It's not without it's reasoning. Without life to replenish it, free oxygen tends to react with other substances and get trapped as oxides such as carbon dioxide, water and oxide minerals. Although other methods of generating oxygen are known (such as the break down of water due to various forms of radiation), these are not currently considered to have been important sources of oxygen on early Earth.Speculation.
Although our understanding of the Earth's early atmosphere has changed over time (and continues to change), we are capable of studying the chemistry of the crust to get an idea that's better than mere speculation.
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