2.4 The atmosphere and polar ice

David A. Rothery Teach Yourself Planets, Chapter 6, pp. 66-75, Hodder Education, 2000, 2003.

Copyright © David Rothery

The Moon's atmosphere is almost as insubstantial as Mercury's, and probably has much the same origin. The Clementine mission returned our first clear views of the lunar poles, showing sites in particular near the south pole that are permanently in shadow, and which could therefore be places where ice might accumulate (Figure 1). Clementine's simple radar gave the first indications that water is present there, and this theory was dramatically backed up by measurements by Lunar Prospector that show a reduction in the average speed of neutrons (produced by cosmic radiation) over both lunar poles. The only reasonable explanation of this seems to be collisions between neutrons and the hydrogen atoms within ice molecules, and it now seems that there may be as much as 3 billion tonnes of ice mixed with the regolith at each pole. This is not a lot in terms of the size of the Moon (it is the equivalent of a 1.5 km cube of ice at each pole), but could be ample to supply the immediate needs of human habitation on the Moon.

Figure 1 a view unobtainable from the Earth, this is a 500-km-wide area centred on the lunar south pole assembled from Clementine images[.] Much of the shadow here is permanent, and corresponds to areas where strong signs of ice have been detected[.] The lunar near side is towards the top, and the far side towards the bottom