The masses and sizes of the Moon and Titan (the satellite of Saturn) are approximately equal, but Titan has an atmosphere, while the Moon cannot. Why?
The equality of the masses and sizes of the Moon and Titan means the approximate equality of the second cosmic velocity required to overcome the body’s gravity and exit into outer space.
Whether gas molecules near the surface of the Moon or Titan will have such a speed depends on temperature conditions. The moon, which is much closer to the sun, is more heated, and the molecules leave it without forming an atmosphere. On cold Titan, the gas envelope is stable and dense enough.
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