ATMOSPHERE.

Imaginary Picture for Jupiter's Atmosphere

 The Atmosphere is a word that goes back it's origins to the ancient Greek, (Atmos) which means Vapor, and the word Sphere means (Spheria) or ball, which by definition means a group of layers that composite on layer of gases that surround a spherical shape or planet that is held in it's place due to the force of gravity of that body or planet.
The atmosphere of any planet stability is a subject of combinations of conditions surrounding planetary conditions, which depends on the planet Mass, Gravity and surrounding planets and it's distance from it's own Star (Sun) whether it's in a habitable zone plus or minus that will absolutely affect the atmosphere condition and stability. Our Earth's atmosphere is somehow stable because of Earth's position in a habitable zone and because of the Mas of our Sun and the suitability of our solar planetary system size variation and also of course gas combination nitrogen (about 78%), oxygen (about 21%), argon (about 0.9%), carbon dioxide (0.04%) and other gases in trace amounts that forms our atmosphere which by it's turn make our planet somehow warm and safe for living creatures.

Photosynthesis Process.

 What keeps these combination fixed is the percentage of each gas consumption on the planet by elements and living organisms The Oxygen is used by the living creatures for Breathing and respiration of the living Plants and animals, the Nitrogen is fixed by the bacteria and by the lightning to produce Ammonia, and the carbon is used by the plants for respiration and for the Photosynthesis process.
The atmospheric pressure Varies between the layers depending on the gases masses and the temperature that diminishes gradually when going up from Earth surface for example if you try to climb a hill top or a mountain top the gas mass decreases exponentially every  specific you climb more.

Earth's Atmosphere.

The earth's Gravity force and the speed of Earth spinning around it's axis makes it not so easy for our atmosphere to escape, otherwise it keeps is in the interactive condition with the planet living and chemical components.
https://en.wikipedia.org/wiki/Atmosphere