Formation of Earth’s Atmosphere
The early Earth was very different from our Earth today. The early Earth experienced
frequent impacts from asteroids and meteorites and had much more frequent volcanic erup-
tions. There was no life on Earth for the first billion years because theatmospherewas
not suitable for life. Earth’s first atmosphere had lots ofwater vaporbut had almost no
oxygen. Later, frequent volcanic eruptions put several different gases into the air (Figure
12.13). These gases created a new type of atmosphere for Earth. The volcanic eruptions
spewed gases such as nitrogen, carbon dioxide, hydrogen, and water vapor into the atmo-
sphere—but no free oxygen. Without oxygen, there was still very little that could live on
Earth.
Figure 12.13: Volcanic eruptions occurred almost constantly on the early Earth. Eruptions
put water vapor, carbon dioxide and other gases into the air that helped create Earth’s early
atmosphere. ( 6 )
Slowly, two processes changed Earth’s atmosphere to one that is more oxygen-rich—like the
one we have today. First, radiation from the Sun caused water vapormoleculesto split
apart. Remember that a molecule of water is made of the elements hydrogen and oxygen, or
H 2 O. Radiation from the Sun split some of the water molecules into hydrogen and oxygen.
The hydrogen escaped back to outer space. The oxygen accumulated in the atmosphere. The
second process that changed Earth’s early atmosphere was photosynthesis (Figure12.14).
About 2.4 billion years ago, a type of organism called cyanobacteria evolved on the early
Earthandbegancarryingoutphotosynthesis. Photosynthesisusescarbondioxideandenergy
from the Sun to produce sugar and oxygen. The cyanobacteria were very simple organisms
but performed an important role in changing Earth’s early atmosphere. They carried out
photosynthesis to produce the materials they needed to grow. They gave off oxygen to the
atmosphere as they did this.