10. Could the Hadean Eon Have Been Habitable?

Given the absence of a macroscopic Hadean rock record, evaluating terrestrial habitability is largely a thought experiment, but data from Hadean zircons can provide some constraints. We are certain that life as we know it would not be possible without four requirements; soluble bioactive elements (carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorous), free energy, liquid water, and time. Beyond these essential ingredients, there is broad agreement that there are ten secondary factors that separate us from the other, uninhabited terrestrial planets and maintain our planet’s homeostasis. They are: (1) a galactic and planetary sanctuary for life; (2) liquid water at the planetary surface to mediate biochemistry and efficiently cool the planet; (3) dissolved water in the deep planetary interior to enhance mantle circulation and catalyze the eclogite transition; (4) a broadly solar chemical composition to provide sufficient metallicity for a stable surface platform; (5) sufficient planetary mass to retain an atmosphere and heat; (6) planetary satellite(s) to stabilize climate zones; (7) extra-planetary impactors to introduce organic building blocks and water and to create satellites; (8) long-term interior heat generation to maintain mantle circulation and the geodynamo; (9) a self-sustaining dynamo to protect the atmosphere is erosion; and (10) a mechanism to recycle surface carbon into the interior and back. Evaluating how these various factors interact is complicated but our speculations can be guided by inferences from Hadean zircon geochemistry which potentially bear on six of the ten ingredients for life—the presence of surface and interior water, the role of impacts on early Earth, internal heat generation, surface recycling, and the existence of a Hadean geodynamo. Knowledge of the geochemistry and inclusion population of Hadean zircons also permits constraints to be placed on whether mineral phases and trace elements key to biopoiesis were present during the Hadean eon.