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Question
Explain why there is no atmosphere on moon.
Solution
The moon has a small gravitational force (pull) and hence, the escape velocity is small. The value of escape velocity for the moon is 4.6 km/s.
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 a large range of speeds. Even though the rms speed of the air molecules is smaller than the escape velocity on the moon, a significant number of molecules have speeds greater than escape velocity and they escape.
Now, the rest of the molecules arrange the speed distribution for the equilibrium temperature. Again a significant number of molecules escape as their speed exceeds the escape speed. Hence, over a long time, the moon has lost most of its atmosphere.
At 300 K, `V_(rms) = sqrt((3kT)/m)`
= `sqrt((3 xx 1.38 xx 10^-23 xx 300)/(7.3 xx 10^-26)`
= 1.7 km/s
Vesc for moon = 4.6 km/s ......[Vesc = escape velocity]
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