question_answer 41)

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

Answer:

                  (a) The low escape speed (2.38 km/s) for the Moon is the reason that it has no atmosphere. Even though the rms speed of the air molecules, which lies between 0.4 km/s to 0.8 km/s for its constituents (\[{{O}_{2}},\,{{N}_{2}},\,C{{O}_{2}}\] and water vapours),  is smaller than 2.38 km/s, a significant number of molecules have speeds greater than 2.38 km/s and hence escape. When the rest of the molecules arrange their speed distribution for equilibrium temperature, again there is a significant number of molecules which have speeds greater than 2.38 km/s and escape the have speed greater than 2.38 km/s and escape the Moon in the same way as their predecessors. Due to this continual escape, the Moon has lost its atmosphere. (b) As the altitude increases, the potential energy of air molecules increases and hence their K.E. decrease. Since average K.E. of air molecules is directly proportional to the temperature of the air. Therefore, temperature of the air decreases with altitude.