4.1. COMPOSITION

HISTORICAL COMPOSITION

Composition of the atmosphere has changed throughout the time:

• 1) H and He (when the Earth began to form)
• 2) Volcanic gases (primitive atmosphere)
• 3) About 3.5 billion years ago (3,500,000,000 years), some bacteria began to produce oxygen through photosynthesis.

CURRENT COMPOSITION

The atmosphere is made up of a mixture of gases called air:

• 78% nitrogen (N₂)
• 21% oxygen (O₂)

YouTube: Frank Gregorio “Introduction to Photosynthesis”

• 1% trace gases: argon, carbon dioxide (CO₂), others (methane or ozone (O₃)).