The Sun, the Earth, and the Evolution of the Earth's Atmosphere

Abstract

Anthropogenic pollution occurs when gas and aerosol particle concentrations rise above natural, background concentrations. This chapter examines the evolution of the background atmosphere. The discussion requires a description of the sun and its origins because sunlight has affected much of the evolution of the Earth's atmosphere. The description also requires a discussion of the Earth's composition and structure because the inner Earth affects atmospheric composition through outgassing, and the crust affects atmospheric composition through exchange processes, including wind-blown soil dust, volcanic, and sea spray emissions. Earth's earliest atmosphere contained mostly hydrogen and helium. Carbon dioxide replaced these gases during the onset of the Earth's second atmosphere. Today, nitrogen and oxygen are the prevalent gases. Processes controlling the changes in atmospheric composition over time include outgassing from the Earth's interior, microbial metabolism, and atmospheric chemistry. These processes still affect the natural composition of the air today. The Sun and Its Origin The sun provides the energy to power the Earth. Most of the sun's energy reaching the Earth originates from the sun's surface, not from its interior. The evolution, structure, and relevant radiation emissions from the sun are discussed here.