that originally surrounded Earth was probably much different from
The atmosphere the air we breathe today. Earth's first atmosphere (some 4.6 billion years ago) was most
likely hydrogen and helium the two most abundant gasses found in the universe
as well as hydrogen compounds, such as methane and a
Most scientists feel that this
early atmosphere escaped into space from the Earth's hot surface.
The second, more dense atmosphere, however, gradually enveloped Earth as
es molten rocks within its hot interior escaped through volcanoes and steam vents.
from We assume that volcanoes spewed out the same gasses then as they do today: mostly
water vapor (about 80 percent), carbon dioxide (about ten percent), and up to a few
percent nitrogen. These same gasses probably created Earth's second atmosphere.
As million years passed, the constant outpouring of gasses from the hot interior
known as outgassing provided a rich supply of water vapor, which formed into clouds. Rain
fell upon Earth for many thousands of years, forming the rivers, lakes, and oceans of the
world. During this time, large amounts of carbon dioxide were dissolved in the oceans.
rough chemical and biological processes, much of the carbon dioxide became locked up
in carbon sedimentary rocks, such as limestone. With much of the water vapor already
condensed into water and the concentration of carbon dioxide dwindling, the atmosphere
gradually became rich nitrogen
It appears that oxygen, the second most abundant gas in today's atmosphere,
probably began an extremely slow increase in concentration as energetic rays from the sun
split water vapor into oxygen during a process called photodissociation. The hydrogen, being
lighter, probably rose and escaped into space, while the oxygen remained in the
atmosphere.
This slow increase in oxygen may have provided enough if this gas for primitive
plants to evolve, perhaps two to three billion years ago. Or the plants may have evolve in
an almost oxygen-free (anaerobic) environment.
At any rate, plant growth greatly enriched
our atmosphere with oxygen. The reason for this enrichment is that plants, in the presence
of sunlight, process carbon dioxide and water to produce oxygen.