The Earth's atmosphere is held by gravity and is composed mainly of nitrogen and oxygen, with smaller amounts of other gases. It becomes thinner with altitude, and the boundary with space is generally considered to be at 100 kilometers.

What force keeps the Earth's atmosphere in place?

• Gravity

• Magnetism

• Friction

• Inertia

What is the primary component of the Earth's atmosphere?

• Oxygen

• Carbon Dioxide

• Nitrogen

• Argon

Which gas makes up about 21% of the Earth's atmosphere?

• Helium

• Carbon Dioxide

• Oxygen

• Nitrogen

As altitude increases, what happens to the Earth's atmosphere?

• It remains the same

• It becomes denser

• It disappears completely

• It becomes thinner

At approximately what altitude is the boundary between the Earth's atmosphere and space generally considered to be?

• 200 kilometers

• 150 kilometers

• 100 kilometers

• 50 kilometers