The blanket of air
Air is all around us. Why does it not escape into space? Gravity is the pull of a planet on things near it. UV is short for ultraviolet.
- Gravity pulls on the air. It stays close to Earth.
- The ozone layer blocks harmful UV rays from the Sun.
- The greenhouse effect traps some heat. Earth stays warm.
The rule
Gravity pulls the air toward Earth, so the air stays.
The atmosphere is the layer of air around a planet. Ozone is a form of oxygen. UV rays are harmful rays from the Sun. Density is how tightly matter is packed.
Earth's gravity is strong enough to keep its air. A much smaller planet, of the same density, has weak gravity. It can lose its air. The ozone layer blocks harmful ultraviolet rays.
Mars has air more than 100 times thinner than Earth's. Mercury has almost no air. The greenhouse effect is heat trapped by some gases in air. Without air, Earth would lose heat and turn too cold.
Watch out!
A bigger planet is not always better. Gravity that is too strong could crush our bones.
Pick a size. Switch the ozone layer on and off.
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