Question

There is no atmosphere on the moon because:
A) It is closer to the earth
B) It revolves around the earth
C) It gets light from the sun
D) The escape velocity of gas molecules is less than their root mean square velocity on the moon.

Answer 1

We have an atmosphere on the earth because of the gravitational force and the nonexistence of atmosphere on the moon is because of the absence of the gravitational force. The escape velocity of the gas molecules present on the earth is greater than their root means square velocity whereas it is not on the moon. So, there is no atmosphere.

Complete step by step answer:
Many planets in our solar system have an atmosphere. In fact, our planet Earth itself has a very thin skin of atmospheric surrounding. Gravitational force of the earth holds on to this thin skin of the atmosphere. The minimum speed with which an object should be thrown such that it can escape a gravitational field of the planet or satellite is called as escape velocity of that planet or satellite. The escape velocity of a moon is 2.38 km/s. This is less than the root mean square velocity of molecules at the moon. So they easily escape from the moon's gravitational field. Thus, the right answer is Option D.

Note: Lighter molecules like hydrogen can escape easily from earth’s gravitational field as their root mean square velocity is more than the escape velocity of the earth. Escape velocity of a molecule must be greater than the molecule’s root mean square velocity, so that they cannot escape from the gravitational field.