6.1 Composition, structure, and evolution of planetary atmospheres

Planetary atmospheres are complex systems that shape a world's climate and habitability. Their composition, structure, and evolution are influenced by factors like outgassing, escape, and chemical reactions. These processes determine the atmosphere's ability to retain heat and protect the surface.

Comparing atmospheres across planets and moons reveals diverse conditions. Earth's atmosphere supports life, while Venus's runaway greenhouse effect creates extreme heat. Mars's thin atmosphere can't retain heat well. Understanding these differences helps us grasp the importance of atmospheres in planetary science.

Planetary Atmosphere Composition

Chemical Composition

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• The chemical composition of a planetary atmosphere determined by the relative abundances of various gases (nitrogen, oxygen, carbon dioxide, methane, noble gases)
• Earth's atmosphere composed primarily of nitrogen (78%) and oxygen (21%), with trace amounts of other gases (argon, carbon dioxide)
• Venus has a dense, CO2-rich atmosphere (96%) with high surface temperatures and pressures due to a runaway greenhouse effect
  • Venus's atmosphere also contains thick sulfuric acid clouds
• Mars has a thin atmosphere composed mainly of carbon dioxide (95%), with small amounts of nitrogen and argon
• Titan, Saturn's largest moon, has a dense nitrogen-rich atmosphere with a significant presence of methane
  • Titan's atmospheric methane participates in a hydrological cycle similar to Earth's water cycle, forming clouds, rain, and surface liquid reservoirs

Vertical Structure

• The vertical structure of an atmosphere typically divided into layers based on temperature gradients and chemical composition changes
  • The troposphere is the lowest layer, where most weather phenomena occur, and temperature decreases with altitude
  • The stratosphere is the layer above the troposphere, characterized by a temperature inversion caused by the absorption of UV radiation by ozone
  • The mesosphere is the layer above the stratosphere, where temperature decreases with altitude, and meteors often burn up
  • The thermosphere is the uppermost layer, where temperature increases with altitude due to absorption of high-energy radiation by atmospheric particles
• Atmospheric pressure decreases exponentially with altitude, following the barometric formula, which relates pressure to altitude, temperature, and mean molecular mass of the atmosphere
• The scale height of an atmosphere is the vertical distance over which the atmospheric pressure decreases by a factor of e (approximately 2.718)
  • Scale height depends on the atmospheric temperature and the mean molecular mass of the gases

Atmospheric Evolution Processes

Outgassing and Escape

• Planetary atmospheres evolve through various processes, including outgassing, escape, and chemical reactions
• Outgassing is the release of gases from a planet's interior through volcanism, sublimation of surface ices, or chemical reactions between the surface and the atmosphere
  • Outgassing can enrich the atmosphere with volatiles (water vapor, carbon dioxide, sulfur dioxide)
• Atmospheric escape occurs when gas molecules gain enough kinetic energy to overcome the planet's gravitational pull and escape into space
  • The rate of escape depends on factors such as the planet's mass, the atmospheric temperature, and the intensity of solar radiation
  • Jeans escape is a thermal escape mechanism that occurs when individual gas molecules in the high-energy tail of the Maxwell-Boltzmann distribution exceed the escape velocity of the planet
  • Hydrodynamic escape is a rapid, bulk escape of the atmosphere that can occur when the atmosphere is heated to very high temperatures (early stages of a planet's formation, extreme solar activity)

Chemical Reactions and Impact Events

• Chemical reactions in the atmosphere can alter its composition over time
  • Photochemical reactions driven by solar UV radiation can break down molecules and create new compounds (formation of ozone in Earth's stratosphere)
• Impact events can significantly alter atmospheric composition by delivering new materials or ejecting portions of the existing atmosphere
  • Large impacts can cause global changes in atmospheric composition and climate (Chicxulub impact on Earth, 66 million years ago)

Terrestrial Atmospheres: Comparisons

Terrestrial Planets

• Earth's atmosphere has a thick troposphere and a stratosphere with an ozone layer that protects the surface from harmful UV radiation
• Venus's atmosphere has a slow rotation rate, leading to global super-rotation of the atmosphere
• Mars's atmosphere is too thin to retain heat effectively, resulting in large diurnal temperature variations and a mostly frozen surface

Moons with Atmospheres

• Some moons (Jupiter's Io and Europa) have tenuous atmospheres composed of gases released by volcanic activity or sublimation of surface ices
  • These atmospheres are often transient and can vary in composition depending on the moon's geological activity and interaction with its host planet's magnetosphere

Atmosphere's Influence on Surface Conditions

Greenhouse Effect and Ozone Layer

• The greenhouse effect, caused by atmospheric gases that absorb and re-emit infrared radiation, can significantly impact a planet's surface temperature
  • The strength of the greenhouse effect depends on the abundance of greenhouse gases (carbon dioxide, water vapor, methane)
• The presence of an ozone layer in a planet's stratosphere can protect the surface from harmful UV radiation, which is crucial for the development and sustainability of life as we know it

Atmospheric Pressure and Composition

• Atmospheric pressure affects the boiling point of liquids and the stability of liquid water on a planet's surface
  • Higher atmospheric pressures can allow liquid water to exist at higher temperatures, while lower pressures may cause water to sublimate directly from solid to gas
• The atmospheric composition can influence the rate of weathering and erosion on a planet's surface
  • An atmosphere rich in carbon dioxide can lead to increased chemical weathering through the formation of carbonic acid in rainwater
• Atmospheric gases can participate in biogeochemical cycles (carbon cycle), which regulate the exchange of carbon between the atmosphere, oceans, and biosphere
  • These cycles can have long-term effects on a planet's climate and habitability

Aerosols and Albedo

• The presence of atmospheric aerosols (dust particles, ice crystals) can affect a planet's albedo (reflectivity) and influence its energy balance by scattering or absorbing incoming solar radiation
  • Aerosols can have a cooling effect by reflecting sunlight back to space or a warming effect by absorbing and re-emitting infrared radiation