Why is Saturns moon Titan the only moon in the solar system with an atmosphere?
Some other moons have faint traces of gas above their surfaces, but Titan is the only one with a proper atmosphere. It is several times’ denser than Earth’s, and made mostly of nitrogen and methane. It is thought that all large bodies in the solar system once had an atmosphere, but in most cases it soon leaked away into space. Titan, however, has relatively powerful gravity, helping it to hold onto its gas. It is also much cooler than many other moons, such as Earth’s moon where the higher temperatures closer to the Sun would have rapidly evaporated an ancient atmosphere. But one mystery remains: two moons of Jupiter – Ganymede and Callisto – lost their atmospheres, even though they are as large and almost as cool as Titan. It’s possible that the high rate of asteroid and comet impacts stripped them away.
