One topic looked at in GCSE Science is the changes which have and do occur on the Earth and in its atmosphere. This is the second of three quizzes on the subject and it focusses specifically on the Earth's atmosphere and the gases found there.
Something that you probably take for granted is the atmosphere around us and the gases that we breathe. But the atmosphere doesn't only supply the oxygen that our bodies need for respiration, it also protects us from space. High level ozone protects us from the effects of the most harmful ultra-violet rays from the Sun and carbon dioxide ensures that the Earth's temperature stays within the limits needed for life. The atmosphere also protects us from most fragments of space debris by causing them to burn up well before they hit the Earth's surface and it allows water and carbon to be recycled through food chains.
But how did the atmosphere get here and how has it ended up as being perfect to sustain life? These are questions that can't be answered with certainty as it all happened long ago in the distant past. It seems that our atmosphere has been pretty much the same mixture of gases for about 200 million years!
Scientists have many theories as to how the atmosphere was formed and how changes have occurred over time. The most widely accepted is the one that suggests it came from volcanic activity. Molten magma contains a lot of gas dissolved in it which is released when it erupts at the surface. The early atmosphere was probably mostly carbon dioxide with little or no oxygen. There were smaller proportions of water vapour, ammonia and methane. As the Earth cooled down, most of the water vapour condensed and formed the oceans. The atmospheres of Venus and Mars are thought to be like the Earth's early atmosphere but the changes which happened on Earth never took place on those planets.