Question

The Faint Young Sun Paradox

Standard astrophysical models indicate that during the Archean eon ($3.8$ to $2.5$ billion years ago), the Sun's energy output was only about $70\%$ to $75\%$ of its current value. Under these conditions, Earth's surface temperature should have been well below freezing, yet geological evidence confirms the continuous presence of liquid water oceans. Two models attempt to explain this paradox.

*Model 1*
Early Earth's atmosphere contained significantly higher concentrations of greenhouse gases, primarily carbon dioxide ($CO_2$) and methane ($CH_4$), than it does today. Active volcanism continuously released large quantities of $CO_2$ into the atmosphere. Additionally, because continental landmasses were small, the rate of silicate weathering (a chemical process that removes atmospheric $CO_2$ and stores it in crustal rocks) was extremely low. The abundance of these gases trapped outgoing infrared radiation, maintaining surface temperatures above freezing.

*Model 2*
The primary driver of early Earth's warmth was a lower planetary albedo (reflectivity), which allowed the Earth to absorb a larger fraction of the Sun's incoming radiation. During the Archean eon, continental landmasses occupied less than $5\%$ of Earth's surface, leaving the planet dominated by dark oceans that absorbed solar energy. Furthermore, the absence of land plants and certain marine organisms meant there were fewer biogenic aerosols to act as cloud condensation nuclei. This resulted in fewer, thinner clouds, allowing more solar radiation to reach and warm the surface.

Based on Model 2, which of the following statements best describes the hypothesis explaining how early Earth maintained liquid water?