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.

The Atmosphere: An Introduction

The atmosphere is the gaseous blanket surrounding the entire planet. The word atmosphere is derived from the Greek word Atmo meaning air. 

Pie chart showing the chemical composition of the atmosphere.
Credits: Patti Isaacs Management and Production

As the pie chart above illustrates, oxygen is the second most abundant gas in our atmosphere. Oxygen is vital for life on planet Earth; all living organisms require this gas for aerobic respiration in order produce the energy required for metabolic reactions. 0.17% of the atmosphere consists of other gases including neon, helium, methane, krypton, hydrogen, nitrous oxide, xenon, ozone, iodine, carbon monoxide and ammonia. Water vapour is also present at low altitudes in the troposphere. The stratosphere, which is the layer above the troposphere, contains a layer of ozone gas. The ozone layer is responsible for filtering high energy ultraviolet (UV) rays which have enough energy to damage the DNA inside cells. This is why exposure to high levels of UV radiation can cause skin cancer. 

Aerobic respiration

The atmosphere is also a shield against meteoroids which are the solid debris any comets or asteroids leave in their path whilst travelling through space. Meteoroids travel at high speeds of up to 70 kilometres per second. Upon entering the Earth's upper atmosphere, the mesosphere, a meteoroid is forced to slow down as the molecules of atmospheric gases provide the drag force friction. The friction has a heating effect and causes the meteoroid to ignite; it is now formally known as a meteor.

This time-lapse photograph shows a meteor shower. A meteor shower occurs when the Earth passes through a belt of debris from a passing comet or asteroid. As a result, many meteors (also known as shooting stars) can be seen in the sky. 
Credits: Kenneth Brandon

Our atmosphere is responsible for the weather we experience on Earth. Different areas of the globe experience different weather patterns which are essentially driven by energy from the sun. Weather is caused by movement of air masses; warm air is carried from the equator to the poles. Areas of mid-high latitudes such as the United Kingdom can experience both cool air from the Arctic and warm air from the equator. Also, it is due to the wind plants were able to colonise land and evolve; wind is responsible for the transportation of seeds and minerals across great distances. Today, wind carries sand and minerals for fertilisation from the Sahara to the Amazon rainforest across the Pacific Ocean.

Air masses moving towards the British Isles. 
Credits: Met Office

What is Earth Science?

Image via NASA

Earth Science is a multidisciplinary field of study; it applies Chemistry, Physics, Biology and even Maths to planet Earth. It involves looking at rocks (as boring as that may sound to some, they're a lot more interesting than they look!), volcanoes, earthquakes, oceans, ecosystems, atmospheric processes, glaciers, soils.. and the list goes on. It is quite fascinating to see how our planets systems work. 
Due to the currently changing climate, there has never been a more important time to learn more about planet Earth. The more we know and learn about our own planet, the easier it will be for us to understand other planets (so yes planetary astronomers, this blog may be useful for you too!). The blog posts will focus on a certain topic, with several posts for each specific topic. For example, if looking at rocks, one post may focus on dating rocks whilst another will be based on plate tectonics.
I hope you find this useful whether you require the information for your homework project or simply have an interest in the world around you.
Enjoy!