A2.1.1 Conditions on early Earth and the pre-biotic formation of carbon compounds (HL only) Notes

The Pre-Biotic Atmosphere

1. The atmosphere of early Earth bore little resemblance to the oxygen-rich air we breathe today.
2. Geological evidence suggests that free oxygen was nearly absent.
3. Instead, the atmosphere likely consisted of gases such as methane (CH₄), carbon dioxide (CO₂), ammonia (NH₃), and water vapor (H₂O), released through intense volcanic activity and constant meteorite bombardment.

Key Atmospheric Factors

1. Lack of Free Oxygen (O₂):
   1. Without free oxygen, organic molecules were not degraded by oxidation.
   2. This allowed their accumulation and further chemical reactions.
2. No Ozone Layer:
   1. The absence of an ozone layer (O₃) exposed the surface to intense UV radiation.
   2. This radiation provided energy for the formation of organic molecules.
3. High Carbon Dioxide (CO₂) Levels:
   1. The atmosphere contained higher concentrations of CO₂, methane, and ammonia, creating a chemically reactive environment.

Spontaneous Formation of Carbon Compounds

1. The combination of these conditions enabled chemical processes that no longer occur today.
2. Under these conditions, simple carbon-based molecules like aldehydes, carboxylic acids, and formaldehyde formed spontaneously.
3. UV radiation provided the activation energy needed for chemical reactions that would not normally occur today.

Why These Processes No Longer Occur

1. Presence of Oxygen: Modern Earth’s atmosphere contains about 21% oxygen, which reacts with and breaks down organic molecules.
2. Ozone Layer: The ozone layer now absorbs most of the Sun’s UV radiation, preventing it from reaching the surface.