Detailed Explanation:

The Moon lacks an atmosphere because its escape velocity (2.38 km/s) is lower than the root mean square (rms) velocity of gas molecules like hydrogen and helium (1–1.5 km/s).

When the rms velocity exceeds escape velocity, gas molecules possess enough kinetic energy to overcome the Moon's weak gravitational pull and escape into space. In contrast, Earth's escape velocity (11.2 km/s) is sufficiently high to retain atmospheric gases.