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Which Came First???

Has Earth always had an atmosphere? Has it always been made of the same materials? What factors could have promoted changes in the gases that make up our "safety blanket" between Earth and the cold vacuum of outer space?

Scientists believe that the gases that made up our initial atmosphere came from volcanic eruptions early in the life of the planet. The atmosphere has changed vastly over time into one that is more hospitable for life to evolve and flourish.

Let's compare the components of Earth's early and current atmosphere. The gases are listed in the order of abundance - the higher on the list, the more of that gas there was. No percentages of the early atmosphere were given, since no one was around 4.4 billion years ago to measure them.

Earth's Early Atmosphere Earth's Current Atmosphere Percent Abundance
Carbon Dioxide (CO2) Nitrogen (N2)
Water Vapor (H20) Oxygen (O2)
Nitrogen (N2) Argon (Ar)

Methane (CH4)

Carbon Dioxide (CO2)
Ammonia (NH4) Neon (Ne)
Sulfur Dioxide (SO2) Helium (He)
Hydrochloric Acid (HCl) Methane (CH4)
  Ozone (O3)

There was no ozone in the early atmosphere that we know of. There is not that much ozone now. What had to happen in the early atmosphere to produce the ozone layer?

We have stated earlier that ozone is harmful to plants and lung tissue. How could life have evolved on the surface without the ozone layer? If plants make oxygen that formed ozone, how did the plants stay alive long enough to make the oxygen?

Ozone is produced everyday when UV radiation from sunlight hits a molecule of oxygen (O2) and breaks it into two atoms of oxygen (O):

O2 (g) ------> 2 O (g)

One of the two atoms of oxygen (O) combine with a molecule of oxygen (O2) to get one molecule of ozone (O3):

O (g) + O2 (g) ------> O3(g)

If ozone is made from oxygen in the atmosphere, where did the oxygen come from in order to make the ozone???

2.5 million years ago, the first primitive life forms, blue-green bacteria, began producing oxygen through the process of photosynthesis. After a long time, oxygen began to accumulate in the atmosphere. After this accumulation occurred, and there was a supply of oxygen to work with, ozone began to be produced through the process above.

Once the ozone layer was formed, plants could survive on the land because of this protective layer. Once plants started to flourish on the land, our atmosphere began its long process of changing from mostly carbon dioxide and harmful gases to the atmosphere we know and love today.

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Updated October 24, 2008 by: Glen Westbroek

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