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

Lecture 29:
The Earth's Atmosphere

Key Ideas:

• Nitrogen, Oxygen, Argon & Water Vapor
• Lack of Hydrogen or Helium

Greenhouse Effect:

• Explains why the Earth is as warm as it is.

Structure of the Atmosphere

Origin of the Atmosphere

• Primordial atmosphere: mostly CO₂
• Evolution of the atmosphere

The Earth's Atmosphere

• 77% N₂ (molecular nitrogen)
• 21% O₂ (molecular oxygen)
• 1% H₂O (Water Vapor)
• 0.93% Argon
• CO₂ (0.035%)
• Traces of CH₄ (methane), Inert Gases (Ne, He, Kr, Xe)
• Particulates (silicate dust, sea salt, sulfates, etc.)

Why is there so little Hydrogen?

• H & He are small and light, and so moves very fast at a given atmospheric temperature.
• The mean speeds are greater than the escape velocity from the Earth.

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

Why is the Earth so warm?

• The energy in sunlight absorbed by the Earth
• The energy radiated as infrared photons by the warm Earth.

Equilibrium Temperature should be T=260 K

• Water freezes at 273 K, so
• You would expect no liquid water

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

Where does all the sunlight go?

• Absorbs IR & UV photons direct from the Sun
• Absorbs IR photons radiated by the ground
• Clouds can reflect or scatter sunlight.

The ground absorbs sunlight

• Absorbs UV, visible, and IR photons.
• Some light is reflected (clouds, ground, snow etc.)

The "Energy Budget" for Sunlight:

• 45% is absorbed by the ground (heats the ground)
• 22% is absorbed in the atmosphere (heats the air)
• 26% is reflected back into space by clouds and lost
• 7% is reflected back into space by the ground, oceans, snow & ice.

Greenhouse Effect

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

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

Zoom into the near-Infrared (1-6micron) showing specific molecular bands

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

• Photons are absorbed by the ground, heating it up
• The warm ground radiates infrared photons (Wein's Law)

• Most of the infrared photons emitted by the warm ground get absorbed by the atmosphere on their way out, heating the 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.

Note:

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

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

• Pressure drops 50% for each 5.5 km of altitude
• At Sea Level: ~1 kg/cm² (14 pounds/inch²)

Structure of the Atmosphere

• Troposphere - weather layer
• Stratosphere - heated by UV absorption, primarily in the ozone (O₃) layer
• Mesosphere - cooler intermediate region
• Thermosphere - heated by UV & X-ray photons

Origin of the Earth's Atmosphere

• Mostly H₂O and CO₂
• Small amounts of N₂ and sulfates
• No oxygen (O₂).

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

Where did all the CO₂ go?

• H₂O condensed to form the oceans.
• CO₂ dissolved into the oceans and precipitated out as carbonates (e.g., limestone).

By contrast, N₂ is chemically inactive

• It stayed a gas in the atmosphere and become its dominant constituent.

Where did the O₂ come from?

• Molecular Oxygen (O₂) comes primarily from photosynthesis in plants and algae.
• The O₂ content of the atmosphere has increased from 1% to 21% during the past 600 Myr.

Ozone (O₃):

• Forms in stratosphere from O₂ interacting with solar UV photons.
• Blocks UV photons from reaching the ground.

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

Atmospheric Evolution

Past Evolution:

• Condensation of H₂O into the oceans.
• Locking up of CO₂ into carbonaceous rocks
• Formation of O₂ by photosynthesis in plants & algae

• CO₂ content of the atmosphere is regulated by a complex balance cycle.
• Increases in O₂ and methane (CH₄) from "biomass" (plants and animals)
• Human activity (fuel burning & agriculture)

Human Impact on the Atmosphere

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

• Increasing amounts of atmospheric greenhouse gases, in the amount of 20 billions tons/year, from industrial and agricultural activity.
• Ozone layer destruction by industrial emission of Chlorofluorocarbons (CFCs).
• Increasing particulates from a combination of industrial pollution, cooking fires, and rainforest burning.

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.