1. The PowerPoint Evolution of the Atmosphere
Earth Science for KS4
Earth Science Education Unit
© The Earth Science Education Unit
Copyright is waived for original material contained herein if it is required for use within the laboratory or classroom. Copyright material included from other publishers rests with them. 1

2. The PowerPoint evolution of the atmosphere2

3. The PowerPoint evolution of the atmosphere
Think through the evolution of the atmosphere and oceans through these sections:
The evolving atmosphere
Atmospheric evolution
- the ball model
Planetary survival
An Excel model
Atmospheric evolution
- the evidence
This PowerPoint presentation was prepared by Mick de Pomerai and edited by Chris King, based on original ideas by: Vicki Chuter, Mick de Pomerai, Elizabeth Devon, Paul Grant, Chris King, Kevan Taplin, Ros Todhunter and Dave Williams.

4. The evolving atmosphere
What makes our planet unique in the Solar System?
© NASA
The atmosphere is mostly nitrogen and oxygen
What are the main gases in the atmospheres of our planetary neighbours, Mars and Venus?
Mostly carbon dioxide (>95%)
How has our atmosphere become so different?

5. What was Earth’s early atmosphere like?
Most scientists think that the Earth’s atmosphere was formed from the volcanic gases released by the volcanic activity that formed the early crust.
Reproduced with kind permission of U.S. Department of Interior, U.S. Geological Survey
What would be the likely composition of those gases?
On average, gases produced by modern volcanoes comprise:
Water %
Carbon dioxide %
Sulfur oxides %
Nitrogen %
Others %
Further Notes: Assumptions.
There are actually a number of snags in assuming that the Earth’s early atmosphere had exactly the same composition as “modern” volcanic gases, but this assumption does work well as a starting point. It is also likely that cometary material has added considerably to the presence of water and other volatiles in the Earth’s atmosphere and oceans.
This is very different from the composition of the present-day atmosphere.
How might things have changed?

6. Atmospheric evolution (1)
Early atmosphere
We can use 100 coloured balls to represent the composition of the Earth’s early atmosphere:
Water (green) balls
Carbon dioxide (black) balls
Sulfur oxides (yellow) balls
Nitrogen (blue) balls
Hydrogen (white) ball
Argon, etc (brown) ball
Oxygen (red) balls
Water
Carbon dioxide
Sulfur oxides
Nitrogen
Hydrogen
Argon, etc.
Oxygen
Key
Note: there was no oxygen in the early atmosphere

7. Atmospheric evolution (2)
Early atmosphere
As the early Earth cooled, what would have happened to all that water vapour?
Evolving atmosphere
Atmosphere
Ocean
Earth became cool enough for most of the water vapour to condense as rain, so that, by about 4000 million years ago, our planet already had oceans. So we can move most of the green ‘water vapour’ balls into the ocean.
Water
Carbon dioxide
Sulfur oxides
Nitrogen
Hydrogen
Argon, etc.
Oxygen
Key
Note: there was no oxygen in the early atmosphere

8. Atmospheric evolution (3)
What would have happened to the
carbon dioxide and the sulfur oxides?
Evolving atmosphere
Atmosphere
Ocean
Evolving atmosphere
Atmosphere
Ocean
Carbon dioxide and sulfur oxides are both soluble, so we can ‘dissolve’ some black ‘carbon dioxide’ balls and some yellow ‘sulfur oxide’ balls in our ‘Ocean’.
Water
Carbon dioxide
Sulfur oxides
Nitrogen
Hydrogen
Argon, etc.
Oxygen
Key

9. Atmospheric evolution (4)
Early bacteria evolved soon after the oceans formed. What effect did they have?
The early bacteria began, together with chemical precipitation, to remove carbon and sulfur compounds that had dissolved in the ocean water, locking them up in the ocean-floor sediments.
So more C and S compounds could move from the atmosphere into the ocean.
Evolving atmosphere
Ocean
Atmosphere
Ocean floor sediments
Water
Carbon dioxide
Sulfur oxides
Nitrogen
Hydrogen
Argon, etc.
Oxygen
Key

10. Atmospheric evolution (5)
Early life also began to photosynthesise, changing carbon dioxide into oxygen. Where did this go?
Ocean
Atmosphere
Ocean floor sediments
Ocean
Atmosphere
Ocean floor sediments
Early oxygen reacted with iron dissolved in sea water and precipitated out to form ocean floor sediments.
Water
Carbon dioxide
Sulfur oxides
Nitrogen
Hydrogen
Argon, etc.
Oxygen
Key

11. Atmospheric evolution (6)
Evolving atmosphere
Where did the hydrogen go?
Atmosphere
Ocean floor sediments
Evolving atmosphere
Atmosphere
Ocean floor sediments
Since hydrogen is the lightest element, it rose into the upper atmosphere and was lost into space.
Water
Carbon dioxide
Sulfur oxides
Nitrogen
Hydrogen
Argon, etc.
Oxygen
Key

12. Atmospheric evolution (7)
Evolving atmosphere
Atmosphere
Ocean floor sediments
Evolving atmosphere
Where did the oxygen in the atmosphere come from?
Atmosphere
Ocean floor sediments
By 2000 million years ago, the vast amounts of algae photosynthesising in the oceans were absorbing lots of carbon dioxide and releasing oxygen
Meanwhile the iron in the oceans had absorbed all the oxygen it could
So “free” oxygen began to appear in the atmosphere for the first time
Water
Carbon dioxide
Sulfur oxides
Nitrogen
Hydrogen
Argon, etc.
Oxygen
Key

13. Atmospheric evolution (8)
Evolving atmosphere
Atmosphere
Ocean floor sediments
Ocean floor sediments
Atmosphere
Evolving atmosphere
What was happening to the carbon dioxide and the sulfur oxides in the atmosphere?
More and more of the CO2 and sulfur oxides became dissolved in the ocean and then locked up in the ocean floor sediments
Water
Carbon dioxide
Sulfur oxides
Nitrogen
Hydrogen
Argon, etc.
Oxygen
Key

14. Atmospheric evolution (9)
Today’s atmosphere
This is the outer Earth today:
An atmosphere mostly of nitrogen with oxygen and a little argon etc.
An ocean of water with some dissolved carbon and sulfur oxides
Ocean floor sediments and
rocks containing carbon,
sulfur and oxygen compounds
Ocean floor sediments
Atmosphere
Water
Carbon dioxide
Sulfur oxides
Nitrogen
Hydrogen
Argon, etc.
Oxygen
Key

15. Planetary survival - key questions for the future
Scientists are very worried about the effects of burning fossil fuels, which is rapidly adding more carbon dioxide to our atmosphere. Why are they worried?
Although Venus is similar in size and overall composition to Earth, its atmosphere is very different. Try to find out more, including why it’s so hot on Venus (430oC), and why its atmosphere evolved in a very different way from Earth’s.
Physicists believe that the Sun has got much hotter since it first started to shine, yet Earth scientists believe that the Earth’s temperature has changed by only a few oC at most. Why might this be?

16. Simplifying and summarising
The evolutionary model of the atmosphere described is simplified, but can be further simplified by assuming that all of the carbon dioxide and sulfur oxides become dissolved in the ‘early ocean’. In reality, this would have resulted in the Earth becoming frozen solid!
Further Notes: “Snowball Earth”.
In fact, volcanoes are always producing CO2 so it can never all be removed from atmosphere. Once continents had formed then CO2 from rain and streams and attacked and weathered the silicates, removing the CO2 from the atmosphere – see earlier notes – around 1200 to 800 million years ago this very nearly did result in a “Snowball Earth” Massive weathering of the super continent of Rodinia and a shut down of many volcanoes allowed CO2 removal from atmosphere and unprecedented global cooling, along with a mass-extinction of the many soft-bodied organisms that lived in those ancient seas. Fortunately, continued volcanic emission, especially as the supercontinent began to break up, replenished atmospheric CO2 allowing temperatures to rise and life to re-establish – ultimately resulting in the “Cambrian explosion” of 500 million years ago.
1. Earth’s early atmosphere is formed by volcanic outgassing
2. Water condenses to form the first oceans. Carbon dioxide and sulfur oxides become dissolved (over time). Hydrogen escapes into space.
3. Living organisms lock up most sulfur and carbon compounds in rocks. Oxygen is released by photosynthesis

17. A computer model (Excel)
This computer model for the Earth’s evolving atmosphere is available from ESEU.
It is based on the same data as described in previous slides.

18. Planetary survival - key questions for the future: some answers
Scientists are very worried about the effects of burning fossil fuels, which is rapidly adding more carbon dioxide to our atmosphere. Why are they worried?
Although Venus is similar in size and overall composition to Earth, its atmosphere is very different. Try to find out more, including why it’s so hot on Venus (430oC), and why its atmosphere evolved in a very different way to Earth’s.
Physicists believe that the Sun has got much hotter since it first started to shine, yet Earth scientists believe that the Earth’s temperature has changed by only a few oC at most. Why might this be?
More carbon dioxide is likely to lead to an increased greenhouse effect and global warming
It is so hot on Venus because of the large percentage of carbon dioxide in the atmosphere leading to a ‘runaway greenhouse effect’. It is thought that Venus never became cool enough for water to condense to form oceans. The water vapour in the atmosphere was broken down by UV light, the H was lost to space and the O combined with other gases and surface rocks, so increasing the percentage of CO2 in the atmosphere.
Earth scientists believe that as the Sun became hotter the different cycles operating on Earth tended to cool the Earth down resulting in a fairly stable temperature over time. This stability is described in the Gaia Hypothesis - see:
Re Venus: Point here is that Venus could never have cooled enough for water to condense and form oceans. Consensus seems to be that, over time, water has been lost from the atmosphere due to dissociation in the upper atmosphere caused by solar UV, resulting in hydrogen loss to space and oxygen combining with other gases (e.g. sulphur compounds to produce sulphuric acid clouds) and surface rocks. Thus, over time water has been lost and CO2 has increased, making Venus ever hotter. Interestingly, Venus not only lacks oceans, but also shows no evidence of Plate Tectonic motions like those on Earth… a truly “dead” planet.

19. Atmospheric evolution - the evidence
© NASA
The Pleiades
© Luc Viatour
The Moon
Atmospheric evolution - the evidence
© NASA
© The UC Museum of Paleontology

20. Atmospheric evolution - the evidence
Origin of the solar system
About 4600 million years ago, the Solar System was formed as a mass of interstellar gas (mainly hydrogen and helium) and “dust” (ice and other solids – the remains of an even more ancient exploded star) which began to collapse under its own gravitational pull. As it collapsed, the slowly swirling mass began to rotate faster, like a spinning skater pulling in her arms – this is why all the planets orbit in the same direction as the Sun’s rotation. The vast bulk of the material gathered to form the Sun itself,
whose core became sufficiently dense and hot to initiate nuclear fusion, at which point it burst violently into life.
The Pleiades
Supporting evidence: Young stars, like the Pleiades (visible to the naked eye) are surrounded by “nebulae” of gas and dust from which planets are forming.
© David Malin (AAO), ROE, UKS Telescope

21. Atmospheric evolution - the evidence
The proto-Earth
The remaining matter of the Solar system gathered together to form larger and larger “planetesimals” that collided with increasing violence, the largest ones pulling in the smaller ones through gravitational attraction. The heat generated on the “proto-Earth” was probably enough to melt the planet, allowing the heavier elements (mainly iron) to sink and form the core, with the lighter ones (e.g. silicon and oxygen, as silicate compounds) rising and forming the early crust, which would have been frequently disrupted by meteor impacts and volcanic outpourings.
The Moon
Supporting evidence: The paler, heavily cratered areas of the moon are around 4200 million years old. The dark areas of the moon are basalt lava flows that formed around 3600 million years ago (dates from samples brought back by Apollo astronauts). There are very few impact craters in the dark areas, suggesting that, by this time, such impacts had become rare events.
© Luc Viatour

22. Atmospheric evolution - the evidence
Earth’s “earliest” atmosphere
The Earth’s “earliest” atmosphere was probably stripped away by the intense solar wind from the initially unstable Sun
Supporting evidence: Only the outer planets have huge atmospheres. Also, most of the “noble gases” (He, Ne, Kr, Xe) are rare on Earth compared to elsewhere in the solar system.
Earth’s second atmosphere
Most scientists assume that Earth’s second atmosphere was produced largely by “outgassing” during the many volcanic outpourings that formed the early crust.
Water %
Carbon dioxide %
Sulfur oxides %
Nitrogen %
Others %
Supporting evidence: Volcanic eruptions today produce huge volumes of gas. On average, these are:
DA Swanson, USGS

23. Atmospheric evolution - the evidence
The first oceans
Earth was sufficiently distant from the early Sun for most of the water vapour to condense so that, by about 4000 million years ago, our planet already had oceans.
Supporting evidence: The metamorphosed remains of the Earth’s oldest known sedimentary rocks (3850 million years old) have been found in Greenland, as shown opposite, and contain evidence that they were laid down in an early ocean.
© Jelte Harnmeijer

24. Atmospheric evolution - the evidence
Life on Earth
The earliest bacteria appeared not long after the oceans formed. In combination with chemical precipitation, they had already begun to remove sulfur and carbon compounds from the ocean water, locking them up in the sea-floor sediments, and thus drawing more of these compounds into solution from the atmosphere (via rain).
Supporting evidence: Some of the early bacteria formed stromatolites - the oldest fossil stromatolites are over 3500 million years old. Stromatolites like these living today in Australia, are formed by cyanobacteria, the earliest known photosynthesisers. These deposit calcium carbonate (forming the mounds seen in the picture) as they produce oxygen.
© The UC Museum of Paleontology

25. Atmospheric evolution - the evidence
The first oxygen
The first oxygen produced by photosynthesis was taken up by chemical reaction (e.g. with iron) to form oxides on the ocean floor.
By 2000 million years ago, “free” oxygen had begun to appear in the atmosphere, released by the vast amounts of photosynthetic algae now existing in the oceans.
Supporting evidence: Evidence for the presence of “free” oxygen comes from the appearance of the first oxidised iron-rich sedimentary rocks on land. Such red-coloured deposits need warm, oxidising conditions to form.
Red sandstone rocks in the Valley of Fire, Nevada, USA

26. Atmospheric evolution - the evidence
The removal of carbon dioxide
Over time, living organisms have removed most of the carbon dioxide from the atmosphere, fixing much of the carbon in the form of limestone (calcium carbonate).
Supporting evidence: Limestones, formed mainly of calcium carbonate, are common rocks on Earth
To today
Meanwhile, plants have absorbed carbon dioxide and released oxygen through photosynthesis, a process that has continued to the present.
Simple animal life may have appeared in the oceans around 1000 million years ago, but the land remained barren until plants began to colonise it about 400 million years ago, after Earth had acquired its protective ozone layer to shield the land from UV rays.
Whilst, in the last few hundred million years, there have been no major changes to the bulk composition of the atmosphere and oceans, geological evidence does indicate that the amount of carbon dioxide in the atmosphere has varied somewhat, sometimes resulting in hot ‘greenhouse’ conditions and at other times in cold ‘icehouse’ conditions.
First Para implies carbonate formation necessarily involves release of oxygen – which would be incorrect as CO2 + O gives CO3 – i.e.extracting oxygen from the atmosphere.

27. Copyright The Earth, by NASA, this image is in the public domain
Lava Flow © USGS (see image for further details)
The Pleiades Star Cluster © David Malin (AAO), ROE, UKS Telescope
Australian Stromatolites © UC Museum of Paleontology,
Valley of Fire Rock Arch, Parks of Southern Nevada © Bert Katzung,
Full Moon © Luc Viatour,
Earth’s oldest rocks (Rock of Isua) © Jelte Harnmeijer
Lava Cascade, by DA Swanson, USGS, this image is in the public domain
Chalk cliffs © Peter Kennett

28. Atmospheric evolution - the evidence
©NASA
The Moon
© NASA
The Pleiades
Atmospheric evolution - the evidence
© NASA
© University of California Museum of Paleontology

29. The PowerPoint Evolution of the Atmosphere
Earth Science for KS4
Earth Science Education Unit
© The Earth Science Education Unit
Copyright is waived for original material contained herein if it is required for use within the laboratory or classroom. Copyright material included from other publishers rests with them. 29