6.3 Atmospheric chemistry of planets and moons

Planetary atmospheres are complex mixtures of gases that shape a world's climate and habitability. Their composition and structure are influenced by factors like mass, distance from the star, and geological history. Understanding these atmospheres is key to unlocking the secrets of planetary evolution.

From Earth's life-sustaining mix to Venus's scorching greenhouse, atmospheric chemistry plays a vital role in determining surface conditions. It affects everything from temperature regulation to UV protection, making it crucial for assessing a planet's potential to host life as we know it.

Composition and Structure of Atmospheres

Chemical Composition of Planetary Atmospheres

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• Planetary atmospheres consist of a mixture of gases, with the most common being hydrogen (H2), helium (He), carbon dioxide (CO2), methane (CH4), and nitrogen (N2)
• The specific atmospheric composition varies based on factors such as the planet's mass, distance from the sun, and geological history
• Examples of atmospheric composition:
  • Earth: 78% N2, 21% O2, 0.9% Ar, 0.04% CO2
  • Mars: 95% CO2, 2.7% N2, 1.6% Ar, 0.13% O2
• The chemical composition of a planet's atmosphere is influenced by its initial formation, outgassing from the interior, escape processes, and chemical reactions

Atmospheric Structure and Layers

• Atmospheric structure is typically divided into layers based on temperature gradients
  • Troposphere: lowest layer, contains most of the atmosphere's mass and is characterized by decreasing temperature with height
  • Stratosphere: layer above the troposphere, characterized by increasing temperature with height due to absorption of UV radiation by ozone
  • Mesosphere: layer above the stratosphere, characterized by decreasing temperature with height
  • Thermosphere: uppermost layer, characterized by increasing temperature with height due to absorption of high-energy radiation
• Each atmospheric layer has distinct chemical and physical properties that influence the behavior of chemical species within them

Lunar Atmospheres and Exospheres

• Lunar atmospheres, also known as exospheres, are extremely tenuous and primarily composed of gases released from the lunar surface
• Processes that contribute to the release of gases in lunar atmospheres include:
  • Solar wind sputtering: bombardment of the lunar surface by high-energy solar wind particles, ejecting atoms and molecules
  • Meteoroid impacts: collisions of meteoroids with the lunar surface, vaporizing surface material
  • Thermal desorption: release of gases adsorbed on the lunar surface due to temperature changes
• Examples of lunar atmospheres:
  • Moon: He, Ar, Na, K, Rn
  • Europa: O2, H2O, CO2, SO2

Atmospheric Chemical Processes

Photochemical Reactions

• Photochemical reactions are initiated by the absorption of solar ultraviolet (UV) radiation by atmospheric molecules
• These reactions can lead to the formation of new compounds, such as ozone (O3) in Earth's stratosphere through the Chapman cycle:
  • O2 + hν (λ < 242 nm) → O + O
  • O + O2 + M → O3 + M (M is a third body, usually N2 or O2)
• Photochemical reactions play a crucial role in the chemistry of planetary atmospheres and can influence the abundance and distribution of various chemical species

Chemical Equilibrium and Non-Equilibrium Processes

• Chemical equilibrium is achieved when the rates of forward and reverse reactions are equal
• Many atmospheric reactions are out of equilibrium due to factors such as solar radiation, atmospheric transport, and surface interactions
• Non-equilibrium processes can drive the production or loss of certain chemical species, leading to deviations from expected equilibrium concentrations
• Examples of non-equilibrium processes:
  • Photodissociation of O2 in Earth's stratosphere, leading to higher O3 concentrations than predicted by equilibrium chemistry
  • Transport of chemical species by atmospheric circulation, such as the global Brewer-Dobson circulation in Earth's stratosphere

Catalytic Cycles and Atmospheric Chemistry

• Catalytic cycles involve species that participate in reactions but are regenerated, allowing them to contribute to atmospheric chemistry without being consumed
• An example is the catalytic destruction of ozone by chlorine radicals in Earth's stratosphere:
  • Cl + O3 → ClO + O2
  • ClO + O → Cl + O2
  • Net: O3 + O → 2O2
• Catalytic cycles can have significant impacts on the abundance of certain chemical species in planetary atmospheres, such as the depletion of stratospheric ozone by chlorofluorocarbons (CFCs) on Earth

Condensation and Evaporation Processes

• Condensation and evaporation processes affect the distribution of chemical species in atmospheres, particularly in the case of volatile compounds like water vapor
• Condensation occurs when a gas phase species reaches its saturation vapor pressure and forms liquid or solid particles, such as the formation of water droplets in Earth's troposphere
• Evaporation occurs when a liquid or solid species gains sufficient energy to overcome intermolecular forces and enter the gas phase, such as the evaporation of water from Earth's oceans
• These processes can influence the vertical distribution of chemical species in an atmosphere and contribute to the formation of clouds and aerosols

Atmospheric Escape Processes

• Atmospheric escape processes can lead to the loss of lighter atmospheric constituents over geological timescales
• Examples of atmospheric escape processes:
  • Jeans escape: thermal escape of atoms or molecules with velocities exceeding the planet's escape velocity
  • Hydrodynamic escape: rapid escape of atmospheric gases driven by intense solar heating, particularly important for hydrogen-rich atmospheres of early Earth and Mars
  • Nonthermal escape: escape of atmospheric species due to processes such as photochemical reactions, sputtering, and charge exchange
• Atmospheric escape can have significant implications for the long-term evolution and habitability of planets and moons, as it can lead to the loss of water and other volatile compounds

Photochemistry and Ion Chemistry in Atmospheres

Photochemistry and Reactive Species

• Photochemistry is driven by the absorption of solar radiation by atmospheric molecules, leading to photodissociation (breaking of chemical bonds) and photoionization (ejection of electrons)
• These processes generate reactive species, such as radicals and ions, which participate in further chemical reactions
• Examples of photochemically generated reactive species:
  • Hydroxyl radical (OH): formed by photodissociation of ozone followed by reaction with water vapor, OH + O3 → HO2 + O2
  • Chlorine radical (Cl): formed by photodissociation of chlorine-containing compounds, such as CFCs, CF2Cl2 + hν → CF2Cl + Cl

Ionospheres and Photoionization

• Photoionization of atmospheric constituents by extreme ultraviolet (EUV) and X-ray radiation leads to the formation of ionized layers, such as the ionosphere on Earth and ionospheres on other planets with substantial atmospheres
• The ionosphere is divided into regions based on the dominant ion species and the density of free electrons:
  • D region: 60-90 km, dominated by NO+ and O2+ ions
  • E region: 90-150 km, dominated by O2+ and NO+ ions
  • F region: 150-500 km, dominated by O+ ions
• Ionospheres play a crucial role in the propagation of radio waves and can influence the behavior of charged particles in a planet's magnetosphere

Ion Chemistry and Complex Organic Molecules

• Ion chemistry involves reactions between charged species (ions) and neutral molecules
• These reactions can be important in the formation of complex organic molecules in the upper atmospheres of planets and moons, such as Titan
• Examples of ion-neutral reactions in Titan's atmosphere:
  • N2+ + CH4 → CH3+ + N2 + H
  • CH3+ + CH4 → C2H5+ + H2
• Ion-neutral reactions can lead to the formation of nitriles, such as hydrogen cyanide (HCN) and cyanoacetylene (HC3N), which are precursors to more complex organic compounds

Dissociative Recombination

• Dissociative recombination is a key process in ion chemistry, where a molecular ion recombines with an electron, leading to the dissociation of the molecule into neutral fragments
• This process can be a significant source of neutral species in the upper atmospheres of planets and moons
• Example of dissociative recombination in Earth's ionosphere:
  • O2+ + e- → O + O
• Dissociative recombination can influence the composition and chemistry of planetary atmospheres by generating reactive neutral species and altering the balance between ions and neutrals

Habitability and Atmospheric Chemistry

Greenhouse Effect and Surface Temperature Regulation

• Atmospheric chemistry plays a crucial role in regulating the surface temperature of planets and moons through the greenhouse effect
• Greenhouse gases, such as CO2, CH4, and H2O, absorb infrared radiation emitted by the surface and re-emit it, warming the lower atmosphere
• The strength of the greenhouse effect depends on the abundance and distribution of these gases in the atmosphere
• Examples of greenhouse gas warming:
  • Earth: CO2 and H2O contribute to an average surface temperature of 15°C, compared to an expected -18°C without the greenhouse effect
  • Venus: a runaway greenhouse effect, driven by high CO2 levels, leads to surface temperatures of 460°C

Ozone Layer and Ultraviolet Radiation Shielding

• The presence of an ozone layer in a planet's stratosphere, as found on Earth, can shield the surface from harmful ultraviolet (UV) radiation
• Ozone (O3) absorbs strongly in the UV-B and UV-C wavelengths, protecting life on the surface from the damaging effects of these high-energy photons
• The ozone layer is maintained by the Chapman cycle, a photochemical process involving the formation and destruction of ozone by UV radiation and atomic oxygen
• Shielding from UV radiation is essential for the survival of life as we know it, as it can cause DNA damage, mutations, and other deleterious effects on biological systems

Liquid Water and Atmospheric Composition

• The presence of liquid water on a planet's surface is a key requirement for habitability, as all known life requires liquid water to survive and reproduce
• Atmospheric composition affects the potential for liquid water to exist by regulating surface temperature and pressure
• A planet's atmosphere must have sufficient pressure to allow for the existence of liquid water, typically around 6.1 millibars (the triple point of water)
• The temperature range at which liquid water can exist is also influenced by atmospheric composition, as greenhouse gases can warm the surface and expand the habitable zone around a star

Abiotic Synthesis of Organic Compounds

• Photochemical reactions in the atmosphere can produce organic compounds, such as amino acids and nucleotides, which are the building blocks of life
• This process, known as abiotic synthesis, may have contributed to the origin of life on Earth and could potentially occur on other planets or moons with suitable atmospheric conditions
• Examples of abiotic synthesis experiments:
  • Miller-Urey experiment: simulated early Earth's atmosphere (CH4, NH3, H2O, H2) and produced amino acids and other organic compounds when subjected to electric discharges
  • Hydrogen cyanide (HCN) polymerization: HCN, a common product of atmospheric photochemistry, can polymerize to form nucleotide bases and other complex organic molecules
• The presence of abiotically synthesized organic compounds in a planet's atmosphere or on its surface could potentially provide the raw materials necessary for the emergence of life

Long-Term Atmospheric Evolution and Habitability

• Atmospheric escape processes can lead to the loss of water and other volatiles over geological timescales, affecting the long-term habitability of a planet or moon
• The presence of a magnetic field and sufficient atmospheric mass can help mitigate atmospheric escape by deflecting high-energy particles and reducing the rate of thermal escape
• Examples of atmospheric escape and its effects:
  • Mars: loss of a thicker, warmer atmosphere due to hydrodynamic escape and sputtering, leading to the present-day cold, dry conditions
  • Titan: a thick atmosphere and low escape rates due to its distance from the Sun and the shielding effect of Saturn's magnetosphere, allowing for the persistence of methane and other organic compounds
• Understanding the long-term evolution of planetary atmospheres is crucial for assessing the potential for habitability and the development and survival of life on other worlds