Free elemental O 2 only began to accumulate in the atmosphere about 2.5 billion years ago during the Great Oxygenation Event, about a billion years after the first appearance of these organisms. Oxygen is the most abundant element by mass in the Earth's crust as part of oxide compounds such as silicon dioxide, making up almost half of the crust's mass. At even higher low earth orbit altitudes, atomic oxygen is a significant presence and a cause of erosion for spacecraft.
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