Habitability of the early Earth: Liquid water under a faint young Sun facilitated by strong tidal heating due to a closer Moon (2007.03423v3)

Abstract: Geological evidence suggests liquid water near the Earth's surface as early as 4.4 billion years ago when the faint young Sun only radiated about 70% of its modern power output. At this point, the Earth should have been a global snowball if it possessed atmospheric properties similar to those of the 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 the early Earth material have been proposed as reservoirs or traps for heat. For now, the faint-young-sun paradox persists as an important problem in our understanding of the origin of life on Earth. Here we use the constant-phase-lag tidal theory to explore the possibility that the new-born Moon, which formed about 69 million years after the ignition of the Sun, generated extreme tidal friction - and therefore heat - in the Hadean and possibly 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 tidal heat. Tidal heating of roughly 10 W/m$^{-2}$ through the surface on a time scale of 100 million years could have accounted for a temperature increase of up to 5 degrees Celsius on the early Earth. This heating effect 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 the early Earth could help in solving the faint-young-sun paradox.

• The paper demonstrates that tidal heating from a closer Moon contributed approximately 10 W/m² extra heat, potentially raising early Earth’s temperatures by up to 5°C for about 100 million years.
• It employs constant-phase-lag tidal theory to quantify that nearly 99% of the Earth-Moon system’s mechanical energy was dissipated as tidal heat.
• The findings suggest that while tidal heating does not solely resolve the faint young Sun paradox, it likely complemented greenhouse effects to sustain liquid water and early habitability.

Habitability of the Early Earth Facilitated by Tidal Heating

The paper presented in the paper explores an often overlooked potential contributor to the habitability of the early Earth: tidal heating due to the Moon's proximity during the planet's formative years. The investigation is primarily motivated by the faint young Sun paradox, which postulates that the Earth's temperature should have been too low to sustain liquid water given the ancient Sun's lower luminosity.

Early Earth and the Faint Young Sun Paradox

Geological evidence indicates the presence of liquid water on Earth as early as 4.4 billion years ago, despite the Sun emitting only about 70% of its current energy. Conventional solutions to the paradox, such as a higher atmospheric concentration of greenhouse gases or a more massive Sun, remain speculative and insufficient according to this paper. Instead, the authors explore tidal heating as a mechanism that, in conjunction with other effects, may have played a substantial role in maintaining habitable temperatures.

Tidal Heating: A Potential Contributor to Earth’s Warmth

The paper applies the constant-phase-lag tidal theory to estimate the tidal heating effect generated by a much closer early Moon. The authors quantify that the Earth-Moon system has dissipated approximately $3 \times 10^{31}$ joules (99% of its mechanical energy) as tidal heat. This dissipated energy may have contributed an additional $\sim10\mathrm{W/m}^2$ of heat through Earth's surface, potentially raising surface temperatures by up to $5^\circ$C for approximately 100 million years. Although this heating alone is insufficient to resolve the faint young Sun paradox, it could have combined with greenhouse effects to sustain liquid water and possibly early life.

Implications and Future Directions

The findings bolster the hypothesis that tidal interactions were a non-negligible factor in Earth's early thermal history. This mechanism is proposed to have strengthened Earth's capability to support liquid water on the surface, thus playing an indirect role in fostering early life. Tidal heating could also explain observed Archean geothermal gradients and accompanying geological phenomena.

Future research could further elucidate the complex interplay between tidal dynamics, atmospheric conditions, and solar evolution, providing a more comprehensive understanding of early Earth habitability. Such investigations may also extend to broader astrogeophysical contexts, possibly informing exoplanet habitability studies by highlighting the potential significance of tidal interactions in sustaining life-supporting conditions.

Conclusion

Thus, while tidal heating from a closer Moon does not singularly resolve the faint young Sun paradox, it represents a compelling component of early Earth’s climate narrative. By potentially augmenting atmospheric and geothermal processes, tidal heating contributed to conditions that defy traditional astrophysical predictions, warranting continued exploration into its role in Earth’s evolutionary history.