An atmosphere is a layer of gas that surrounds a planet or moon, held in place by gravity. On M-class planets, the lowest layer of the atmosphere is called the troposphere.

The atmosphere of Earth is primarily composed of nitrogen and oxygen, with notable amounts of water vapor, carbon dioxide, and noble gases.

In an alternate version of 1944, the NX-class starship Enterprise entered Earth's atmosphere in order to destroy a Na'kuhl temporal conduit. (ENT: "Storm Front, Part II")

In 2152, while sending a shuttlepod to meet with the mining colonists of Paraagan II, Captain Jonathan Archer and Lieutenant Malcolm Reed apparently ignited the tetrazine-filled atmosphere, wiping out the entire colony. (ENT: "Shockwave")

In 2161, Ensign Travis Mayweather piloted Shuttlepod 2 from Enterprise to Rigel X, a planet in the Rigel system. Upon entering the planet's troposphere, he warned the shuttlepod's occupants, an away team from Enterprise, that the craft might experience some minor turbulence. (ENT: "These Are the Voyages…")

In 2269, Doctor Leonard McCoy discovered that the former immortal Flint had lost his immortality because he had left the Earth's atmosphere. The Earth's atmosphere is what had made it responsible for Flint to have instant tissue regeneration that made him, on Earth, immortal. (TOS: "Requiem for Methuselah")

In 2364, the USS Enterprise-D was able to defeat a cloaked drone of the Echo Papa 607 weapons system by luring it into the atmosphere of Minos, where it became visible. (TNG: "The Arsenal of Freedom")

In 2369, the space station Deep Space 9 lost its atmosphere due to several power failures. Commander Benjamin Sisko ordered the decompression protocol and Chief Miles O'Brien stabilized the systems. (DS9: "Q-Less")

In 2372, a Karemma vessel attempted to hide from two Jem'Hadar attack ships in the atmosphere of a gas giant. The USS Defiant followed and was able to destroy both Jem'Hadar ships, despite its systems being compromised by the ambient conditions. (DS9: "Starship Down")

In 2376, Captain Rudy Ransom of the USS Equinox evaded the USS Voyager by taking his ship into the atmosphere of an M-class planet. (VOY: "Equinox, Part II")

The graviton ellipse, discovered by the crew of Voyager in 2376 was found to have a primitive atmosphere. However, according to Ensign Tom Paris, it wasn't anything anybody would want to breathe in. (VOY: "One Small Step")
An atmosphere (New Latin atmosphaera, created in the 17th century from Greek ἀτμός [atmos] "vapor"[1] and σφαῖρα [sphaira] "sphere"[2]) is a layer of gases surrounding a material body of sufficient mass[3] that is held in place by the gravity of the body. An atmosphere is more likely to be retained if the gravity is high and the atmosphere's temperature is low.

Earth's atmosphere, which contains oxygen used by most organisms for respiration and carbon dioxide used by plants, algae and cyanobacteria for photosynthesis, also protects living organisms from genetic damage by solar ultraviolet radiation. Its current composition is the product of billions of years of biochemical modification of the paleoatmosphere by living organisms.

The term stellar atmosphere describes the outer region of a star, and typically includes the portion starting from the opaque photosphere outwards. Relatively low-temperature stars may form compound molecules in their outer atmosphere.

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