Introduction

Any discussion of Earth's early atmosphere and ocean must be somewhat uncertain because we do not have samples of ancient air or water to analyze and because even the indirect record preserved in rocks is not very informative. We know from radiometric dating that Earth and the Solar System itself are both about 4.5 billion years old (Ga). The oldest preserved sedimentary rocks are from Isua, West Greenland, and have been dated at ∼3.8 Ga. Some igneous rocks are older than this, but they contain little information about the atmosphere and oceans. Thus, conditions during the first 0.7 Ga of Earth history must be inferred largely from theoretical models. One might be tempted to forego trying to deduce what the Earth was like at that time, were it not for the fact that this is also the time interval during which life seems to have originated. If we hope to place any physical constraints on this process, we need to consider the formation and evolution of the early atmosphere and ocean.

During the first half of this century, models of the early atmosphere were shaped by the ideas of A. I. Oparin in Russia and Harold Urey in the United States. Urey, a geochemist, was aware that the atmospheres of the giant planets, Jupiter and Saturn, were dominated by H2, CH4, and NH3. He reasoned that these gases had been captured from the solar nebula and that they had been retained since that time because the giant planets were too large to lose hydrogen by escape to space.