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16.4: Early Atmosphere and Oceans

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    What did Earth look like at first?

    The earliest Earth did not have an atmosphere or free water. The planet was far too hot for gases and water to collect. The atmosphere and oceans that we see today evolved over time. The gases came from volcanic eruptions and from comets.

    Formation of the Atmosphere and Oceans

    An atmosphere is the gases that surround a planet. The early Earth had no atmosphere. Conditions were so hot that gases were not stable.

    Earth's First Atmosphere

    Earth’s first atmosphere was different from the current one. The gases came from two sources. Volcanoes spewed gases into the air. Comets carried in ices from outer space. These ices warmed and became gases. Nitrogen, carbon dioxide, hydrogen, and water vapor were in the first atmosphere (Figure below). Take a look at the list of gases. What's missing? The early atmosphere had almost no oxygen.

    Gases from Earth’s interior came through volcanoes and into the atmosphere

    Gases from Earth’s interior came through volcanoes and into the atmosphere.

    The Early Oceans

    Earth’s atmosphere slowly cooled. Once it was cooler, water vapor could condense. It changed back to its liquid form. Liquid water fell to Earth’s surface as rain. Over millions of years water collected to form the oceans. Water began to cycle on Earth as water evaporated from the oceans and returned again as rainfall.

    Streams carry dissolved minerals. These ions make the oceans salty. Four billion-year-old marine sedimentary rocks have been found. The first oceans could have formed as early as 4.2 to 4.4 billion years ago!

    The Later Atmosphere

    Eventually life evolved. Single-celled organisms were the first life forms to appear, and some of them produced oxygen as a byproduct of photosynthesis. Oxygen spread around the planet about 2.5 billion years ago (though photosynthesis evolved around this time, plants wouldn't appear until much later). Many organisms died off because they could not handle the oxygen. But this development was extremely important for other life. Animals need oxygen to breathe. If photosynthesis had not evolved, there would be no animals.

    Oxygen is needed to make ozone. Ozone is a molecule made of three oxygen ions. Ozone in the upper atmosphere blocks harmful solar radiation from reaching the surface. Without oxygen, life on Earth would have been very simple.


    • Earth's first atmosphere came from the planet's interior. Gases also came from asteroids and comets from elsewhere in the solar system.
    • There was very little oxygen in the atmosphere until plants evolved. Oxygen allowed animals to evolve and ozone to form.
    • The first oceans formed when water rained out of the atmosphere into basins.


    1. What were the sources of gases in Earth's early atmosphere?
    2. Where did oxygen come from? Why is oxygen so important for life on Earth?
    3. How did the first oceans form?

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