As understandings of the solar life cycle developed, scientists were confronted by a nagging question: If, early in the history of Earth's planetary formation, the sun was significantly dimmer than it is now, how do we explain evidence for the presence of liquid water on the planet's surface during this period? It is understood that during the early Earth period our sun would have shone at about 70 percent of its current luminosity, significantly below the level necessary to sustain the presence of liquid oceans on the planet's surface. But paleontological and geological records indicate with substantial certainty that liquid water was present on the Earth's surface during this time. If scientific models of solar development and the early geological history of Earth are to be reconciled, there must have been some other, intervening factor affecting surface temperatures during this early period of geologic history.
Our method of reconciling these two apparently contradictory bodies of evidence has been to analyze the characteristics and chemical composition of Earth's atmosphere during its various periods of geological development. We have known for decades that the presence of certain vapors in the air, commonly known as “greenhouse gases,” increases the amounts of heat retained by an atmosphere to a degree proportionate to the concentration of greenhouse gases within it. If the Earth was warmer in the past than solar luminosity would suggest—and we have strong evidence that it was—then we can expect that the distribution of greenhouse gases in its atmosphere would be of paramount value in explaining that discrepancy. Heightened greenhouse gas levels would trap and magnify the sun's heat as it struck the atmosphere, enabling liquid water to exist on the surface of Earth. However, we have discovered little evidence that greenhouse gases during the early Earth period were present in larger quantities than previously assumed. These surprising findings indicate that the discrepancy between solar-astronomical and geological evidence related to the development of Earth's oceans cannot be explained by the presence of carbon in the atmosphere.
One likely alternative is that another, non-solar source for atmospheric heat during the early Earth period. Earth and its moon were much closer together in the early geologic history of the planet, and tidal heating caused by the moon's rotation around Earth was likely much greater. While it is unclear how much influence this phenomenon might have exerted over atmospheric temperatures, scientists are increasingly convinced that it could play a part in explaining the unlikely presence of liquid water on our infant planet, perhaps in combination with indeterminate geological activity, solar flares, or irregularities in Earth's magnetic field.