as the largest impact events vaporized seas and created rock-vapor-rich atmospheres that made surface conditions untenable for life. The impacts also, however, produced vast sub-surface hydrothermal systems that may have provided crucibles for pre-biotic chemistry and/or the refuges for thermophilic and hyperthermophilic life that survived the bombardment and eventually formed the biological roots of life that populated post-impact and re-precipitated seas.