On the possible presence of ice on the Moon

It is generally presumed that gases of low molecular weight escape very rapidly from the moon. As a consequence, it has been assumed that volatile substances, such as water, which possess short relaxation times for escape, do not exist there. Urey [1952] recognized that there may be depressions in which the sun never shines, and in which some condensed volatile substances could be present, but he concluded that no solid or liquid water could exist on the moon for more than very short periods of time. Kuiper [1952], following a suggestion by Herzberg, stated that SO_2 molecules might be concentrated on the night side of the moon.