Atmospheres

Composition, Pressure & the Greenhouse Effect

Atmospheric Comparison

Atmospheric thickness shown is exaggerated 3–5× relative to body radius for visibility — Earth's real atmosphere is paper-thin against the planet. Cloud motion reflects relative super-rotation rates.

Anatomy of an Atmosphere

Pressure falls off exponentially with altitude. The scale height H = kT/(mg) is the altitude at which pressure drops by a factor of e — determined by temperature, gas mass, and gravity.

Layers from surface outward: troposphere (weather), stratosphere (ozone, jet streams), mesosphere, thermosphere (auroras), and the diffuse exosphere bleeding into space.

Whether a planet keeps its atmosphere depends on escape velocity versus molecular thermal speeds — light gases like H₂ leak from any terrestrial body, heavy gases like CO₂ stay put.

Greenhouse & Climate

Mars — Thin CO₂ — weak greenhouse, freezes despite sunlight

Earth — Trace CO₂/H₂O — +33 °C of warming over no-atmosphere baseline

Titan — CH₄ greenhouse + N₂ pressure-broadening

Venus — Runaway: 96% CO₂ → +500 °C of greenhouse forcing

The same physics drives why ELWs are rare — small composition shifts run away in either direction.

ED: Atmospheric Landings

Odyssey introduced landings on bodies with thin atmospheres only — typically pressure under ~0.1 atm. Anything thicker remains no-land.

The system map flags atmospheric type on every body. Common atmospheric compositions in ED: CO₂, SO₂, water, methane, ammonia, helium, and neon. Atmosphere composition is a scan-bonus parameter.

Look for N₂/O₂ on a non-ELW: it usually flags a terraformable body with substantial cartographic value.

Atmosphere Reference — Sol System

Body	Surface P	Composition	Scale H	Surface T	Note
Mercury	10⁻¹⁵ atm	Trace He, Na, K (exosphere only)	—	−180 / +430 °C	Effectively no atmosphere
Mars	0.006 atm	95% CO₂, 3% N₂, 1.6% Ar	11 km	−63 °C avg	Thin, dust storms, no greenhouse
Earth	1.0 atm	78% N₂, 21% O₂, 1% Ar/H₂O	8.5 km	+15 °C avg	Liquid water, biosphere, magnetic field
Titan	1.45 atm	95% N₂, 5% CH₄, ethane haze	50 km	−179 °C	Thickest moon atmosphere; methane cycle
Venus	92 atm	96% CO₂, 3.5% N₂, H₂SO₄ clouds	16 km	+462 °C	Runaway greenhouse, super-rotation 60×
Jupiter (1 bar)	no surface	~90% H₂, 10% He, NH₃ clouds	27 km	−108 °C @ 1 bar	Atmosphere extends to metallic H mantle
Neptune (1 bar)	no surface	~80% H₂, 19% He, 1.5% CH₄	20 km	−201 °C @ 1 bar	Methane absorption gives blue colour