1998 March, NASA announced the discovery of water on the moon. The Lunar Prospector satellite detected the presence of hydrogen near the moon’s poles?and the most likely source of the hydrogen is water.

The satellite was equipped with a neutron spectrometer, an instrument that detects neutrons by capturing the particles as they bounce off the surface of the moon. Neutrons are created when cosmic rays from outer space bombard the surface of the moon. Since neutrons carry no electrical charge, they are difficult to detect. But they lose energy when they collide with another substance. If the substance is a particle of similar size, such as hydrogen nucleus (a proton), they lose so much energy that they become “slow neutrons,” as opposed to “fast neutrons.”

The neutron detector contains helium-3, an isotope of the element helium. When a slow neutron enters the detector, it reacts with the helium-3 to produce a hydrogen-3, or tritium isotope, a proton, and a burst of energy that is measured by the spectrometer. (1) The nuclear reaction is as follows:

The detection of slow neutrons by the neutron spectrometer indicates the presence of hydrogen nuclei on the moon. The source of these nuclei is thought to be water, which is composed of hydrogen and oxygen.

Image courtesy of The Why Files. c 1998, University of Wisconsin, Board of Regents.

Since the hydrogen was detected near the poles, researchers think that the water is in the form of ice inside craters. Some of these craters are 1400 miles across and 8 miles deep. Shaded from the sun, the crevices within these craters would reach temperatures as low as 40°K. (2) If the water were exposed to sunlight, it would quickly boil away. The extreme temperatures also rule out the possibility that the source of hydrogen is ammonia or methane; since they are gases, at these temperatures they would have evaporated on the surface.

The designer of the neutron detector, William Feldman of Los Alamos National Laboratory, estimates that there might be as much as 500 million metric tons of water on the moon, the equivalent of a lake of water 10 kilometers on a side and 5 meters deep.(1) Has the water been present since the moon came into being? No. The poles of the moon have shifted over geological time, which would have exposed any resident water to sunlight boiling it away. Therefore, the water must have come from some other source. The best guess as to this source is comets, possibly comets made mostly of ice. This may be evidence for Frank’s hypothesis, which states that the earth is being bombarded by “small comet snowballs” many times each day (see Chemistry News, Small Comet Snowballs Bombarding the Earth?).

Analyzing samples of ice from the moon could reveal a long history of cosmic events on the moon’s surface. Further, if there is another expedition to the moon, this water would be a valuable resource?not only of water itself but also of hydrogen and oxygen, which could be obtained by the electrolysis of water: 2H₂O 2H₂ + O₂.

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