Why do we not know the composition of the Earth's early atmosphere?

The correct answer highlights the challenges associated with understanding the Earth's early atmosphere due to the immense time scale of approximately 4.6 billion years since the Earth formed. This vast duration means that direct evidence from that period is scarce and often lost. Geological and atmospheric records from such an early time are not well-preserved, making it difficult for scientists to study the specifics of the Earth's atmosphere when it was newly formed.

Understanding the Earth's early atmosphere relies on indirect evidence, such as the study of ancient rocks, minerals, and meteorite compositions, which can provide clues, but these are often fragmented and not perfectly representative of the atmosphere itself. The limited availability of data from such a distant past means that reconstructions are often based on scientific models and assumptions, rather than direct measurement or observation.

Other factors, such as rapid changes in atmospheric conditions, the absence of historical records, and the challenges of measuring atmospheric composition so far back in time, do play roles in our understanding, yet they do not encompass the primary reason for our limited knowledge as effectively as the significance of the extensive time scale.