Venus Greenhouse Effect

Venus vs Earth

Greenhouse mechanism

1. Sun's visible/UV light passes through atmosphere, reaches surface (75% reflected by clouds; 25% absorbed).
2. Surface warms; emits IR radiation.
3. CO₂ strongly absorbs IR (especially around 15 µm).
4. Re-emission goes in all directions; some back to surface.
5. Net heating; surface T much higher than equilibrium without atmosphere.

For Venus: greenhouse warming = ~470°C above no-atmosphere equilibrium. (Earth: ~33°C above equilibrium.)

JavaScript — Venus greenhouse calculations

// Equilibrium temperature without greenhouse
function equilibriumTemp(albedo, distance_AU, sun_temp = 5778) {
  const solar_flux_earth = 1361;  // W/m²
  const flux = solar_flux_earth / (distance_AU * distance_AU);
  const sigma = 5.67e-8;
  return Math.pow(flux * (1 - albedo) / (4 * sigma), 0.25);
}

// Venus equilibrium (no greenhouse)
console.log(`Venus equilibrium: ${equilibriumTemp(0.77, 0.723).toFixed(0)} K = ${(equilibriumTemp(0.77, 0.723) - 273).toFixed(0)}°C`);

// Venus actual surface T
console.log(`Venus actual: 735 K = 462°C`);
console.log(`Greenhouse warming: ~${(735 - equilibriumTemp(0.77, 0.723)).toFixed(0)} K`);

// Compare Earth
console.log(`Earth equilibrium: ${equilibriumTemp(0.30, 1.0).toFixed(0)} K`);  // 254 K = -19°C
console.log(`Earth actual: ~288 K (15°C)`);
console.log(`Earth greenhouse: ~33 K`);

// Pressure scale height (atmosphere)
function pressureAtHeight(P_surface, height_m, T_K, M_kgPerMol, g_mPerS2) {
  const R = 8.314;
  const H = R * T_K / (M_kgPerMol * g_mPerS2);
  return P_surface * Math.exp(-height_m / H);
}

// Venus: M_CO2 = 0.044 kg/mol, T = 735 K, g = 8.87 m/s²
console.log(`Venus 50 km up: ${pressureAtHeight(92, 50000, 735, 0.044, 8.87).toFixed(1)} atm`);

// Heat conduction in CO₂ atmosphere
function thermalConductivity(T_K) {
  // Approximate for CO₂
  return 0.024 + (T_K - 300) * 5e-5;
}

// Solar wind / surface insolation
const I_venus = 2615;  // W/m²
const albedo = 0.77;
const absorbed = I_venus * (1 - albedo);
console.log(`Venus absorbed: ${absorbed.toFixed(0)} W/m²`);
// 600 W/m² absorbed — vs 956 W/m² for Earth (different albedos)

Why Venus matters

• Climate science. Most extreme greenhouse effect — informs Earth climate models and feedback mechanisms.
• Comparative planetology. Earth twin diverged dramatically — what factors lead to habitability vs uninhabitability.
• Atmospheric science. Super-rotation, dynamics of dense CO₂ atmosphere.
• Astrobiology. Past habitability? Atmospheric microbes? Recent phosphine claims (debated).
• Future missions. NASA DAVINCI, VERITAS, Russia Venera-D upcoming.
• Surface geology. Volcanism, tectonics very different from Earth's.
• Solar evolution. Venus tells us about Sun's increasing brightness over billions of years.

Common misconceptions

• Venus is hot because close to Sun. Mercury is closer and isn't as hot at night. Venus's heat is from greenhouse trapping, not direct solar radiation alone.
• Sulfuric acid rain hits the surface. Drops sublimate before reaching surface (too hot). No surface acid rain.
• Venus is just like a hot Earth. Differs in many ways — retrograde rotation, no plate tectonics, no global magnetic field, no moons.
• Earth could become Venus. Different scenarios. Earth could undergo runaway greenhouse, but only with ~100× more CO₂ (currently nowhere close).
• Cloud cover means dim surface. Surface receives ~17 W/m² — about as bright as overcast Earth day. Despite the clouds, sufficient light reaches surface.
• Venus had similar history to Earth. Probably had habitable phase, but timeline diverged due to runaway greenhouse and lack of plate tectonics.