The Sun, the Earth, and the Evolution of the Earth’s Atmosphere - Air Pollution and Global Warming: History, Science, and Solutions - page 29

Chapter 2

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

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 sun-
light 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 composi-
tion 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 control-
ling the changes in atmospheric composition over time
include outgassing from the Earth's interior, microbial
metabolism, and atmospheric chemistry. These pro-
cesses still affect the natural composition of the air
today.

compressed into a single point, estimated to have a
density of 10 9 kg m − 3 and a temperature of 10 12
K(Kelvin). With the Big Bang ,thispoint of mass
exploded, ejecting material in all directions. Aggre-
gates ofejected material collapsed gravitationally to
form the earliest stars. When temperatures in the
cores of early stars reached 10 million K, nuclear
fusion of hydrogen (H) into helium (He) and higher
elements began, releasing energy that powered the
stars. As early stars aged, they ultimately exploded,
ejecting stellar material back into space. Table 2.1
gives the abundance of hydrogen in the universe
today relative to the abundances of other interstellar
elements.
About 4.6 b.y.a., some interstellar material aggre-
gated to form a cloudy mass, the solar nebula ,inour
current solar system. The composition of the solar neb-
ula was the same as that of 95 percent of the other
stars in the universe. Gravitational collapse of the solar
nebula resulted in the formation of the sun.
Today, the sun, which comprises 99.86 percent of
the mass of the solar system, is divided into concentric
layers, including interior and atmospheric layers. About
90 percent of the atoms in the sun are hydrogen, and
9.9 percent are helium. The remaining atoms are the
other natural elements of the periodic table.
The sun's interior composition is a plasma ,which is
afourth state of matter aside from solid, liquid, and gas,
in which temperatures are so high that gas molecules
break apart into positively charged atoms or molecules
and negatively charged free electrons.

2.1. 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.
Common theory suggests that, about 15 billion years
ago (b.y.a.), all mass in the known universe was