There is no atmosphere on the moon. It is because

The reason is The gravitational force is comparitively small. So, the escape velocity is small and gases have lesser gravitational force. 

Concept:

Escape velocity

• It is the minimum velocity with which a body should be projected from the surface of the planet so that the body reaches infinity.
• It is given as,

\(⇒ v_{e}=\sqrt{\frac{2GM}{R}}\)     

where ve = escape velocity, G = gravitational acceleration, M = mass of the planet and R = radius of the planet

Explanation:

Given - mass of the moon (Mm) = 7.4×1022 kg, radius of the moon (Rm) = 1.75×106 m and G = 6.67×10-11 Nm2/kg2

• So the escape velocity of the moon is given as,

\(⇒ v_{e}=\sqrt{\frac{2GM_{m}}{R_{m}}}\)

⇒ ve = 2.38 km/s

• Since the escape velocity of the moon is less than the root means square velocity of the gas molecules on the moon, so there is no atmosphere on the moon.