Cartoon illustrating the effect of the runaway greenhouse transition on atmospheric composition for two rocky planets with identical He, H₂O, and CO₂ inventories. With a thin He atmosphere, planets below the runaway greenhouse limit (right) will have He-rich upper atmospheres and may support surface liquid water. For planets inside the inner edge of the habitable zone (left), H₂O from liquid water oceans and CO₂ from carbonates are transported into the atmosphere, dramatically decreasing the scale height and making the transit spectrum of the planet flatter.