Earth's early air and oceans

Grade 9 Science Worksheet

Circle the best answer.

1. Read this first

   Oxygen in the air over time (Smithsonian, NASA)

   Time	Oxygen in the air
   before about 2.7 billion years ago	almost none
   about 2.4 billion years ago	first fills the air (NASA)
   about 600 million years ago	about one-fifth of today's level
   today	21%

2. 1.

   Using the table, about how long after cyanobacteria began (about 2.7 billion years ago) did oxygen first fill the air (about 2.4), by Bo's math?

   • About 5.1 billion years
   • No time; it was instant
   • About 300 million years
   • About 3 years
3. 2.

   Using the table, about how long did it take from the first oxygen in the air (2.4 billion years ago) to one-fifth of today's level (600 million), by Esme's math?

   • About 1.8 billion years
   • About 600 million years
   • About 3 billion years
   • About 1.8 million years
4. 3.

   Why did oxygen take hundreds of millions of years to build up in the air, by Jun's reading (Smithsonian)?

   • Volcanoes kept taking oxygen in
   • Photosynthesis used up oxygen
   • The air already held plenty
   • It first reacted with methane and with iron in rocks
5. 4.

   Drawing of reddish rock layers. Smithsonian: after about 300 million years of oxygen-making, reddish redbeds formed. What do they show, by Kira's reading?

   • Oxygen had built up enough to rust iron in rocks
   • There was still no oxygen
   • The rocks were heated by the Sun
   • Volcanoes were giving off oxygen
6. 5.

   About when did the methane haze clear and the sky turn blue, by Luz's reading (Smithsonian)?

   • At Earth's start
   • About two billion years ago
   • It was blue from the start
   • About two million years ago
7. 6.

   Carbon dioxide + water + sunlight → sugar + oxygen. Cyanobacteria take in carbon dioxide and give off oxygen. How did they change the early air, by Milo's reading?

   • Less carbon dioxide, more oxygen
   • No change
   • More carbon dioxide, less oxygen
   • More methane from their breathing
8. 7.

   Today's air and a lifeless model. If volcanoes had kept adding gas but no life had made oxygen, what would Earth's air be like today, by Noah's reading?

   • Just like today's, 21% oxygen
   • Rich in carbon dioxide, with almost no oxygen
   • No gases
   • Pure oxygen
9. 8.

   Earth formed about 4.6 billion years ago, and its surface cooled enough for water about half a billion years later (Smithsonian). About when could oceans begin to collect, by Pia's math?

   • About 4.1 billion years ago
   • About 0.5 billion years ago
   • Right at 4.6 billion years ago
   • About 5.1 billion years ago
10. 9.

    The young Sun gave about 70% of today's light. A dimmer Sun means a colder Earth unless what, by Ren's reading (Smithsonian)?

    • Unless greenhouse gases trap extra heat
    • Unless the oceans freeze
    • Unless oxygen fills the air
    • Unless volcanoes make oxygen

11. Oxygen in the air over time (Smithsonian, NASA)

    Time	Oxygen in the air
    before about 2.7 billion years ago	almost none
    about 2.4 billion years ago	first fills the air (NASA)
    about 600 million years ago	about one-fifth of today's level
    today	21%

12. 10.

    Earth is about 4.6 billion years old. For about what fraction of that time has its air held oxygen (since about 2.4 billion years ago), by Ari's math?

    • The whole time
    • About half
    • About 2.4%
    • About one-tenth