But also water played an important role. Particurlarly its origin must have taken place once such atmosphere originated so that water didn't leave the Earth as vapor.

It is also important to have a big enough magnetic field to protect the

Atmosphere that is generated from the internal workings of the planet.

Earth atmosphere has not been like it is now all the time - initially Earth had a very CO2 rich atmosphere (changing to a very acidic O2 dominant atmosphere, actually resulted in a lot of deaths of species). The main factor behind the evolvement has been simply plants: They change (and still do ;-) ) CO2 to O2, which is their way of creating energy (while we do it vice versa, in both cases with various by products). So the short answer is: it has been life form induced.

A very detailed desscription of the early Earth and the development of its planetary body and also atmosphere can be found in this book:

http://www.springer.com/astronomy/astrobiology/book/978-3-642-22551-2

I can highly recommend it.

The atmosphere of a planet comes up from inside the planet.

I suspect that all planets produce an atmosphere. The question is,

can the planet keep the atmosphere with the solar wind effectively stripping the atmosphere away.

The Earth has developed an atmosphere, and has developed a means of keeping

its atmosphere by developing a protective magnetic shield.

The process is one of " Fractional Distillation " where lighter materials migrates

upward in the planet, and denser materials, like iron, migrate

downward, and concentrate in layers of similar density.

The concentration of iron is beneficial in that the size of the planet,

the amount of iron, the confining pressure at depth, and the applied torque of the

Sun's magnetic field all work together to apply torque of the Earth's accumulating,

and growing magnetic iron core, which creates internal fluid motions

that generate the protective magnetic field.

This has created two + questions in my mind. Why did it take nearly 4,000

million years for life to evolve to larger life forms on this planet ? AND

Why has life managed to stay large and prosper for roughly 630 million years, and

why has Earth's life sustaining processes managed to continue

within a fairly narrow atmospheric temperature and pressure range for

630 million years ?

It does not seem probable that the Earth could manage to stay in such

a narrow , improbably life sustaining " Goldilocks " zone for such a

totally improbably long period of time.

The only possible mechanism that I can envision is one that is cyclical,

or semi cyclical and allows the atmosphere to vary in depth, density. and surface

pressure and temperature in a way that allows it to vary on average

between a low of 6 C and a high of 26 C, with a long term dominate

temperature average of 22 C.

Small pulsed growths and expansions of the Earth would accomplish this

including a glacial period following a pulsed expansion, that gradually returns to

normal over several thousand years, as continents sink, and sea floors rise.

and the atmosphere is replenished from inside the planet and as it becomes thicker

and gradually rises along. with rising sea levels and sinking continents.

It would be a dynamic form of isostacy in concert with a dynamic form of

atmospheric equilibrium replenishment from inside the planet, which

also is working in concert with a growing and expanding planet.

Perhaps a gas giant stage occurs when the planet reaches a point where it can no longer grow and expand, so it the heats up too much internally, and releases

gas at too fast a rate, and the atmosphere grows essentially without bounds.

Thank you K. Towe, M. Clark and V. Maiwaid for info and references. Really contributing!

There are many factors that have influenced the atmosphere of the Earth to reach to the present composition and form:

The distance of the Earth from the Sun is just appropriate for the moisture content to be retained in the atmosphere (and of course many other contents). Imagine the Earth nearer or farther from the Sun, the temperature would not be what it is now.

The size of the Earth is just appropriate. Imagine it bigger or smaller vis-a-vis effect of gravity. (Had Moon not separated from the Earth, Its size would not have allowed present atmosphere). Many lighter harmful gases escaped from the Earth. We know that H2O is also getting dissociated in the atmosphere and hydrogen being lighter is escaping from the atmosphere. The time will come when no hydrogen would be left (no cause for alarm because it will take billions of years).

The heat engine of the earth (operative most probably because of radioactivity) is operating just at right speed. Had it been a little faster there would be many volcanic eruptions causing not so clean air. The nitrogen -oxygen ratio is being maintained because of such factors. Imagine even slight perturbation in this ratio because of various causes!!

Asteroidal impacts did gradually add important elements on the Earth, for instance phosphorus, nickel, iron etc that is supposed to have improved or may be deteriorated the atmosphere. Photosynthesis, a crucial process for more or less shaping present atmosphere came later on the Earth. Some scientists say: the water on the earth was brought due to cometary impact.