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

Geological evidence suggests liquid water on the earth's surface as early as 4.4 Ga when the faint young Sun only radiated about 70 % of its modern power output. At this point, Earth should have been a global snowball if it possessed atmospheric properties similar to those of modern Earth. An extreme atmospheric greenhouse effect, an initially more massive Sun, release of heat acquired during the accretion process of protoplanetary material, and radioactivity of early Earth material have been proposed as reservoirs or traps for heat. We explored the possibility that the new-born Moon, which formed about 69 Ma after the ignition of the Sun, generated extreme tidal friction - and therefore heat - in the Hadean and the Archean earth. We show that the Earth-Moon system has lost about 3 × 10^31 J (99 % of its initial mechanical energy budget) as tidal heat. Tidal heating of about 10 W/m^2 through the surface on a time scale of 100 Myr could have accounted for a temperature increase of up to 5 degrees Celsius on early Earth. Tidal heating alone does not solve the faint-young-sun paradox but it could have played a key role in combination with other effects. Future studies of the interplay of tidal heating, the evolution of the solar power output, and the atmospheric (greenhouse) effects on early Earth could help in solving the faint-young-sun paradox, particularly if tied to geologic evidence.

Details published in Heller et al. (2021) accepted by Paläontologische Zeitung, PDF pre-print: https://arxiv.org/abs/2007.03423