Why is it hard to know the exact evolution ofe eearths atmosphere?
Because it critically depends on when life first appeared on Earth. We know what happened, but not how fast. The first atmosphere was hydrogen and helium, as those gases are most abundant in the Solar system. The Earth is too small to retain those gases, but how fast we lost them depends on how much there was in the first place. The next atmosphere was volcanic out gassing. It contained gases like H2O, CO2, SO2, CO, S2, Cl2, N2, H2 and NH3 and CH4, that we can still find in volcans today. A lot more water came from outer space brought by comets bombarding the proto-Earth. Again, what we don’t know how fast these processes occurred. Oxygen started to buildup as soon as the Archean era, more than 2.5 billion years ago, by means of photochemical dissociation. That is breakup of water and ammonia molecules by ultraviolet. The O2 level rose to 1-2% of the current level. At these levels O3 (Ozone) can form to shield the Earth surface from ultraviolet light. Once the Earth was properly shield
