Definition of Formation and Evolution of the Atmosphere

There is considerable uncertainty about the origin and composition

of the Earth’s earliest atmosphere. Many models

assume that methane and ammonia, instead of nitrogen and

carbon dioxide, dominated the planet’s early atmosphere as it

is presently. The gases that formed the early atmosphere

could have come from volcanic outgassing by volcanoes,

from extraterrestrial sources (principally cometary impacts),

or, most likely, both. It is also likely that comets brought

organic molecules to Earth. A very large late impact is

thought to have melted outer parts of the Earth, formed the

Moon, and blown away the earliest atmosphere. The present

atmosphere must therefore represent a later, secondary atmosphere

formed after this late impact.

During the early Archean, the Sun was only about 70

percent as luminous as it is presently, so the Earth must have

experienced a greenhouse warming effect to keep temperatures

above the freezing point of water, but below the boiling

point. Increased levels of carbon dioxide and ammonia in the

early atmosphere could have acted as greenhouse gases,

accounting for the remarkable maintenance of global temperatures

within the stability field of liquid water, allowing the

development of life. Much of the carbon dioxide that was in

the early atmosphere is now locked up in deposits of sediatmosphere

mentary limestone, and in the planet’s biomass. The carbon

dioxide that shielded the early Earth and kept temperatures

in the range suitable for life to evolve now forms the bodies

and remains of those very life-forms.

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