The Habitability of Earth

The astronomical and cosmogeochemical context for Earth formation created the necessary conditions for the origin of life (biogenesis), and the emergence of biogeo-chemical cycles. This context included the mass ofthe Sun (and hence its luminosity history as a main sequence star), the Earth's distance from the Sun, the Earth's composition, and early impact history. The presence ofliquid water was likely a critical necessary condition for biogenesis, which occurred plausibly during the Hadean, from 4.4 to 3.8 billion years ago, since there is suggestive evidence for liquid water from the oxygen-isotopic record of mineral inclusions in Hadean zircons, dated by the U-Pb method.

Would the Earth's surface have been habitable if it were lifeless (i.e., abiotic)? Surely, it was at the time of biogenesis, but would it have remained habitable over geologic time if life did not emerge? The likely magnitude of the present BEW, about 100, the culmination of the progressive increase in BEW over geologic time, can be used to estimate the likely abiotic temperature history of the Earth's surface. The result is yes, the Earth would have remained habitable for thermophilic life (growing above 50 °C) but not for mesophiles, that is, low-temperature life, including plants and animals. In the future, with rising solar luminosity, the biosphere will terminate in some 1-2 billion years, long before the Sun becomes a red giant star.

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