Passage IV

Introduction

During the early Earth period (the first 2 billion years after Earth formed), the Sun produced only about 70% of the light and heat that it does today. Consequently, if early Earth’s atmosphere had been identical to Earth’s atmosphere today, the average surface temperature would have been well below the freezing point of water. However, geologic evidence indicates that a large amount of liquid water was present on the surface. Two hypotheses were proposed to explain how 3 heat-absorbing greenhouse gases—carbon dioxide (CO₂), ammonia (NH₃), and methane (CH₄)—in early Earth’s atmosphere contributed to the presence of liquid water on the surface.

Hypothesis 1

During the early Earth period, volcanic eruptions released both CO₂ and NH₃ into the atmosphere. In addition, microbes produced CH₄ by metabolizing hydrogen (H₂) gas. Compared with atmospheric greenhouse gas concentrations at present day, those on early Earth were considerably greater: the CO₂ concentration was about 100 times as great, the NH₃ concentration was about 20 times as great, and the CH₄ concentration was about 1,000 times as great. These higher-than-present atmospheric concentrations of CO₂, NH₃, and CH₄ absorbed enough heat to maintain an average surface temperature that allowed for liquid water.

Hypothesis 2

The only source of atmospheric CO₂, NH₃, and CH₄ on early Earth was volcanic eruptions. Compared with atmospheric CO₂ and NH₃ concentrations at present day, those on early Earth were somewhat greater: the CO₂ concentration was about 40 times as great and the NH₃ concentration was about 10 times as great. The CH₄ concentration was about the same as its present value. At those concentrations, the 3 gases by themselves would not have absorbed enough heat to raise the average surface temperature above freezing. However, atmospheric concentrations of both nitrogen (N₂) and H₂ were approximately twice what they are today. These higher-than-present concentrations of N₂ and H₂ greatly enhanced the heat-absorbing effects of the 3 greenhouse gases, maintaining an average surface temperature that allowed for liquid water.