Origins of the Atmosphere (OCR Gateway GCSE Chemistry: Combined Science)

The Early Origins

• Theories on the development of Earth’s atmosphere have developed over time as instrumental analysis has improved
• The surface of the early Earth was molten for millions of years during which time there was no atmosphere surrounding the planet
• Eventually cooling began to take effect and allow for molten materials to slowly solidify forming land masses
• Volcanoes formed on the land masses and released gases from the Earth’s interior through violent eruptions
• Earth’s gravity prevented the gases from escaping into outer space and they formed the atmosphere
• Analysis of the minerals in the earth's crust enables scientists to deduce the gases present millions of years ago that helped form those minerals
• It is thought that the atmosphere at that moment in Earth’s history was similar to that of Venus or Mars today which consist mainly of CO₂
• During a period of intense volcanic activity, large amounts of carbon dioxide and water vapour were released, as well as nitrogen, hydrogen, ammonia, methane and other gases.
• The early atmosphere therefore contained mainly CO₂ and water vapour
• There was little or no oxygen present

Volcanoes spewed out water, carbon dioxide and other gases from the Earth’s interior

• While the surface of the Earth was still very hot the large quantities of water vapour remained in the gaseous state
• When conditions cooled sufficiently, the water vapour later condensed and fell to the surface of the Earth, forming the oceans
• Carbon dioxide is a water soluble gas (it is the gas used in fizzy drinks) and dissolves readily
• When the water vapour condensed large amounts of CO₂ dissolved in the oceans
• Carbonate substances were precipitated during this process which later formed sediments

The Development of an Oxygen Rich Atmosphere

• Primitive plants and algae began photosynthesizing which used up carbon dioxide from the atmosphere and released oxygen:

Carbon dioxide and water producing glucose and oxygen during photosynthesis

• Through these processes over a long period of time, the amount of O₂ in the atmosphere increased  and the amount of CO₂ decreased
• Algae first evolved around 2.7 billion years ago and during the next billion years or so small green plants began to appear
• As more and more plants began to appear the levels of oxygen began to increase which allowed for more complex life forms to evolve
• This trend continued until around 200 million years ago the composition of the atmosphere reached similar characteristics as today: around 20% oxygen, 80% nitrogen and tiny amounts of other gases
• Marine algae are throught to be responsible for about 90% of all the atmospheric oxygen produced