1. Why Hadean?

The development of the geologic timescale arose from early nineteenth century fossil correlations and thus firmly rooted in the rock record. The thinking of that time included the possibility that our planet had forever existed in a quasi-steady state. By the later part of that century, it was broadly understood that Earth must have experienced a discrete origin but that details of that event might never be discerned. The advent of radiometric dating shortly thereafter catapulted this discussion from considering upper bounds of tens of millions of years to several billions. But it was the return of lunar highland rocks in the early 1970s that revolutionized thinking about the early inner solar system, including a hypothesized impact bombardment at ca. 3.9 billion years that was thought to have obliterated planetary surfaces. It was in this context that the term “Hadean” was proposed as the earliest division of geologic time. Since then, the meaning of Hadean evolved to describe the first roughly 500 million years of Earth history, which currently coincides with the period for which we have no macroscopic rock record. The premise of this book is that there are four avenues available to seek the nature of this eon of Earth history: (1) the presumption that physical laws are time dependent, and thus limits can be placed on early Earth’s behavior using mathematical calculations; (2) that isotopic variations resulting from long-lived and extinct radioactivities preserved in mantle rocks can constrain the timing of early, planetary-scale differentiation; (3) that as much as half the rocks on the lunar surface are likely between 4.4 and 3.9 billion years in age and thus can attest to conditions then extant in the inner solar system; and (4) that detrital zircons between 4.0 and nearly 4.4 billion years preserve a lithic record of terrestrial activity in that period. Although the initial choice of the word Hadean was meant in infer that this phase of Earth history was characterized by hellish surface conditions, in classic mythology it was a cool and watery realm. Studies of these ancient zircons over the past 20 years appear to reveal an early history more akin to the latter myth than the former.