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Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 9:30

Lecture 29:
The Earth's Atmosphere

Key Ideas:


Greenhouse Effect:

Structure of the Atmosphere

Origin of the Atmosphere

The Earth's Atmosphere


Why is there so little Hydrogen?

Hydrogen & Helium are the most abundant elements in the Universe, yet they are very rare in the Earth's atmosphere. Why? As a consequence, most of the H and He escaped from the atmosphere a long time ago.

The Earth is too small to retain atmospheric H & He.

Why is the Earth so warm?

If there was no atmosphere, the Earth's temperature could be calculated by balancing:

Equilibrium Temperature should be T=260 K

Why is this not the case? In other words, why is the Earth as warm as it is?

Where does all the sunlight go?

The Earth's atmosphere strongly absorbs infrared and UV radiation:

The ground absorbs sunlight

The "Energy Budget" for Sunlight:

Only 67% of sunlight actually heats the Earth.

Greenhouse Effect

The atmosphere is transparent to visible light, but mostly opaque to infrared.

Infrared "opacity" comes from absorption bands of H2O, CO2, CH4 and others molecules.

Calculated Atmospheric Absorption, 1-28 microns
Infrared Absorption of the Atmosphere from 1-28 microns (calculated)

Calculated Atmospheric Absorption from 1-6 microns
Zoom into the near-Infrared (1-6micron) showing specific molecular bands

Visible Sunlight passes through the Earth's atmosphere to the ground:

The atmosphere, however, is mostly opaque to infrared photons: The Greenhouse Effect is responsible for making the Earth about 35K warmer than it would be if there it had no atmosphere.

The implications are very interesting: Without the Greenhouse Effect, there would be no liquid water on the Earth, only ice. Since life as we understand it requires liquid water, if there was no Greenhouse Effect, the Earth would be inhospitable to life.


The term "greenhouse effect" is something of a misnomer. Greenhouses (glass buildings mean to stay warm through winter for growing plants) work primarily by inhibiting convection (boiling motions of air warmed by solar-heating of the ground inside the greenhouse), rather than by radiative trapping of sunlight behind glass. Radiative trapping does contribute to how a garden greenhouse works, but only to a fraction of the total heating experienced inside a greenhouse.

Atmospheric Pressure

Weight of air above makes the surface pressure higher.

The atmosphere's pressure falls as you climb in altitude:

For example, at the summit of Mt. Everest at 8850 meters, the air pressure is about 1/3 what it is at sea level.

Structure of the Atmosphere

The Earth's atmosphere is divided vertically into several thermal layers, from lowest to highest above the ground: The atmosphere does not so much stop as get thinner until it is nearly indistinguishable from interplanetary space.

Origin of the Earth's Atmosphere

After losing most of its original H and He, the Primordial Atmosphere of the Earth was built up by outgassing of the crust by volcanos:

This is very different than our present atmosphere. How did our atmosphere get the way it did?

Where did all the CO2 go?

The primordial atmosphere had ~1000 times more CO2 than it does now. Where did it all go? Most of the present-day CO2 is locked up in crustal rocks and dissolved in the oceans.

By contrast, N2 is chemically inactive

Where did the O2 come from?

The second major constituent of the present-day atmosphere is Oxygen (O2), but it was absent in the Primordial Atmosphere. Where did all the O2 come from?

Ozone (O3):

This made land life possible as solar UV radiation is hazardous to life.

The presence of O2 and O3 in our atmosphere is a sign of life (photosynthesis).

Atmospheric Evolution

Atmospheres are complex, dynamic systems that evolve over time.

Past Evolution:

This evolution continues into the present day.

Human Impact on the Atmosphere

The impacts of human activity on the Earth's atmosphere include:

The impact of human activity (industrial or otherwise) upon the Earth's atmosphere is real and it is measurable.

What is currently being debated, both scientifically and politically, is the long-term implications of this impact, and what it may mean for the future evolution of our atmosphere. This topic is formally beyond the scope of this class, but of great general interest.

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Updated: 2006 October 28
Copyright Richard W. Pogge, All Rights Reserved.