Why is the composition of the Earth's early atmosphere not well understood?

The composition of the Earth's early atmosphere is not well understood primarily due to the long geological time scale of approximately 4.6 billion years. Over such an extended period, numerous geological and atmospheric processes have occurred that have altered the original atmosphere substantially. This extensive period makes it challenging for scientists to gather direct evidence and accurately reconstruct the specific composition of the atmosphere at that time.

Furthermore, the limited evidence available comes from rock formations, ice cores, and meteorite studies, which can only provide indirect information about the conditions and composition of the early atmosphere. As a result, the lengthy timeline and the gradual changes that have taken place complicate our understanding and lead to uncertainties about the exact make-up of the early atmosphere when the Earth first formed.