Question

why the temperature of moon falls to 90 degree Celsius

Answer 1

There is nothing to hold the heat in the soil and rocks that compose the surface, so the surface is exposed to the temperature of empty space, which is extremely cold. The side of the moon facing the sun is broiling hot also because there is no atmosphere ... nothing to reflect any portion of the sun's energy before it reaches the surface.

Answer 2

Because moon has no atmosphere. Earth atmosphere protects life on the earth by absorbing ultraviolet radiation coming from the sun and maintaining the average temperature on the surface ( not too hot and not too cold) through green house effect. 

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