ATMOSPHERIC COMPOSITION

ORIGIN OF THE ATMOSPHERE:

The origin of our earth's atmosphere is still subject to much speculation. However, one theory seems fairly certain; that when the earth was formed some five billion years ago, it was probably too hot to retain any of the atmosphere it had to begin with. Based on our knowledge of gases in the universe, this first atmosphere probably consisted of helium, hydrogen, ammonia and methane.

If we assume that volcanoes five billion years ago emitted the same gasses as they do today, the earth's second atmosphere probably consisted of water vapor, carbon dioxide, and nitrogen. These gasses were expelled from the earth's interior by a process known as outgassing.

The vast amounts of water vapor expelled by the volcanic earth resulted in the formation of clouds which, in turn, produced rain. Over a period of thousands of years, the rain accumulated as rivers and lake and ocean basins.

EXPLORES! ASKS?

Why is some water considered "fresh water" and some water considered "salt water" and how might the differences have come about?

During this time, the water reservoirs acted as sinks for the large amounts of carbon dioxide (as they do today) and through chemical and biological processes became locked up in sedimentary rocks as limestone. The nitrogen, which is not very chemically active continued to accumulate in the atmosphere. In addition, evidence exists that the Sun was stronger several billion years ago, strong enough to radiatively split (called photodissociation) water vapor molecules into hydrogen and oxygen. The hydrogen, being very light, escaped into space, while the heavier oxygen accumulated in the atmosphere.

These processes acting sequentially and simultaneously appear to have produced the delicate balance of 78% nitrogen (N2) and 21% oxygen (O2) we observe today.

PRESENT COMPOSITION OF THE ATMOSPHERE:

The atmosphere is comprised of gases which are considered to be permanent (gases which remain essentially constant by percent) and gases considered to be variable (gases which have changing concentrations over a finite period of time).

PERMANENT gases in the atmosphere by percent are:

	Nitrogen    78.1%
	Oxygen	    20.9%

(Note that these two permanent gases together comprise 99% of the atmosphere)

Other permanent gases:

	Argon	             0.9%
	Neon	             0.002%
	Helium	             0.0005%
	Krypton              0.0001%	
	Hydrogen	     0.00005%

VARIABLE gases in the atmosphere and typical percentage values are:

	Water vapor     0 to 4%
	Carbon Dioxide	0.035%
	Methane	        0.0002%
	Ozone	        0.000004%

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