When Did Earth Become Habitable

One of the great challenges in science is discovering under what conditions, when, and how life arose. Morphological and isotope fossil evidence support the view that by 3.8-3.5 Ga our planet hosted microbial life leaving open the possibility, as suggested by fossil molecular clocks, that our planet became habitable and life emerged during its first 500 million years – the Hadean Eon. Life would not be possible without soluble bioactive elements, energy and liquid water. As the first two were almost certainly then available, the key unresolved issue is – and seemingly the rate limiting step in making our planet habitable – when liquid water become available. The absence of a Hadean rock record appears to leave but a single strategy – examination of >4 Ga zircons – with which to directly assess the surface conditions, and thereby habitability, of Hadean Earth. But it has long been argued that continental crust mutually requires, and is required for, the long-term stability of liquid water at Earth’s surface – a feedback system whereby tectonic transport of water to the mantle via oceanic crust enables the production of felsic magmas. In turn, subaerial weathering of buoyant, felsic crust captures CO2 preventing a runaway greenhouse atmosphere and corresponding loss of surface H2O. Thus while knowing when felsic crust emerged is key to assessing terrestrial habitability, the aforementioned lack of a pre-4.02 Ga rock record has seriously limited our ability to understand the growth history of continental crust, particularly in Earth’s most formative stages. The limitations of the rock record are driven home by recent estimates of Hadean continental crust volume which range from essentially zero to its present size. We show that the paradigm of a longstanding, early mafic crust is either based on deeply flawed estimates using redox sensitive trace elements proxies across the period during which atmospheric oxygen rose dramatically or the problematic assumption that the degree of mantle melting has remained constant over Earth history. The path forward is a mission-scale effort to greatly extend what we’ve learned from the geochemical characterization of Hadean Jack Hills zircons to the other 15 locations where their presence has been detected.