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Planetary atmospheres vary widely in composition (Venus: CO₂-dominated, Earth: N₂-O₂, Jupiter: H₂-He) and vertical structure (troposphere, stratosphere, thermosphere). Composition reflects primary outgassing during formation, secondary outgassing from volcanism, and long-term atmospheric escape and chemical processes.
A planet's atmosphere is not a static envelope—it is the cumulative product of formation history, interior activity, and billions of years of chemical and physical processing. From your study of planetary formation, you know that the initial atmospheric composition depends on when and where a planet accreted. Gas giants like Jupiter captured enormous hydrogen-helium envelopes directly from the solar nebula during the first few million years, preserving roughly solar composition. Rocky planets like Earth and Venus were too small and too warm to retain these light gases gravitationally, so their primary atmospheres were largely lost. What we see today on terrestrial worlds is a secondary atmosphere, built up later through volcanic outgassing of heavier molecules—CO₂, N₂, H₂O, and SO₂—from the planet's interior.
The vertical structure of an atmosphere follows from thermodynamics and hydrostatic balance, concepts you have encountered as prerequisites. Atmospheric pressure decreases exponentially with altitude because each layer must support the weight of all the gas above it. Temperature, however, does not decrease monotonically. In the troposphere, convective mixing drives temperature down with altitude at the adiabatic lapse rate. Above this, the stratosphere can be isothermal or even show a temperature inversion—on Earth, ozone absorbs ultraviolet radiation and heats the stratosphere from above. Higher still, the thermosphere is heated by absorption of extreme ultraviolet radiation, reaching temperatures of over 1,000 K on Earth despite being nearly a vacuum. Each planet's specific layering depends on which absorbing species are present and how solar energy is deposited at different altitudes.
Why do Venus, Earth, and Mars have such different atmospheres despite starting from similar materials? The answer lies in divergent evolutionary pathways. Venus, closer to the Sun, could not sustain liquid water; without oceans to dissolve CO₂ and sequester it as carbonate rock, carbon dioxide accumulated to produce a massive 90-atmosphere greenhouse. Earth's oceans and biological activity drew down CO₂ while photosynthesis injected O₂—a composition unique in the solar system and diagnostic of life. Mars, being smaller, lost its internal heat early, shutting down the volcanic outgassing that replenishes atmospheric gases, while its weak gravity allowed atmospheric escape to strip away much of what remained. These comparisons illustrate that atmospheric composition encodes a planet's geological, chemical, and potentially biological history.
Understanding atmospheric structure also requires recognizing the role of chemical equilibrium and disequilibrium. In a chemically inert atmosphere, composition would settle to thermodynamic equilibrium. But active processes—photochemistry driven by stellar radiation, volcanic injection of reduced gases, and biological metabolism—continuously push atmospheres away from equilibrium. Detecting chemical disequilibrium in an exoplanet's spectrum (such as the simultaneous presence of O₂ and CH₄, which should rapidly react to form CO₂ and H₂O) is one of the leading proposed biosignatures for identifying life beyond Earth.