Explain why there is no atmosphere on moon

cbse
kinetictheoryofgases

#1

Explain why
(i) there is no atmosphere on moon.
(ii) there is fall in temperature with altitude.


#2

(i) The moon has small gravitational force and hence the escape velocity is small. As the moon is in the proximity of the earth as seen from the sun, the moon has the same amount of heat per unit area as that of the earth. The air molecules have large range of speeds. Even though the rms speed of the air molecules is smaller than escape velocity on the moon a significant number of molecules have speed greater than escape velocity and they escape.
Now, rest of the molecules arrange the speed distribution for the equilibrium temperature. Again a significant number of molecules escape as their speeds exceed escape speed. Hence, over a long time the moon has lost most of its atmosphere.
(ii) As the molecules move higher their potential energy increases and hence kinetic energy decreases and hence temperature reduces.
At greater height more volume is available and gas expands and hence some cooling takes place.