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Geological evidence suggests liquid water near the Earth's surface as earlyas 4.4 billion years ago when the faint young Sun only radiated about 70% ofits modern power output. At this point, the Earth should have been a globalsnowball if it possessed atmospheric properties similar to those of the modernEarth. An extreme atmospheric greenhouse effect, an initially more massive Sun,release of heat acquired during the accretion process of protoplanetarymaterial, and radioactivity of the early Earth material have been proposed asreservoirs or traps for heat. For now, the faint-young-sun paradox persists asan important problem in our understanding of the origin of life on Earth. Herewe use the constant-phase-lag tidal theory to explore the possibility that thenew-born Moon, which formed about 69 million years after the ignition of theSun, generated extreme tidal friction - and therefore heat - in the Hadean andpossibly the Archean Earth. We show that the Earth-Moon system has lost about$3~ \times ~10^{31}$ J (99% of its initial mechanical energy budget) as tidalheat. Tidal heating of roughly 10 W/m$^{-2}$ through the surface on a timescale of 100 million years could have accounted for a temperature increase ofup to 5 degrees Celsius on the early Earth. This heating effect alone does notsolve the faint-young-sun paradox but it could have played a key role incombination with other effects. Future studies of the interplay of tidalheating, the evolution of the solar power output, and the atmospheric(greenhouse) effects on the early Earth could help in solving thefaint-young-sun paradox.

Habitability of the early Earth: Liquid water under a faint young Sun facilitated by strong tidal heating due to a closer Moon