Oxygen is necessary for all humans, animals, and plant life to survive. When Earth was first formed, its atmosphere had no oxygen the colorless, odorless and tasteless gas that makes up about 20 percent of the air we breathe. It had only a deadly combination of hydrogen, methane, ammonia, and hydrogen cyanide. The hydrogen escaped into space and ultraviolet radiation from the Sun broke down the mixture, leaving only nitrogen and carbon dioxide. Only when life began and photosynthesis (the conversion of light energy into chemical energy by living organisms) occurred did oxygen first appear about 3.4 billion years ago.
- Why is the ozone layer important to Earth?
- What is air made of?
- Why is Earth mostly crater-free compared to the pockmarked Moon?
- Why does the Moon follow us when we’re driving?
- How does the Moon affect the ocean tides?
- Does the Moon really have volcanoes?
- How high could you jump on the Moon?
- What are moonquakes?
- Is there a Man in the Moon?
- Which planets experience the greenhouse effect?
- What is the Moon?
- What is the Arctic tundra?
- Where are the world’s rain forests?
- Why do deserts have special plants and animals?
- Why did life develop on Earth and not on the other planets?
- Is it true that parts of Earth are always covered in ice?
- Why does Earth look blue from outer space?
- Did the continents always look like they do now?
- Is there a place where two continents almost touch?
- How many continents are there?
