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.

20. A supporter of Hypothesis 1 and a supporter of Hypothesis 2 would be likely to agree that, during the early Earth period, magma from beneath Earth’s crust contained:

Answer and Explanation

Your Answer is

Correct Answer is H

Explanation

Hypothesis 1 supports volcanic eruption to bring CO2 and NH2, Hypothesis 2 supports volcanic eruption to bring CO2, NH3 and CH4, CO2 are carbon compounds, NH2 and CH4 are Nitrogen compounds