History of our/our evolving atmosphere

The early atmosphere contained gases such as : ammonia ,carbon dioxide, methane ,water vapor and nitrogen

🌍 Stage 1: The Early Atmosphere (Volcanic Activity)- 3,500M years ago

• The Earth was extremely hot with constant volcanic eruptions.

• Volcanoes released gases like:

  • Carbon dioxide (CO₂)

  • Water vapour (H₂O)

  • Nitrogen (N₂)

  • Small amounts of methane (CH₄) and ammonia (NH₃)

• No oxygen present = toxic atmosphere, unsuitable for life.

• Similar to atmospheres on Mars or Venus today.

☁ Stage 2: Oceans and Early Life- 3500 to 3000M years ago

• The Earth cooled → Water vapour condensed into oceans 🌧

• Some CO₂ dissolved in oceans, reducing its levels.

• Bacteria (like cyanobacteria) evolved in oceans and began photosynthesis:

This started adding oxygen to the atmosphere and removing more CO₂.

oxygen levels thus began to rise

🪨 How CO₂ got trapped in/forms sedimentary rock (Stage 2):

1. CO₂ dissolves in water first and then(in step 2) reacts with calcium ions (Ca²⁺), which are found in calcium-rich minerals (like limestone):

   Dissolving CO₂ in water:

   CO₂ + H₂O —> H₂CO₃(carbonic acid)

2. Carbonic acid (H₂CO₃) reacts with calcium ions (Ca²⁺) in the water to form calcium carbonate (CaCO₃), which precipitates out as solid sediment on the ocean floor:

   Formation of calcium carbonate (CaCO₃):

   Ca²⁺ + H₂CO₃ —> CaCO₃ + H₂O

3. Over time, calcium carbonate (CaCO₃) accumulates and hardens, eventually becoming sedimentary rock like limestone.

   Complete Balanced Symbol Equation:

   Ca²⁺ + CO₂ + H₂O —> CaCO₃ + H₂O

🌬 Stage 3: Oxygen Levels Rise- 3000 to 500M years ago

• Oxygen levels slowly built up in the atmosphere over millions of years.

• This allowed the formation of the ozone layer (O₃):

  • The ozone layer protected life from harmful UV radiation, making land-based life possible.

• Oxygen reacted with methane and ammonia, causing their levels to drop.

• More complex life forms evolved thanks to the increasing oxygen.

• Carbon dioxide (CO₂) continued to decrease because:

  • It got trapped in sedimentary rocks (e.g. limestone).

  • It was used by plants during photosynthesis.

• Nitrogen (N₂) levels remained relatively stable:

  • Slightly increased due to the breakdown of ammonia (NH₃).

  • Since nitrogen is very unreactive, it stayed in the atmosphere and still makes up about 78% today.

1.Removal of Methane (CH₄):

When oxygen (O₂) builds up in the atmosphere, it reacts with methane to form carbon dioxide (CO₂) and water (H₂O). This is a combustion reaction.

Balanced symbol equation: CH₄ + 2O₂ —> CO₂ + 2H₂O

2. Removal of Ammonia (NH₃):

Ammonia reacts with oxygen in the presence of sunlight (in the upper atmosphere), forming nitrogen (N₂) and water (H₂O).

Balanced symbol equation: 4NH₃ + 3O₂ —> 2N₂ + 6H₂O

Final Composition of Today’s Atmosphere:

• Nitrogen (N₂) – ~78%

• Oxygen (O₂) – ~21%

• Trace gases (1%) – e.g.

  • Carbon dioxide (CO₂)

  • Argon (Ar)

  • Water vapour (H₂O) (varies with weather)

Limits of the Theory of Atmospheric Development:

• There were no humans around at the time, so we rely on indirect evidence like volcanic rocks, ice cores, and fossils.

• This evidence can be incomplete or hard to interpret.

• Different scientists may have slightly different versions of the theory because new discoveries can update our understanding.

• For example, some theories suggest the exact timing of oxygen increase or how much methane was present is still debated.

SUMMARY FOR MY EXAM:

The theory is based on evidence from rocks and gases on other planets, but we weren’t there 4 billion years ago, so we can’t be 100% sure.
There’s no direct proof, only assumptions from things like volcanic activity, ice cores, and fossil records.
The theory could change if new evidence is discovered.

Why the Earth's atmosphere hasn’t changed much in 200 million years:

Over the last 200 million years, the Earth's atmosphere has remained stable because:

1. Photosynthesis and Respiration Are Balanced

   • Plants and algae constantly take in CO₂ and release O₂ through photosynthesis.

   • Animals and decomposers breathe in O₂ and release CO₂ through respiration.

   • These processes keep CO₂ and O₂ levels fairly steady.

2. Carbon Stores Are Stable

   • A lot of CO₂ is locked away in sedimentary rocks like limestone and in fossil fuels.

   • It doesn’t enter the atmosphere unless those rocks are eroded or fuels are burned.

3. No Major Catastrophic Changes

   • There have been no mass volcanic events or asteroid impacts recently to drastically change the gas levels.

   • So the atmosphere has stayed in a kind of dynamic balance.

SUMMARY FOR EXAM:

The composition hasn’t changed much because the processes that add and remove gases (like photosynthesis, respiration, and rock formation) are in balance. Plus, a lot of CO₂ is locked in rocks and fossil fuels, keeping it out of the air.