1. 1
   A science museum is designing a new exhibit timeline showing how the Earth's atmosphere has changed over billions of years. The exhibit begins with a description of the atmosphere roughly four and a half billion years ago, when intense volcanic activity is believed to have released large amounts of gas.

   (a)

   According to this theory, which gas made up the majority of the early atmosphere?

   [1 mark]

   • ACarbon dioxide
   • BOxygen
   • CNitrogen
   • DHydrogen

   (b)

   The exhibit's next panel explains that, as the Earth cooled, water vapour released by volcanic activity condensed to form the oceans. What happened to much of the carbon dioxide in the atmosphere once the oceans had formed?

   [1 mark]

   • AIt reacted with nitrogen to form ammonia
   • BIt dissolved into the oceans and carbonates were precipitated
   • CIt was immediately converted into oxygen by sunlight
   • DIt escaped into space and was permanently lost

   (c)

   A visitor asks the museum curator why scientists say the theory of the early atmosphere's composition is uncertain rather than definitely proven. Explain why this is the case.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A marine biologist is studying ancient fossilised algae mats found in layered rock formations, which scientists believe were among the first organisms to release a significant new gas into the atmosphere around 2.7 billion years ago.

   (a)

   Which process were these early algae carrying out that released this gas?

   [1 mark]

   • ACombustion
   • BRespiration
   • CPhotosynthesis
   • DFermentation

   (b)

   The biologist explains to students that the gas released by these algae is the same gas that later increased steadily as plants evolved and spread across the land. Which gas is this?

   [1 mark]

   • AAmmonia
   • BMethane
   • CCarbon dioxide
   • DOxygen

   (c)

   The biologist also explains that carbon dioxide levels in the atmosphere fell steadily over the same long period. Describe two separate processes, other than photosynthesis by algae and plants, that contributed to the removal of carbon dioxide from the early atmosphere.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A geography teacher is preparing a lesson on limestone quarrying near a village, and wants to explain to students how the limestone in the local hills is connected to the history of the Earth's atmosphere.

   (a)

   Describe how the carbon dioxide once present in the early atmosphere became locked into limestone rock over geological time.

   [3 marks]

   (b)

   A student asks the teacher why scientists believe fossil fuel deposits found beneath the village, such as coal seams, are also evidence for changes in the ancient atmosphere. Explain the connection between the formation of fossil fuels and the reduction of carbon dioxide in the early atmosphere, and describe why this evidence, while useful, does not perfectly prove the exact original composition of the atmosphere.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A science communicator is writing a public article explaining, from start to finish, the widely accepted theory of how the Earth's atmosphere changed from its earliest volcanic beginnings to its present-day composition. A reader later writes in pointing out that some scientists have proposed alternative theories about the early atmosphere.

   (a)

   Describe the full sequence of changes, from the release of gases by volcanic activity through to the roughly stable modern atmosphere, explaining the key processes responsible for each major change in gas composition, and referencing approximately when in Earth's history each stage occurred.

   [6 marks]

   (b)

   A reader writes to the science communicator pointing out that some scientists have proposed alternative theories about the exact composition of the early atmosphere, and asks why there is not just one single, universally agreed theory if the process happened so long ago. Evaluate why uncertainty exists in this area of science, discussing the types of evidence available to scientists, the limitations of that evidence, and the role of peer review and scientific debate in refining these theories over time.

   [6 marks]

   Total for question 4: 12 marks

End of questions