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When the Earth was formed 4.6 billion years ago, Earth’s atmosphere
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was probably mostly hydrogen (H) and helium (He) plus hydrogen
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compounds, such as methane (CH4) and ammonia (NH3).
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Those gases eventually escaped to the space.
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The release of gases from rock through volcanic eruption (so-called
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outgassing) was the principal source of atmospheric gases.
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The primeval atmosphere produced by the outgassing was mostly
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carbon dioxide (CO2) with some Nitrogen (N2) and water vapor (H2O),
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and trace amounts of other gases.
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