The atmosphere is held together by

Earth's gravity

Explanation:

Atmosphere is formed by the set of different layers of gases together, thus forming atmosphere. The Earth's atmosphere protects us by absorbing the UV rays coming from the Sun. These gases are held together forming an envelop because of the gravity of the Earth. It mainly comprises of 78% of Nitrogen, 20.9% of the oxygen, 0.9% of argon, approximately 0.035% of Carbon dioxide, water vapor, and other gases present in negligible amount.

Our atmosphere is a mixture of gases that surround Earth. It is kept in place by the pull of Earth's gravity. If Earth was a much smaller planet, like Mercury or Pluto, its gravity would be to weak to hold a large atmosphere.