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  • Explain why (a) there is no atmosphere on the moon.

Explain why
(a) there is no atmosphere on the moon.
(b) there is a fall in temperature with altitude.

Answers (1)

a) The acceleration on the moon due to gravity is one-sixth that of Earth

escape velocity on the moon can be written as: V es = \sqrt{2gR} = 2.38 km/s

mass of hydrogen, m = 1.67 x 10-24 kg

v (rms) =\sqrt{ \frac{3 Kb T}{m}} = \sqrt{\frac{3 \times 1.38 \times 10^{-23}\times 300}{1.67 \times 10^{-24}}} = 2.72 km/s

v(RMS) is larger than the escape velocity as the gravitational force is small.  The sun-moon distance is fairly equal to that of the earth. The energy of the sun reaches in a larger intensity to the moon due to its lower density of the atmosphere. So, the molecules of sunlight have a larger RMS speed and hence can escape out fairly easily. Hence, the moon has lost its atmosphere over time.

b) The kinetic energy of the air molecule gives the characteristic temperature to the atmosphere.  At greater heights, atmospheric pressure is low, which makes air molecules rise higher, which in turn leads to an increase in their potential energy and decrease in their kinetic energy. Since the overall kinetic energy decreases, the temperature also decreases. Also, the lower atmospheric pressure causes expansion of the gas, which also provide a cooling effect.

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