Which process is considered an important source of Earth's early atmosphere?

• A. Outgassing by volcanoes
• B. Condensation
• C. Meteoritic delivery
• D. Photosynthesis

Answer: (A)

The main concept here is how Earth's first atmosphere formed from internal processes rather than from surface condensation or later biological activity. In the planet’s early history, intense heat from accretion and differentiation caused magmatic activity. Gases dissolved in molten rock—water vapor, carbon dioxide, nitrogen, and other volatiles—were released to the surface through volcanic outgassing. This built up a thick atmospheric layer long before life appeared. As Earth cooled, much of that water vapor condensed to form oceans, leaving an atmosphere initially dominated by greenhouse gases like CO2 and N2. This makes outgassing by volcanoes the primary source of the early atmosphere.

Condensation itself doesn’t create an atmosphere; it removes water from the air by forming liquid oceans. Meteoritic delivery did contribute some volatiles and water, but it wasn’t the main source of the early atmospheric gases. Photosynthesis later altered the atmosphere’s composition by producing oxygen, but it did not generate Earth’s initial atmospheric inventory.