- Webb 1100
The Hadean: Empirical Evidence vs. Convenient Fiction
The ubiquity of origin myths among human societies suggests that our species has an innate need to explain how Earth formed and came to its present state. The controls on myth fabrication include the limitations of the available historical record and the technological capability of the culture in question. Despite our impressive technology and a western cultural bias to watery origins, when the scientific community encountered the limits of its historical record – there are no known rocks older than 4 Ga – it chose the paradigm of a desiccated, continent-free wasteland in which surface petrogenesis was largely due to bollide impact into a basaltic substrate and called it the “Hadean” (hellish time). But the story emerging from geochemical investigations of >4 Ga Jack Hills zircons is of their formation in near-H2O saturated meta- and peraluminous magmas under relatively low (15-30°C/km) geotherms. These results have been interpreted as reflecting chemical weathering and sediment cycling in the presence of both liquid water and plate boundary interactions shortly after Earth accretion. Given general agreement that life could not have emerged until liquid water appeared at or near the Earth’s surface, a significant implication is that our planet may have been habitable as much as 500 Ma earlier than previously thought. Indeed, recent C isotopic evidence obtained from inclusions in Hadean zircons is consistent with life having emerged by 4.1 Ga, or several 100 million years earlier that the hypothesized lunar cataclysm. Perhaps the most remarkable feature of these observations drawn from ancient zircons is that none were predicted from theory. Rather, generations of models essentially innocent of observational constraints fed the longstanding paradigm. What compelled the scientific community to develop its own origin myth – of a hellish beginning – in the absence of direct evidence? While science is clearly distinguished from mythology by its emphasis on verification, its practitioners may be as subject to the same existential need for explanations as any primitive society. In context with high expected Hadean heat production and impact flux, it was nearly irresistible to explain the lack of ancient continental crust by its non-existence rather than the equally or more plausible notion that it has been largely consumed by the same processes we see operating on the planet today.