Where did the first atmosphere and oceans come from?
At first, Earth did not have an atmosphere or free water since the planet was too hot for gases and water to collect. The atmosphere and oceans that we see today evolved over time. The gases came from within the planet and from far out in the solar system.
Earth’s First Atmosphere
Earth’s first atmosphere was made of hydrogen and helium, the gases that were common in this region of the solar system as it was forming. Most of these gases were drawn into the center of the solar nebula to form the Sun. When Earth was new and very small, the solar wind blew off atmospheric gases that collected. If gases did collect, they were vaporized by impacts, especially from the impact that brought about the formation of the Moon.
Eventually things started to settle down and gases began to collect. High heat in Earth’s early days meant that there were constant volcanic eruptions, which released gases from the mantle into the atmosphere (see opening image). Just as today, volcanic outgassing was a source of water vapor, carbon dioxide, small amounts of nitrogen, and other gases.
Scientists have calculated that the amount of gas that collected to form the early atmosphere could not have come entirely from volcanic eruptions. Frequent impacts by asteroids and comets brought in gases and ices, including water, carbon dioxide, methane, ammonia, nitrogen, and other volatiles from elsewhere in the solar system (Figure below).
The gases that create a comet’s tail can become part of the atmosphere of a planet.
Calculations also show that asteroids and comets cannot be responsible for all of the gases of the early atmosphere, so both impacts and outgassing were needed.
Earth’s Second Atmosphere
The second atmosphere, which was the first to stay with the planet, formed from volcanic outgassing and comet ices. This atmosphere had lots of water vapor, carbon dioxide, nitrogen, and methane but almost no oxygen. Why was there so little oxygen? Plants produce oxygen when they photosynthesize but life had not yet begun or had not yet developed photosynthesis. In the early atmosphere, oxygen only appeared when sunlight split water molecules into hydrogen and oxygen and the oxygen accumulated in the atmosphere.
Without oxygen, life was restricted to tiny simple organisms. Why is oxygen essential for most life on Earth?
1. Oxygen is needed to make ozone, a molecule made of three oxygen ions, O3. Ozone collects in the atmospheric ozone layer and blocks harmful ultraviolet radiation from the Sun. Without an ozone layer, life in the early Earth was almost impossible.
2. Animals need oxygen to breathe. No animals would have been able to breathe in Earth’s early atmosphere.
The early atmosphere was rich in water vapor from volcanic eruptions and comets. When Earth was cool enough, water vapor condensed and rain began to fall. The water cycle began. Over millions of years enough precipitation collected that the first oceans could have formed as early as 4.2 to 4.4 billion years ago. Dissolved minerals carried by stream runoff made the early oceans salty. What geological evidence could there be for the presence of an early ocean? Marine sedimentary rocks can be dated back about 4 billion years.
By the Archean, the planet was covered with oceans and the atmosphere was full of water vapor, carbon dioxide, nitrogen, and smaller amounts of other gases.
Earth’s Third Atmosphere
When photosynthesis evolved and spread around the planet, oxygen was released in abundance. The addition of oxygen is what created Earth’s third atmosphere. This event, which occurred about 2.5 billion years ago, is sometimes called the oxygen catastrophe because so many organisms died. Although entire species died out and went extinct, this event is also called the Great Oxygenation Event because it was a great opportunity. The organisms that survived developed a use for oxygen through cellular respiration, the process by which cells can obtain energy from organic molecules. This opened up many opportunities for organisms to evolve to fill different niches and many new types of organisms first appeared on Earth.
What evidence do scientists have that large quantities of oxygen entered the atmosphere? The iron contained in the rocks combined with the oxygen to form reddish iron oxides. By the beginning of the Proterozoic, banded-iron formations (BIFs) were forming. Banded-iron formations display alternating bands of iron oxide and iron-poor chert that probably represent a seasonal cycle of an aerobic and an anaerobic environment.
The oldest BIFs are 3.7 billion years old, but they are very common during the Great Oxygenation Event 2.4 billion years ago (Figure below). By 1.8 billion years ago, the amount of BIF declined. In recent times, the iron in these formations has been mined, and that explains the location of the auto industry in the upper Midwest.
With more oxygen in the atmosphere, ultraviolet radiation could create ozone. With the formation of an ozone layer to protect the surface of the Earth from UV radiation, more complex life forms could evolve.
- Earth's first atmosphere came from outgassing from the planet's interior and from asteroids and comets from elsewhere in the solar system.
- Earth's first and second atmosphere did not contain oxygen so there was no ozone layer to protect life from ultraviolet radiation and no oxygen for animals to breathe.
- Earth's third atmosphere contained oxygen that is a by-product of photosynthesis, allowing the evolution of animals and the formation of an ozone layer.
- What were the first gases to collect in Earth's atmosphere? Where did they come from and where did they go?
- What was the source of gases in Earth's first atmosphere that collected? What were those gases?
- When did oxygen enter the atmosphere in abundance? Where did it come from? What was the effect on life on Earth?
- What are banded-iron formations and why are they important to Earth historians?