Origins of the Atmosphere

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Origins of the Atmosphere

ORIGINS OF THE ATMOSPHERE

About 4.5 billion years ago, Earth formed out of nebula of gases and dust that were to become the solar system

Small objects--called planetoids-- accreted or combined together to build larger objects…such as planets

ORIGINS OF THE ATMOSPHERE

We will discuss planetoids and the formation of planets in greater detail later

However, the Earth, along with all of the other planets in our solar system, began to form out of these semi-solid objects that we call planetoids.

This process is related to what scientists call the Solar Nebula

Theory, which discusses the origins of the solar system.

ORIGINS OF THE

ATMOSPHERE

• As a result of the formation of the planets, this might be how your favorite planet may have looked when it was first born!

Origin of the atmosphere

• The original atmosphere

– Probably made up of

hydrogen and helium

.

– These are fairly common in the universe.

• Original atmosphere stripped away by the solar wind

– H and He are very light

• Hydrogen and helium have the smallest atoms by mass.

The secondary atmosphere

• Formed from degassing of volcanoes

• Gasses emitted probably similar to the gasses emitted by volcanoes today.

– H

2

O (water), 50-60%

– CO

2

(carbon dioxide), 24%

– SO

2

(sulfur dioxide), 13%

– CO (carbon monoxide),

– S

2

– Cl

2

(sulfur),

(chlorine),

– N

2

– H

2

(nitrogen),

(hydrogen),

– NH

3

– CH

4

(ammonia) and

(methane)

WHERE DID THE VOLCANOES COME FROM FOR THE SECONDARY

ATMOSPHERE?

The Earth was initially very hot and molten when it first form 4.5 billion years ago. As the Earth sat for millions of years, the surface started to cool slowly over time. This is when the actual solid surface of the Earth began to form

BUT! It was still very hot inside the Earth, the heat from deep within left cracks in the Earth’s surface. These cracks in the Earth were where the first volcanoes began to form, release magma, heat, and many gasses trapped under the Earths new crust!

Modern (Third) atmosphere

Nitrogen (N2)-

78%,

Oxygen (O2)-

21%,

Carbon Dioxide

(CO2) 0.03 %,

Where did all the oxygen come from?

Modern (Third) atmosphere

Life changes the atmosphere

• With the evolution of life the first cellular organisms began to use the gasses in the early atmosphere

NH

3

– ammonia,

CH

4

– methane,

H

2

O

– water for energy.

Photosynthetic organisms evolve. (cyanobacteria)

These organisms use CO

2 and produce oxygen (O2) as a waste product.

Modern (Third) atmosphere

• Where did the O

2 come from?

– Produced by photosynthetic life

(cyanobacteria).

• Where did the CO

2 go?

– Dissolves in water in the oceans

– Used by life by photosynthesis and buried when plants and micro-organisms die.

• The source of coal and oil

What Are Cyanobacteria?

• Cyanobacteria are microbes that live primarily in seawater. They are believed to have been the first organisms on Earth to perform oxygenic photosynthesis. In this process, they produce organic carbon, the building blocks of life’s molecules, and release oxygen gas (O2). The O2 enters into the seawater, and from there some of it escapes into the atmosphere.

Evidence from the Rock Record

Iron (Fe)-

is extremely reactive with oxygen. If we look at the state of Fe in the rock record, we can learn a great deal about atmospheric evolution.

Banded Iron Formation (BIF)

- Deep water deposits in which layers of iron-rich minerals alternate with iron-poor layers, primarily chert.

Iron minerals include iron oxide, iron carbonate, iron silicate, iron sulfide. BIF's are a major source of iron ore, b/c they contain magnetite (Fe

3

O

4

) which has a higher ironto-oxygen ratio than hematite. These are common in rocks

2.0 - 2.8 B.y. old, but do not form today.

Red beds

-are never found in rocks older than 2.3 B. y., but are common during Phanerozoic time. Red beds are red because of the highly oxidized mineral hematite (Fe

2

O

3

), that probably forms secondarily by oxidation of other Fe minerals that have accumulated in the sediment.

Conclusion

- amount of O

2 in the atmosphere has increased with time.

Current Atmosphere

Characteristics

Permanent gases:

(together 99.96%)

Variable gases:

(N) Nitrogen 78%

(O) Oxygen 21%

(Ar) Argon ~ .9%

(CO

(O

(H

3

2

2

) Carbon dioxide

) Ozone

O) Water vapor

varies from 4% to less than 1%

-

Conclusions!

• The composition of Earth’s atmosphere has changed over geologic time.

• Earth’s atmosphere is unique in the solar system in that it contains substantial oxygen.

The most primitive atmosphere was comprised of mainly helium and hydrogen. The early atmosphere contained mostly CO2, CO, and water vapor. This atmosphere was then modified by early photosynthetic life.

Early photosynthetic life such as cyanobacteria (blue-green algae) consumed carbon dioxide and generated oxygen.

It was only after early photosynthetic life generated oxygen that animal life became possible.

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