Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 28: The Earth's Atmosphere

Key Ideas:

Composition:: Nitrogen, Oxygen, Argon, and Water Vapor; Lacks Hydrogen and 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

Composition:: 77% N₂ (molecular nitrogen); 21% O₂ (molecular oxygen); 1% H₂O (water vapor); 0.93% Argon; 0.035% CO₂ (carbon dioxide); Traces of CH₄, Inert gases (Ne, He, Kr, Xe); Particulates (silicate dust, sea salt, sulfates, etc.)

Why is there so little Hydrogen?

Hydrogen and Helium are the most abundant elements in the Universe, yet they are very rare in the Earth's atmosphere. Why?: H and He are small and light and so move very fast at a given atmospheric temperature.; The mean atomic speeds are greater than the escape velocity from the Earth
Most of the H and He escaped long ago

Earth is too low mass to retain atmospheric H and He

Why is the Earth so warm?

If there was no atmosphere, the Earth's temperature could be found by balancing:: The energy of sunlight absorbed by the Earth; The energy radiated as infrared photons by the warm Earth
Equilibrium Temperature: T = 260 K: Water freezes at 273 K, so; You would expect no liquid water!

Why is this not the case?

Where does all the sunlight go?

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 other molecules
Sunlight heats the ground, warming it up:: The warm ground radiates infrared photons; These infrared photons are absorbed by the atmosphere, heating it.

This makes the Earth ~35K warmer than it would be if there were no atmosphere

Atmospheric Pressure

Atmospheric Pressure drops with altitude:: Sea Level: ~1 kg/cm² (14 pounds/in²); Pressure drops 50% for every 5.5 km in altitude.
Mt. Everest: Altitude: 8850m; Pressure is 1/3 sea-level

Structure of the Atmosphere

The Earth's atmosphere is divided vertically into several Thermal Layers: Troposphere - lowest "weather" layer; Stratosphere - heated by UV absorption in the ozone (O₃) layer; Mesosphere - cooler intermediate region; Thermosphere - heated by UV and X-ray photons
Above this the atmosphere merges smoothly into interplanetary space.

The Origin of the Atmosphere

After losing most of its H and He, the early atmosphere was built by outgassing from volcanoes:: Mostly H₂O and CO₂; Small amounts of N₂ and sulfates; No O₂
Very different from present-day atmosphere
How did it get the way it is now?

Where did all the CO₂ go?

The primordial atmosphere had ~1000 times more CO₂ than it does now.
Where did it go?: H₂O rained out to form the oceans; CO₂ dissolved into ocean water and precipitated out as carbonates (e.g. limestone); Today most CO₂ is locked up in crustal rocks and dissolved in the oceans
N₂ is chemically inactive: Stays as the dominant constituent

Where did the O₂ come from?

Molecular Oxygen (O₂) comes from life:: Photosynthesis in plants and algae; O₂ content increased from 1% to 21% during the past 600 Myr
Ozone (O₃):: Forms in the stratosphere when O₂ interacts with solar UV photons; Blocks UV from reaching the ground; Made life on land possible

O₂ and O₃ are signs of life (photosynthesis)

Atmospheric Evolution

Atmospheres are dynamic and evolving
Past evolution: Condensation of H₂O into the oceans; Locking up CO₂ into carbonaceous rocks; Formation of O₂ by photosynthesis
Continues into the present day:: CO₂ regulated by a complex cycle; Increases in O₂ and CH₄ from "biomass"; Human activity (fuel burning and agriculture)

Human Impact on the Atmosphere

Primary Human Impacts:: Emissions of greenhouse gases (20 billion tons per year) from industry and agriculture; Ozone layer destruction by industrial CFCs; Increase in atmospheric particulates (industrial pollution, cooking fires, and rainforest burning)

Human impact is real and measureable

Implications

The increase in greenhouse gases during the 20th century coincides with an 0.6 C rise in global mean temperature.
Is this causes by human activity?: We are at least a major part of the cause; Natural cycles are also in play
We are significantly altering our atmosphere: Consequences are hard to predict accurately

Atmospheres are complex, dynamic systems

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.