L.O: To know what the earth’s atmosphere was like in the past and what it is like now. Starter: What does the air we live in consist of?

Our atmosphere has remained the same in the last 200million years Carbon 0.04% dioxide Oxygen 21% Nitrogen 78% Others (Argon) 0.9% and trace others

 The Earth was formed about 4500 million years ago.  The very first atmosphere mainly consisted of hydrogen and helium gases.  Frozen giant planets like Saturn and Jupiter still have atmospheres like this. However on the warmer, smaller Earth these light gases were largely lost into space.  Smaller planet = less gravity (couldn’t hold on to the gases) Saturn Jupiter Earth Helium Hydrogen Space

 During the first billion years on Earth there was intense volcanic activity. This produced the next early atmosphere.  It would have contained large quantities of carbon dioxide (CO 2 ), along with methane (CH 4 ), and ammonia (NH 3 ) and steam  This is rather like the atmosphere on Mars and Venus today.  The Earth’s atmosphere would also have contained water vapour which condensed to form the oceans. Venus CO 2 CH 4 NH 3 steam Mars Earth CO 2, CH 4, NH 3, H 2 0

 Carbon dioxide reacted with rocks and much became trapped in them.  The evolution of algae some 3000 million years ago, and subsequently plants which successfully colonised the Earth’s surface, led us towards the present atmosphere.  Their photosynthesis replaced carbon dioxide with oxygen.  Over a period of time billions of tonnes of carbon dioxide became locked up in fossil fuels. Photosynthesis increased oxygen levels CO 2 + ROCK CO2 + Water Oxygen + Glucose Earth CO 2 decreases, O 2 Increases, H 2 0, CH 4, NH 3

 As oxygen levels rose atmospheric ammonia (NH 3 ) reacted with oxygen(O 2 ) to form water(H 2 O) and nitrogen (N 2 ) O 2 + NH 3 H N 2  As oxygen levels rose atmospheric methane (CH 4 ) reacted with oxygen (O 2 ) to form water and carbon dioxide O 2 + CH 4 H CO 2  Also, living organisms, including denitrifying bacteria, broke down nitrogen compounds releasing more nitrogen into the atmosphere.  And so the atmosphere headed towards a composition that has remained fairly constant for the last 200 million years. Earth O 2 increases, H 2 0 increases, NH 3 decreases N 2 increases CH 4 decreases

 Oxygen normally exists as pairs of atoms (O 2 ).  Oxygen can, however, turn into another form that has three atoms joined together. This is ozone (O 3 ).  As oxygen levels rose, so did the amount of ozone.  This layer of ozone in the atmosphere filters out harmful ultraviolet rays from the sun. This will have allowed new organisms to evolve and survive. 3O 2  2O 3 Oxygenozone Earth Harmful UV rays stopped with ozone layer Harmful UV rays reach Earth’s surface without ozone layer

AD 2006 The city of London and oxygen levels are stabilised at about 21% of the atmosphere. The oxygen level hasn’t changed for 200 million years. Ox conc 21.0%

4 Billion years3 Billion years2 Billion years1 Billion yearsPresent day Carbon dioxide MethaneAmmoniaOxygenNitrogenOthers Present day atmosphere contains 78% nitrogen, 21% oxygen, 1% noble gases and about 0.03% CO 2

Gases Explanation Pictures

Evolution of the earth’s atmosphere Answer the questions on the worksheet

1. What was the main gas in the atmosphere around 3500M years ago? 2. Where did this gas come from? 3. What process led to reduction in CO 2 levels? 4. What gas protects life from harmful UV radiation? 5. What % of the present atmosphere is oxygen? Carbon dioxide Volcanoes Photosynthesis Ozone 21% Activity