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WATCH: Earth and the Early Atmosphere

The Earth's atmosphere keeps changing, creating an endless cycle of warming, cooling, and recovery. Discover how Earth's early atmosphere formed and evolved, as violent collisions that created intense heat and pressure, the formation of oceans, and the creation of the moon. Explore the rise of oxygen, the role of cyanobacteria, and the changes that led to today's diverse life forms. Created by Big History Project.

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  • female robot grace style avatar for user Lena Lindström
    How do we know that the Earth was frozen over 3-4 times? Or that the oceans were created by water vapor condensing into liquid, and this happening a couple of times? Or that a huge body slammed into the Earth, releasing debris that formed our moon? I find all of this absolutely fascinating, but HOW DO WE KNOW? By analyzing rocks?
    (9 votes)
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  • blobby green style avatar for user evanthomas
    how did we know how many layers the earth has?
    (2 votes)
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    • duskpin sapling style avatar for user JaguarPaw10
      I think this is why: Scientists use seismic monitoring to understand the Earth's interior structure. They measure sound waves generated by earthquakes and examine how they slow down as they pass through the Earth's layers.
      The sound waves are reflected and refracted
      as they hit layers with different densities.
      By measuring these changes in velocity, scientists can determine the number of layers, their thickness, and their composition
      (1 vote)
  • piceratops seed style avatar for user SharmarkeSheikh
    how did the formation of the Moon impact the Earth?
    (2 votes)
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  • winston default style avatar for user axolotl
    how do we know that the earths first atmosphere was made of steam?
    (1 vote)
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  • blobby green style avatar for user dlately210740
    How do we know that the Earth was frozen over 3-4 times? Or that the oceans were created by water vapor condensing into liquid, and this happening a couple of times? Or that a huge body slammed into the Earth, releasing debris that formed our moon? I find all of this absolutely fascinating, but HOW DO WE KNOW? By analyzing rocks?
    (1 vote)
    Default Khan Academy avatar avatar for user
    • scuttlebug green style avatar for user just a ghost learning
      Oh boy this is gonna be long..

      Listen, the knowledge of Earth's geological history, including events you just asked, can be found below.

      #1 Snowball Earth Hypothesis
      The idea that the Earth experienced global glaciations (known as "Snowball Earth" events) is supported by:

      Geological Evidence:
      - Glacial Deposits: Certain types of rock formations, like diamictites and tillites, are formed by glacial action. These deposits, found at tropical latitudes, suggest that glaciers existed near the equator, indicating a globally frozen Earth.

      - Cap Carbonates:
      Layers of carbonate rock that form directly above glacial deposits suggest rapid melting and a sudden return to warmer conditions.

      - Paleomagnetic Data:
      - Rocks preserve the direction and intensity of Earth's magnetic field at the time they were formed. Analyzing these data helps determine the ancient positions of the continents, showing that glacial deposits were formed near the equator.

      - Geochemical Signatures:
      - Ratios of certain isotopes (like oxygen isotopes) in ancient rocks can indicate past temperatures. Variations in carbon and sulfur isotopes can also reflect changes in the global carbon and sulfur cycles associated with glaciations.


      #2 Formation of the Oceans
      The formation of Earth’s oceans is understood through:

      - Isotopic Analysis:
      - The ratios of hydrogen and its isotope deuterium in ancient minerals suggest the presence of water early in Earth's history. These isotopic signatures help trace the origins and history of Earth's water.

      - Geological Records:
      - Ancient rocks known as zircons can contain water inclusions and provide evidence for liquid water on Earth's surface as far back as 4.4 billion years ago.

      - Modeling:
      - Theories on how water vapor from volcanic outgassing could have condensed to form the oceans are supported by computer models of early Earth’s atmosphere and climate.


      #3 Giant Impact Hypothesis (Formation of the Moon)
      The hypothesis that the Moon formed from debris after a Mars-sized body collided with Earth is supported by:

      - Lunar Rocks:
      - Samples brought back from the Moon by the Apollo missions show isotopic similarities to Earth's mantle, suggesting a common origin.

      -Computer Simulations:
      - Models of the impact scenario can reproduce the current Earth-Moon system, including the Moon’s composition and orbit.

      Geochemical Evidence:
      - The isotopic composition of elements like oxygen, silicon, and titanium in both Earth and Moon rocks are nearly identical, supporting a shared history.

      #4 General Methodology
      - Radiometric Dating:
      - By measuring the decay of radioactive isotopes in rocks, scientists can determine the age of geological formations and the timing of events.

      Stratigraphy:
      - The study of rock layers (strata) allows scientists to reconstruct the sequence of geological events over time.

      Fossil Record:
      - Fossils provide evidence for the types of organisms that existed at different times and can indicate environmental conditions.

      Remote Sensing and Satellites:
      - Observations from space can provide large-scale data on Earth's geology and climate history.

      These methods, combined with an approach involving geology, chemistry, physics, and biology, allow scientists to piece together the complex history of our planet.
      (1 vote)
  • blobby green style avatar for user shayleeappenzeller
    how do we know that the earths first atmosphere was made of steam?
    (1 vote)
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  • duskpin sapling style avatar for user JaguarPaw10
    wait I thought planet Theia collided with Earth, right?
    (1 vote)
    Default Khan Academy avatar avatar for user

Video transcript

NARRATOR: When the Earth first formed, the solar system was a violent place. Giant hunks of rock, metal and ice slammed into the Earth's surface. As material collided and fused, there is intense heat and pressure. Matter vaporized on impact leaving puddles of magma. Many of the collisions released water vapor and other gases, which gradually formed a blanket of steam around the early Earth. This thickened over time becoming the first atmosphere. Some of the lighter gases like hydrogen leaked into space, but the denser steam collected and had a greenhouse effect insulating, heating and melting the surface of the planet. Over time, the Earth, in a process called differentiation, separated into layers and it's crust cooled. Steam in the atmosphere collapsed and formed the oceans, covering much of the planet in chemical-rich waters. The young Earth settled down, but then something about the size of Mars is thought to have slammed into the planet, causing immense change. The two bodies coalesced and material was blasted outward. Debris from the impact formed a ring of matter that orbited the Earth and eventually became the moon. The surface of the Earth became molten again from the intense heat, and the oceans reformed a steamed atmosphere. When things finally cooled down again, the Earth's crust hardened and steam settled back down to reform the oceans. The moon stabilized the Earth's tilt and helped to regulate climate. This part of Earth's history is uncertain because there is no surviving sedimentary rock to offer clues about the environment. There may have been several large asteroid or comet impacts, but none as big as the one that formed the moon. Molten rock or magma oozed in some places and blasted out in others. Volcanic activity released heavier gases like carbon dioxide and methane. There were still very little oxygen in the atmosphere. The oldest sedimentary rocks found in Greenland tell us a lot about the Earth at this time. There were oceans, lands, rivers, and beaches. Deep in the ocean, chemical-rich hydrothermal venting may have contributed to the first forms of life on Earth. These first microbial organisms are thought to have eventually spread throughout the Earth's oceans. Some microbes consumed hydrogen gas and others produced methane as a waste product. Biology began to affect the atmosphere. By about 3.5 billion years ago, stromatolites-- mounds made by microbes-- populated the world's beaches. Some early microbes used the sun's energy for photosynthesis, but the first photosynthesizers didn't release oxygen. However, by 2.8 billion years ago, life forms evolved that could use sunlight to split water molecules and release oxygen as a waste product. These were the cyanobacteria that still prosper in today's oceans. Most of the new oxygen combined with organic carbon to recreate carbon dioxide molecules, and some was used up another chemical reactions. But eventually, oxygen flooded the atmosphere and touched off a mass of ecological disaster from many of the anaerobic life forms that were poisoned by the abundance of oxygen. Other life forms adapted to thrive in the new conditions. The rock record offers proof. Oxidized iron compounds are reddish and rust colored. In certain layers of sedimentary rock, they demonstrate the predominance of oxygen after 2.4 billion years ago. At this time, oxygen formed the ozone layer about 20 to 30 kilometers above the ground, protecting life on the Earth's surface from the sun's harmful ultraviolet rays. The rise of oxygen was coupled with the reduction in greenhouse gases like methane and carbon dioxide, so the Earth retained less of the sun's heat, and the global climate became significantly colder. There was mass glaciation and the Earth was encased in ice, often called a snowball Earth. The icy shell reflected sunlight, making it colder and colder, but volcanoes punched through the ice and volcanic carbon dioxide gradually built up in the atmosphere. When the greenhouse effect became strong enough, the planet warmed and the ice melted. Scientists think that there were three snowball Earth cycles over a period of time from 2.4 to 2.2 billion years ago, and then a period of about one billion years in which the atmosphere and climate were fairly stable. More recently, the planet experienced other snowball Earth events, but some life forms were able to survive the cold. The current eon, the Phanerozoic, brought a proliferation of plant and animal life. Vascular plants with tissues for conducting water and nutrients colonized the land about 400 million years ago and their photosynthesis caused oxygen levels in the atmosphere to rise. By about 300 million years ago, extensive forest covered the Earth. They pushed the oxygen levels higher and enabled an even greater diversity of life. Biology, geology, astronomical events and periodic changes in the Earth's position in orbit influenced the climate, but overall, the atmosphere remained stable enough for life to persist.