C1.1a Oxygen in the early atmosphere How do we know that there was little or no oxygen in the atmosphere when the Earth was young, and that it has increased? Some of the most important evidence comes from iron compounds in rocks. The Earth is thought to be about 4.5 billion years old. The earliest rocks that have been found date from about 4.1 billion years ago. These rocks contain iron compounds that would form if there was no oxygen around – such as iron sulfide and iron carbonate. Scientists therefore believe there was little or no oxygen in the atmosphere at this time. Rocks that contain iron oxide date from around 3.5 billion years ago. These rocks contain a form of iron oxide (Fe3O4) that forms if there is only a little oxygen available. The oldest fossils date from 3.5 million years ago as well and they are of organisms that could photosynthesise. Most scientists think this shows that oxygen was formed by photosynthesis when life began on Earth. Rocks dating from about 1.8 billion years ago contain a different form of iron oxide (Fe2O3). This form of iron oxide forms when there is more oxygen present. Scientists use this as evidence that a significant amount of oxygen had built up in the atmosphere 1.8 billion years ago. 1
Use the information above to write a paragraph to explain why scientists think that there was little or no oxygen in the Earth’s early atmosphere.
2
Use the information above to write a paragraph to explain why scientists think the amount of oxygen in the Earth’s atmosphere has increased.
3
Use the information above to write a paragraph to explain why scientists think that it was the start of life on Earth that led to changes in the amount of oxygen in the atmosphere.
© Pearson Education 2010. Edexcel GCSE Science Activity Pack. This document may have been altered from the original.
C1.1b Other atmospheres Name
Class
Date
Venus and Mars are planets. Titan is a moon of the planet Saturn. The table shows the composition of the atmosphere for each one. It only shows gases with percentages above 1%. Colour on pie charts
Gas
Venus
carbon dioxide (%) nitrogen (%) argon (%) methane (%)
1
3
95.3 2.7 1.6
Titan
98.4 1.5
Complete a pie chart to show the gases on each planet or moon. Use a different colour for each gas. In the table, show a key for the colours you have used.
Venus
2
96.5 3.5
Mars
Mars
Titan
Many scientists believe the early atmosphere on Earth may have been like those on Venus and Mars. Some believe it may have been like the atmosphere on Titan. a
If the Earth’s early atmosphere was like it is on Mars and Venus, what was the main gas in the young Earth’s atmosphere?
b
If the Earth’s early atmosphere was like it is on Titan, what was the main gas in the young Earth’s atmosphere?
Suggest why there is uncertainty among scientists about what the Earth’s atmosphere was like when the planet was young.
© Pearson Education 2010. Edexcel GCSE Science Activity Pack. This document may have been altered from the original.