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One last problem remains. If 300 small comets do hit the moon everyday, where some people wanted to know, was all the water on the moon? One of the strongest conclusions from the study of the lunar rocks brought back by the astronauts is that the moon is remarkably dry. But this problem is very simply resolved. The lunar gravity is such that practically all the water vapour from the impact of small comets simply flies off. But some of the water molecules may wander around and eventually condense in the crevices near the poles. A number of scientists such as James Arnold, a chemist at the University of California in San Diego, have argued that water and ice may be trapped in the permanently shadowy region near the lunar poles. The small comets may be a source of that ice.

Because most of the water flies off, a huge envelope of water should surround the moon. The earth has its own tenuous shroud of hydrogen gas lying beyond the atmosphere. The impact of the small comets on the moon’s surface gives our companion a hydrogen shroud also. The cometary water vapour is hot enough that it describes the moon’s gravity and moves outward into interplanetary space. After travelling for about a day from the moon these water molecules are broken apart by solar radiation into hydrogen and oxygen. These molecules will have traveled on the average, over half a million miles. This means that our planet is sitting in a very tenouse lunar atmosphere, an atmosphere of water, hydrogen and oxygen. Unlike the known comets which expend their own water supply, the moon merely releases the water deposited by small comments that collide with it. Yet for a space traveller viewing the earth from a distant vantage point, the moon should resemble a faint comet.

Excerpted from pages 149-150 of ‘The Big Splash’ by Louis Frank