What causes us to gain our atmosphere?

Unlike the Jovian planets, the terrestrials were too small to capture significant gas from the solar nebula.

• what gas they did capture, was H & He, and it escaped
• present-day atmospheres must have formed at a later time

Sources of atmospheric gas:

• outgassing - release of gas trapped in interior rock by volcanism
• evaporation/sublimation - surface liquids or ices turn to gas when heated
• bombardment - micrometeorites, solar wind particles, or high-energy photons blast atoms/molecules out of surface rock - occurs only if the planet has no substantial atmosphere already