The Proportions of Different Gases in the Atmosphere and the Earth's Early Atmosphere

PAPER 2 • SPEC 4.9.1.1, 4.9.1.2

Earth's Atmosphere — Composition & Early History

Today's proportions, the volcanic theory, and how oceans removed CO₂. The atmosphere module starts here — and so do the most common examiner traps.

Why this lesson matters

This is the foundation of every Module 9 question. AQA expects you to know two specific things — and only one specific theory of how the early atmosphere formed.

What you'll be able to do

State the approximate proportions of nitrogen, oxygen and other gases in today's atmosphere.

Describe Earth's early atmosphere using the volcanic theory — and only the volcanic theory.

Explain how the oceans formed and how this removed carbon dioxide from the atmosphere.

Avoid the two highest-frequency examiner traps: invoking other theories, and bringing photosynthesis in too early.

Spec scope

The spec states explicitly: "No knowledge of other theories is required." Anything beyond the volcanic theory is wasted ink — and can negate correct answers.

Today's Atmosphere — The Numbers

For the past 200 million years, the proportions of gases in the atmosphere have been roughly the same as they are today. The spec gives three specific facts:

~80%

Nitrogen (N₂)

About four-fifths of the atmosphere.

~20%

Oxygen (O₂)

About one-fifth of the atmosphere.

The remaining ~1%

Small proportions of various other gases, including carbon dioxide, water vapour, and noble gases. The spec doesn't ask for an exact figure — just that you can name examples of these "small proportion" gases.

Visualising Today's Atmosphere

The proportions are easier to remember as a pie chart. Notice how dominated it is by just two gases.

Earth's Early Atmosphere — Setting the Scene

Earth is around 4.6 billion years old. The atmosphere we breathe today is nothing like the one that surrounded the planet at the start.

Why the spec hedges its language

"Theories about what was in the Earth's early atmosphere and how the atmosphere was formed have changed and developed over time." The spec is explicit that evidence for the early atmosphere is limited because of the time scale of 4.6 billion years. That uncertainty is why AQA only requires one theory.

Spec hard limit

The spec ends section 4.9.1.2 with: "No knowledge of other theories is required." If you mention comets, asteroids, or anything else as your main answer, you may not score the marks for the volcanic theory.

The Volcanic Theory

This is the one theory the spec asks you to know. The story has three steps.

Step 1 — Intense volcanic activity

During the first billion years of Earth's existence, the planet was geologically violent. Volcanoes were everywhere, and they released gases from inside the Earth into what would become the atmosphere.

Step 2 — Gases released

Volcanoes produced mostly water vapour and carbon dioxide, plus nitrogen, and small amounts of methane and ammonia. Crucially: little or no oxygen.

Step 3 — Oceans form

As Earth cooled, water vapour condensed and fell as rain, eventually collecting to form the oceans.

The Gases from Volcanoes

One picture, four key gases — these are the volcanic outputs you must be able to name.

How the Oceans Formed

The single most abundant gas from volcanoes was water vapour. Once Earth cooled enough, this changed everything.

Condensation

As temperatures fell, water vapour condensed into liquid water and fell as rain. Over millions of years, the rain collected in low-lying areas, forming the oceans.

Why this matters for the atmosphere

Water vapour was removed in vast quantities — but more importantly, the new oceans then started removing carbon dioxide too. That's the next step.

How CO₂ Was Removed

This is the big mechanism the spec wants you to know — and the one Q06.2 of P2 Jun22 asked about directly.

From the spec, 4.9.1.2 (verbatim)

"When the oceans formed carbon dioxide dissolved in the water and carbonates were precipitated producing sediments, reducing the amount of carbon dioxide in the atmosphere."

Two-step removal

1. CO₂ dissolved in the newly formed oceans.

2. Carbonates were precipitated as sediments — locking the carbon away on the sea floor.

Over millions of years, this dramatically reduced the amount of CO₂ in the atmosphere.

Don't say photosynthesis here

In the early atmosphere stage, there were no algae or plants yet. Any answer that uses photosynthesis to explain the early CO₂ drop is wrong. Mark scheme: "do not accept references to photosynthesis".

Why Mars and Venus Matter

The spec gives one comparison hook to help you remember what Earth's early atmosphere was like.

Spec language (4.9.1.2)

"At the start of this period the Earth's atmosphere may have been like the atmospheres of Mars and Venus today, consisting of mainly carbon dioxide with little or no oxygen gas."

Early Earth

Mainly CO₂

+ H₂O vapour (early on)

+ small amounts of CH₄ and NH₃

Little or no O₂

Earth Today

~80% N₂

~20% O₂

~0.04% CO₂

+ noble gases, water vapour

Memory hook

Mars and Venus today: roughly 95% CO₂. That's what Earth probably looked like 4 billion years ago — before life and oceans transformed it.

When AQA Gives You Evidence

The spec doesn't just want you to know the volcanic theory — it also tests your ability to evaluate evidence. AQA can give you data, charts, or a comparison and ask you to reason from it.

Spec-required skill

"Students should be able to, given appropriate information, interpret evidence and evaluate different theories about the Earth's early atmosphere."

How this gets tested

Graph questions: "Use Figure X to describe how the percentage of [gas] changed." Read the data — don't recite memorised facts that contradict the graph.

Comparison questions: "Mars and Venus today have ~95% CO₂. Use this to suggest what early Earth's atmosphere may have been like." (The spec gives you this comparison ready-made.)

Limitations questions: "Suggest why scientists are uncertain about the early atmosphere." Answer: "Evidence is limited because of the time scale of 4.6 billion years" — straight from the spec.

Strategy

If a question gives you data, start your answer with what the data shows, then connect it to a theory. If it asks "why are scientists uncertain", lift the spec phrase about 4.6 billion years and limited evidence.

Examiner Alert — One Theory Only

This is the most common error on Module 9 — and it's been flagged in the examiner report.

P2 Jun22 Q06.1 — Examiner Report

"Some answers were seen which invoked theories about the evolution of the atmosphere other than the single theory given in the specification. Responses therefore in terms of ammonia were insufficient."

"Weaker responses did not identify nitrogen from the provided key, but even so, there were many responses which mentioned volcanic activity."

What this tells you

Stick to the volcanic theory. If a question gives you a graph, name the gas, then say "volcanic activity".

Don't focus on alternative origin theories (e.g. ammonia from comets, asteroid impact theories) — these won't score the AO1 mark for the volcanic theory. Note: ammonia ITSELF is fine to mention as one of the small-proportion gases volcanoes produced.

Always identify the gas by name, even if the graph has a key. Examiners want to see the chemistry, not just graph reading.

Examiner Alert — Photosynthesis Comes Later

The second high-frequency error: dragging photosynthesis into an explanation about a time when there were no plants.

P2 Jun22 Q06.2 — Examiner Report

"There were no algae or plants this early and many students negated correct responses about dissolving in oceans or carboniferous sediments with a further comment about photosynthesis."

"As a result, only around 20% of students scored both marks here."

The geological order matters

Early atmosphere CO₂ drop = dissolving in oceans + carbonate precipitation. No photosynthesis.

Photosynthesis came later, once algae evolved (covered in lesson 9.02).

If a question asks why CO₂ was reduced when oceans first formed — only the ocean / carbonate mechanism scores.

Mark Scheme Language to Use

These are the exact phrases AQA accepts. Lift the wording into your answers.

If asked about...Mark-scoring phrasing (P2 Jun22 MS)

Why nitrogen built up in the early atmosphere"the percentage of nitrogen increased" + "because of intense volcanic activity"

Why CO₂ decreased after oceans formed"the percentage of carbon dioxide decreased" + EITHER "carbon dioxide dissolved in oceans" OR "formation of carbonate (precipitates / sediments)"

What the early atmosphere contained"mainly carbon dioxide" + "little or no oxygen" + "water vapour" + "small amounts of methane and ammonia"

Comparison to today"like the atmospheres of Mars and Venus" — direct quote from spec

Common Errors to Avoid

Three patterns that cost the most marks across past papers.

Wrong / weak answerWhy it loses marksCorrect version

"Volcanoes produced oxygen."Volcanoes produce H₂O, CO₂, N₂, CH₄, NH₃. Not oxygen."Volcanic activity released CO₂, water vapour, nitrogen, and small amounts of methane and ammonia."

"Photosynthesis reduced CO₂ when oceans formed."No algae or plants existed yet. MS rejects photosynthesis at this stage."CO₂ dissolved in the oceans and carbonates were precipitated as sediments."

"The atmosphere came from comets / asteroids."The spec only requires the volcanic theory. Other theories don't score."During the first billion years, intense volcanic activity released gases that formed the early atmosphere."

Practice Question 1

Describe the composition of Earth's early atmosphere, according to one theory.3 MARKS

Point

According to one theory, Earth's early atmosphere was formed by intense volcanic activity during the first billion years of Earth's existence.

Evidence

Volcanoes released gases — mainly water vapour (which later condensed to form the oceans) and carbon dioxide, with small amounts of methane and ammonia, and nitrogen that gradually built up.

Explain

There was little or no oxygen — oxygen only appeared later, once photosynthesising organisms had evolved.

1 mark: volcanic origin / first billion years. 1 mark: main gases (water vapour + CO₂, plus methane / ammonia). 1 mark: little or no oxygen.

Practice Question 2

A scientist plotted a graph of how the percentage of three gases in Earth's atmosphere has changed over the last 4 billion years.

In the earliest region of the graph, the percentage of carbon dioxide falls sharply once the oceans form, even though no algae or plants yet exist.

Explain why the percentage of carbon dioxide decreased in this earliest region.2 MARKS

Mark 1

Identify the change: the percentage of carbon dioxide decreased.

Mark 2

Give a mechanism — either:

"Carbon dioxide dissolved in the oceans"

or

"Carbonates were precipitated as sediments"

The question explicitly tells you no algae or plants yet exist — that's the examiner blocking the photosynthesis answer. If you write "photosynthesis", you negate any correct mechanism.

Inspired by P2 Jun22 Q06.2. Mark scheme phrasing taken verbatim from AQA.

Lesson Summary

Today's Atmosphere

~80% nitrogen (N₂) — about four-fifths

~20% oxygen (O₂) — about one-fifth

Small proportions of carbon dioxide, water vapour, and noble gases

Stable for roughly 200 million years

The Volcanic Theory (one theory only)

First billion years of Earth's existence: intense volcanic activity

Volcanoes released: water vapour , CO₂ , nitrogen , small amounts of methane and ammonia

Little or no oxygen

Like the atmospheres of Mars and Venus today (mainly CO₂)

How CO₂ Was Removed

Water vapour condensed as Earth cooled, falling as rain → forming the oceans

Carbon dioxide then dissolved in the oceans

Carbonates were precipitated as sediments , reducing atmospheric CO₂

Photosynthesis came later (covered in 9.02) — not during this stage

Two Examiner Traps

Don't invoke other theories (comets, asteroids). Spec requires only the volcanic theory.

Don't use photosynthesis to explain the early CO₂ drop — algae didn't exist yet. Mark scheme: "do not accept references to photosynthesis".

Up Next: Lesson 9.02

How Oxygen Increased & Carbon Dioxide Decreased — photosynthesis takes the stage, the algae timeline, and the rate-of-change comparison that examiners love.