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 Our atmosphere has remained the same in the remained the same in the

last 200million yearslast 200million years

Carbon0.04%

dioxide Oxygen21%

Nitrogen78%

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

EarthHelium

HydrogenSpace

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 (CO2), along with methane (CH4) , and ammonia (NH3) 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

CO2 CH4 NH3 steam

Mars

EarthCO2, CH4, NH3, H20

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

CO2 + ROCK

CO2 + Water Oxygen + Glucose

EarthCO2 decreases, O2 Increases, H20, CH4, NH3

As oxygen levels rose atmospheric ammonia (NH3) reacted with oxygen(O2) to form water(H2O) and nitrogen (N2)O2 + NH3 H20 + N2

As oxygen levels rose atmospheric methane (CH4) reacted with oxygen (O2) to form water and carbon dioxideO2 + CH4 H20 + CO2

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

EarthO2 increases, H20 increases, NH3 decreasesN2 increases

CH4 decreases

Oxygen normally exists as pairs of atoms (O2).

Oxygen can, however, turn into another form that has three atoms joined together. This is ozone (O3).

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.

3O2 2O3

Oxygen ozone

Earth

Harmful UV rays stopped

with ozone layer

Harmful UV rays reach Earth’s surface

without ozone layer

AD 2006The 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 years 3 Billion years 2 Billion years 1 Billion years Present day

Carbondioxide

Methane Ammonia Oxygen Nitrogen Others

Present day atmosphere contains 78% nitrogen, 21% oxygen, 1% noble gases and about 0.03% CO2

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 CO2 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