The Atmosphere: Evolution of the Atmosphere

A digital image showing a cross section of the Earth.
Credits: Discovery Channel

You may wonder why we have an atmosphere and where the gases came from. The Earth was formed 4.6 billion years ago through particles of dust being bound together by gravity. The heavy elements sank to the core as they experienced the greatest gravitational pull whilst lighter ones surrounded the core forming the mantle. The lightest elements of them all, such as hydrogen and helium, formed the primitive atmosphere. This was a very thin layer of gas which disappeared with time. One hypothesis states that because the early Earth was very hot, the atmospheric particles had a lot of kinetic energy. This kinetic energy may have been enough for the particles to overcome the gravitational forces bounding them to the Earth. The velocity required for the atmospheric particles to overcome the Earth's gravitational field is known as the escape velocity. Another hypothesis mentions that this primitive atmosphere was stripped by the solar wind which is a stream of charged particles released from the sun with high velocities. The solar wind may have transferred enough energy to the atmospheric particles for them to achieve escape velocity.Today, we are not affected by the solar wind because of the magnetic field generated by the churning of the molten iron core inside the Earth. The magnetic field deflects any charged particles away from Earth. 
Want to know how the Earth was formed?

Illustration of the magnetic field lines surrounding Earth.
Credits: National Geographic

The primitive atmosphere was soon replaced by the secondary atmosphere. This was formed from frequent volcanic activity which was the Earth's mechanism for removal of volatile gases dissolved in the magma (liquid rock beneath the Earth's crust). The gases released by the eruptions consisted mainly of water vapour as well as nitrogen, carbon dioxide, hydrogen, sulphur dioxide, chlorine, hydrogen sulphide, methane, ammonia and carbon monoxide. Although no oxygen was released from the volcanoes, it is thought that there were traces of oxygen in the early secondary atmosphere. The oxygen was a product of the photodissociation (also known as photolysis) of water molecules and carbon dioxide molecules. Photodissociation is a process by which UV rays from the sun break down a molecule into its components:

The OH (hydroxide) and O (oxygen) chemically bond to form H (hydrogen) and O₂ (dioxygen/oxygen gas). However, it was not until 3.5 billion years ago (3.5 x 10⁹ years ago) that this oxygen started to become increasingly abundant. This was when primitive micro-organisms that had adaptations to withstand the extreme environments on early Earth began to photosynthesise. Photosynthesis is the process by which autotrophic organisms produce their own food in order to survive. These early life forms hadn't yet evolved to survive the oxic (oxygen-containing) environment they had started to create. For this reason, they remained below the ground and retreated within sediments which are now known as stromatolites.

Basic photosynthetic reaction

Peter Sawyer's painting illustrates the hostile environments on the primitive Earth. In the foreground lay stromatolites containing the earliest life forms which were able to withstand the environment caused by volcanic eruptions and an anoxic (low oxygen) atmosphere.

The graph above shows how the amount of atmospheric oxygen changed with time. As we already know, the first photosynthesis release oxygen into the atmosphere 3.5 billion years ago, you may wonder why the graph does not show an increase in atmospheric oxygen. The reason there is no initial increase is because this oxygen was absorbed in the ocean and by seabed rock which consisted mainly of iron. This caused the iron to become oxidised to form an iron oxide known as magnetide.

Banded iron formation in seabed rock. The variation between each layer of magnetide sediment is caused by the fluctuations in the oxygen and iron levels in the ocean.
Credits: Stefan Lalonde

It was not until 1 billion years ago that atmospheric oxygen began to increase until it reached the current amount of 20.9%. As the atmospheric oxygen levels increased during this time, organisms began to evolve and begin to respire aerobically (using oxygen). Today, the Earth is dominated by oxygen-tolerant photosynthesisers on which the entire biosphere depends.