radiative equilibrium
Definition: This section is included to make clear that the single-point equilibrium-temperature formula derived earlier in this lecture, while genuinely useful and correct as a global energy-balance constraint, is a simplification: it gives the correct emitting temperature averaged over the whole planet, but a full radiative-convective model is required to say anything about how that temperature is distributed with altitude or between day and night hemispheres, an important caveat when this formula is later applied to TRAPPIST-1 planets in Lecture 14.
Why it matters here: A planet's zero-atmosphere equilibrium temperature follows directly from global radiative energy balance: absorbed stellar flux equals emitted thermal flux.
Relationship: Radiative-Convective Equilibrium Models
Boundary or common confusion: This is the same radiative energy-balance logic used throughout astrophysics (a star's photosphere emits, in steady state, exactly the luminosity generated in its interior); here it is applied to a planet's balance between absorbed stellar flux and its own thermal emission, and it makes no reference at all to what is happening inside the planet -- it depends only on the planet's size, albedo, and distance from its star.
Bond albedo
Definition: A planet of radius R, at distance a from a star of luminosity L, intercepts starlight over its cross-sectional area πR² and absorbs a fraction (1-A) of it, where A is the Bond albedo.
Why it matters here: A planet's zero-atmosphere equilibrium temperature follows directly from global radiative energy balance: absorbed stellar flux equals emitted thermal flux.
Relationship: Bond albedo is measured from spacecraft observations at multiple phase angles (only achievable by a spacecraft that can see a planet from angles Earth-based telescopes cannot) or, historically, inferred from disk-integrated photometry combined with phase-function models.
Boundary or common confusion: This is the same radiative energy-balance logic used throughout astrophysics (a star's photosphere emits, in steady state, exactly the luminosity generated in its interior); here it is applied to a planet's balance between absorbed stellar flux and its own thermal emission, and it makes no reference at all to what is happening inside the planet -- it depends only on the planet's size, albedo, and distance from its star.
equilibrium temperature
Definition: Venus: computed zero-albedo, zero-greenhouse equilibrium temperature is far below its observed 737 K surface temperature -- the largest greenhouse excess of any solar system body, driven by its 92-bar, 96.5% CO₂ atmosphere.
Why it matters here: A planet's zero-atmosphere equilibrium temperature follows directly from global radiative energy balance: absorbed stellar flux equals emitted thermal flux.
Relationship: This section is included to make clear that the single-point equilibrium-temperature formula derived earlier in this lecture, while genuinely useful and correct as a global energy-balance constraint, is a simplification: it gives the correct emitting temperature averaged over the whole planet, but a full radiative-convective model is required to say anything about how that temperature is distributed with altitude or between day and night hemispheres, an important caveat when this formula is later applied to TRAPPIST-1 planets in Lecture 14.
Boundary or common confusion: This is the same radiative energy-balance logic used throughout astrophysics (a star's photosphere emits, in steady state, exactly the luminosity generated in its interior); here it is applied to a planet's balance between absorbed stellar flux and its own thermal emission, and it makes no reference at all to what is happening inside the planet -- it depends only on the planet's size, albedo, and distance from its star.
greenhouse effect
Definition: This pattern -- good agreement for airless or thin-atmosphere bodies, systematic excess for thick-atmosphere bodies -- is itself the primary quantitative evidence for the greenhouse effect: it is not merely that Venus is "hot," but that Venus is hundreds of kelvin hotter than energy balance alone (with its measured, quite high albedo of 0.76) can explain, and that specific, quantified gap is what the greenhouse effect must explain.
Why it matters here: A planet's zero-atmosphere equilibrium temperature follows directly from global radiative energy balance: absorbed stellar flux equals emitted thermal flux.
Relationship: Greenhouse warming, ΔT_gh = T_surface - T_eq, is therefore a genuine, quantifiable radiative-transfer effect, not merely a qualitative "blanket" analogy.
Boundary or common confusion: This is the same radiative energy-balance logic used throughout astrophysics (a star's photosphere emits, in steady state, exactly the luminosity generated in its interior); here it is applied to a planet's balance between absorbed stellar flux and its own thermal emission, and it makes no reference at all to what is happening inside the planet -- it depends only on the planet's size, albedo, and distance from its star.
runaway greenhouse
Definition: Venus's current bone-dry surface and its atmosphere's deuterium-to-hydrogen ratio (roughly 100-150 times Earth's, indicating strong preferential loss of light hydrogen from photodissociated water) are considered strong indirect evidence that Venus underwent a runaway greenhouse early in its history.
Why it matters here: A planet's zero-atmosphere equilibrium temperature follows directly from global radiative energy balance: absorbed stellar flux equals emitted thermal flux.
Relationship: Whether Venus's runaway greenhouse was triggered primarily by its closer distance to the Sun, by a brighter early Sun's evolution, or by some combination is an active research question revisited in Lecture 09's climate-evolution treatment.
Boundary or common confusion: This is the same radiative energy-balance logic used throughout astrophysics (a star's photosphere emits, in steady state, exactly the luminosity generated in its interior); here it is applied to a planet's balance between absorbed stellar flux and its own thermal emission, and it makes no reference at all to what is happening inside the planet -- it depends only on the planet's size, albedo, and distance from its star.
radiative-convective model
Definition: This section is included to make clear that the single-point equilibrium-temperature formula derived earlier in this lecture, while genuinely useful and correct as a global energy-balance constraint, is a simplification: it gives the correct emitting temperature averaged over the whole planet, but a full radiative-convective model is required to say anything about how that temperature is distributed with altitude or between day and night hemispheres, an important caveat when this formula is later applied to TRAPPIST-1 planets in Lecture 14.
Why it matters here: A planet's zero-atmosphere equilibrium temperature follows directly from global radiative energy balance: absorbed stellar flux equals emitted thermal flux.
Relationship: Radiative-Convective Equilibrium Models
Boundary or common confusion: This is the same radiative energy-balance logic used throughout astrophysics (a star's photosphere emits, in steady state, exactly the luminosity generated in its interior); here it is applied to a planet's balance between absorbed stellar flux and its own thermal emission, and it makes no reference at all to what is happening inside the planet -- it depends only on the planet's size, albedo, and distance from its star.