Earth's early atmosphere and oceans
Where did Earth's air and water come from? Volcanoes released gases, and meteorites show that asteroids may also have delivered water.
- Describe how Earth's early atmosphere formed and changed.
- Explain how carbonaceous chondrites contain water.
- Evaluate evidence that asteroids delivered some of Earth's water.
Links to: GCSE chemistry, chemistry of the atmosphere: the evolution of the atmosphere and oceans, and evaluating different theories; maths: percentages.
From volcanoes to oxygen
In its first billion years, Earth had intense volcanic activity that released mainly carbon dioxide and water vapour, with some nitrogen and small amounts of other gases. As Earth cooled, the water vapour condensed to form the oceans. Carbon dioxide dissolved in the oceans and was locked into carbonate rocks and later fossil fuels.
About 2.7 billion years ago, algae and later plants began producing oxygen by photosynthesis, which built up gradually to today's level of about 21%.
Water locked in rock
Some meteorites contain a surprising amount of water. Carbonaceous chondrites come from asteroids where ice melted early on, and the water reacted with the rock to form clay minerals. In the common CM type, water bound in these minerals makes up roughly 10% of the mass. The Winchcombe meteorite that fell in England in 2021 is one of these.
Testing the idea
Water contains a little deuterium, a heavier isotope of hydrogen with one neutron. The ratio of deuterium to ordinary hydrogen acts as a fingerprint. Water in carbonaceous chondrites, including Winchcombe, has a ratio similar to Earth's oceans. Water measured in most comets studied so far, such as the comet visited by the Rosetta spacecraft, has a higher ratio.
This suggests that water-rich asteroids, rather than comets, supplied much of the water that did not come from Earth's own rocks. Scientists still debate how much came from each source, which is a good example of competing theories tested with measurements.
Water from a meteorite
A 50 g carbonaceous chondrite contains about 10% water by mass. How much water does it contain, and how many such meteorites would contain 1 kg of water?
- Water = 10% of 50 g = 0.10 × 50 = 5 g.
- 1 kg = 1,000 g, and 1,000 ÷ 5 = 200.
Answer: 5 g of water each, so 200 such meteorites would contain 1 kg. Delivering oceans this way needed enormous amounts of material during the early bombardment of the young Earth.
Make a two-column table: evidence that Earth's water came from inside Earth (volcanic outgassing), and evidence that some came from asteroids or comets. Use it to write a short paragraph answering: why do scientists still disagree about where Earth's water came from?
Quiz
5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.
-
Question 1Which gases did early volcanic activity mainly release?
Volcanoes released mostly carbon dioxide and water vapour, with some nitrogen.
-
Question 2How did the oceans form?
As the early Earth cooled, water vapour in the atmosphere condensed into liquid water.
-
Question 3How is water held in carbonaceous chondrites?
Melted ice reacted with the rock, locking water into clay minerals.
-
Question 4What does the deuterium-to-hydrogen ratio of water in chondrites suggest?
The ratio in carbonaceous chondrites matches ocean water better than most comets measured.
-
Question 5A 200 g meteorite is 12% water by mass. How much water does it contain?
0.12 × 200 = 24 g.