Why an Atmosphere of N2 and O2?

Earth's atmosphere -- the gaseous envelope that surrounds the rocky crust and water oceans -- is a bit of an oddball in the Solar System. The Sun is mostly H2 and He. The gas giants, Jupiter, Saturn, etc., are H2, He, CH4 (methane), and NH3 (ammonia); although one of Jupiter's moons does have frozen oceans of water. Venus, often called Earth's twin, has an atmosphere off CO2, H2O, and sulfuric acid. So why should the Earth have mostly nitrogen and oxygen. Actually that is quite a story, and in learning the answers, geologists and climatologists have learned a lot about the history of our planet.

I. Composition of the current Atmosphere

A. Constant components

B. Variable components

1. Local variations in time and space (H2O)

2. Changes resulting from human activity (CO2, CFCs, and CH4)

C. More likely atmosphere

1. The sun

2. Gas Giants

3. Venus

4. Comets

II. Primordial Atmosphere

A. Formed with the Sun

B. Escape velocity and the lost gases

C. Post formation events

1. Cometary collisions

2. Volcanic eruptions

3. Biological Activity

III. Why not like Venus?

A. Runaway greenhouse effect

1. Energy Balance

2. Greenhouse gases and the effect of clouds

B. Earth's CO2

1. Limestone

2. Coal

Questions: 1. Describe how the Earth's atmosphere is distinct from other atmospheres of the Solar System.

2. Discuss the escape of the Earth's primordial atmosphere and explain why similar escape did not occur from the gas giants.

3. Comets and volcanoes added a secondary atmosphere of CO2, CH4, NH3, and H2O. What happened to these gases as the present N2-O2 atmosphere formed?

4. Describe the runaway greenhouse effect of Venus. Discuss the potential for a similar runaway greenhouse here on Earth.

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