[AntonioLao]

But how soon, maybe never, is it possible to make it rain on the moon? The answers are yes and no depending on the definition of the word rain. If it is defined as water droplets then the answer is no. On the other hand, if it is defined as micrometeorite showers then the answer is yes.

Earth’s atmosphere spares it from impacts of these micrometeorites or even larger chunks of these interstellar gravity’s strays. Annually, about 100 000 000 kilograms entered the earth increasing its mass by 1% every 600 000 000 000 000 years. Since the moon is 1/81 of earths mass it is reasonable that about 1 million kilograms of meteorite materials is added to its mass each year. However, the airless moon could not prevent larger meteorites from creating lunar impact craters. Some are large enough to be seen thru terrestrial telescopes. Fortunately, the impact frequency has diminished since the era of the planetesimals http://en.wikipedia.org/wiki/Planetesimal. Therefore, as the moon grows older there are less and less of these larger impacts analogous to facial pimples of older adolescents that practically disappear upon reaching young adulthood. http://en.wikipedia.org/wiki/Moon