In any solar system, there are planets and these planets have got their own natural satellites surrounding them which keep on orbiting these planets.The Moon is actually a natural satellite that is closest to the earth and this natural satellite keeps on orbiting the planet earth all around the year. Now earth being a planet has an atmosphere, but moon being a satellite has got no atmosphere and various reasons can be given in order to support this answer some of which can be explained as follows:-
Why Moon Has No Atmosphere
1. Moon is actually surrounded By a Vacuum:-
Talking about the moon, it has no atmosphere and instead of the atmosphere, there is a total vacuum surrounding it. The gases need some medium to form atmosphere and the barrier of gravity proves to lag behind the moon in terms of having an atmosphere because of which even the astronauts are not able to talk with each other over the moons as there is no medium for sound to travel and thus their suits are fitted with special devices to make this happen.
2. The Value of Acceleration Due To Gravity is small:-
Talking about the moon, its acceleration due to gravity is too much small that makes it unable to have a hold over the gases that may lead to the formation of the atmosphere and thus moon does not have any atmosphere at all. The gases get escaped and moon has got vacuumed.
3.The Value of Escape Velocity is also small:-
The value of escape velocity is also small due to the small value of acceleration due to gravity and thus there is no presence of the atmosphere over the moon as it is not able to have a hold over the gases needed to make the atmosphere.
4. All the Molecules of Gases Actually Get Escaped:-
The value of the R.M.S velocity of the molecules of different gases is much above the value of escape velocity on the moon and thus the molecule of gases that were needed to make atmosphere or result in the formation of atmosphere actually got escaped from the moon and thus it became surrounded by vacuum.
5. Actually Atmosphere Is Composed Of Gases That Can Be Held only With the Help of Gravity:-
In order to make atmosphere possible with the moon, we will need to provide it artificial gravity that has not been possible till yet.The Atmosphere is actually nothing but a collection of gases held together that only gravity can make possible and as gravity is not possible with the moon, even the atmosphere is not possible.
6. The Gravity Depends On Mass:-
Actually, it is not that the moon does not have any gravity of its own, but it is that gravity depends on the mass of the object and the size of the moon is small as compared to the earth what proves to keep it a step back than earth in terms of having an atmosphere over it.