What was in the early earths atmosphere?

Question

What was in the early earths atmosphere?

Answer 1

First Atmosphere

Composition - Probably H2, He

These gases are relatively rare on Earth compared to other places in the universe and were probably lost to space early in Earth's history because

Earth's gravity is not strong enough to hold lighter gases

Earth still did not have a differentiated core (solid inner/liquid outer core) which creates Earth's magnetic field (magnetosphere = Van Allen Belt) which deflects solar winds.

Once the core differentiated the heavier gases could be retained

Second Atmosphere

Produced by volcanic out gassing.

  

Gases produced were probably similar to those created by modern volcanoes (H2O, CO2, SO2, CO, S2, Cl2, N2, H2) and NH3 (ammonia) and CH4 (methane)

No free O2 at this time (not found in volcanic gases).

Ocean Formation - As the Earth cooled, H2O produced by out gassing could exist as liquid in the Early Archean, allowing oceans to form.

Evidence - pillow basalts, deep marine seds in greenstone belts.

Addition of O2 to the Atmosphere

Today, the atmosphere is ~21% free oxygen. How did oxygen reach these levels in the atmosphere? Revisit the oxygen cycle:

Answer 2

The early earth's atmosphere was reducing. It contained no free oxygen at all. It instead contained primarily carbon monoxide and hydrogen sulfide (the gas that smells like rotten eggs). It likely also contained nitrogen and some carbon dioxide. The reason that it wouldn't have contained alot of CO2 is because in reducing environments CO2 is usually reduced to CO because its more energetically favorable. However, this process is not a particularly fast one so it is likely that some CO2 would accumulate in the atmosphere.

Cheers,

Andrew