Astronomy: Dr. Sun & the Moon

Generations of schoolboys who have been taught that moonlight is nothing more than reflected sunlight may well have been misinformed. More and more scientists have become convinced that the moon occasionally generates light of its own. During periods of intense solar activity, say modern astronomers, high-energy protons expelled from the sun strike luminescent meteorite material on the lunar surface, and the collisions cause some areas of the moon to glow. Now a Chinese-born, Westinghouse Electric Corp. scientist has gone a step further. An ever-shifting, narrow strip of the moon, he believes, constantly emits a glow of its own.

Lunar Bombardment. Writing in Nature, Physicist Kuan-Han Sun suggests that a combination of the solar wind, meteorites, and lunar temperature changes provide ideal conditions for thermoluminescence—the release of stored-up energy in the form of visible light during a rapid temperature rise. Like other bodies in the solar system, Sun points out, the moon is constantly bombarded by a solar wind consisting of charged, low-energy particles boiled off the solar surface and "blown" into space. Because these particles, which are mostly protons, follow magnetic lines of force, they can strike the moon from all directions, hitting its dark side as well as the side that faces the sun.

On the sunny side, where temperatures rise as high as 250°F., the luminescent meteorite particles that litter the lunar surface give off a small amount of light as soon as they are struck by the solar protons. On the dark side, the meteorites cannot luminesce because of the—240°F. cold; instead, they absorb the energy of the protons. During the two-week lunar night, Sun estimates, one pound of meteorite particles would soak up more than enough energy to burn ten 100-watt light bulbs for one hour.

The revolving moon brings lunar dawn, and temperatures rise quickly. The meteorites give up their stored energy in the form of visible light. Thus, Sun suggests, in a strip less than 100 miles wide alongside the lunar terminator—the line that divides the moon's areas of day and night—the moon emits light of its own, which may be almost as intense as its reflected sunlight.

Simulated Solar Wind. To test his theory, Sun borrowed samples of meteorites believed to be similar to those on the moon. Using liquid nitrogen, he cooled them to—320°F. and bombarded them with high-energy electrons that simulated the impact of solar-wind protons for a 14-day period. No glow was produced. When Sun removed the liquid nitrogen and rapidly heated the samples, however, they began to give off vivid and pulsating light. The Westinghouse physicist is now working on further laboratory tests to support his theory. He believes that it can also be confirmed by careful telescopic analysis of light emanating from the vicinity of the lunar terminator. If, as he suggests, that band of moonlight is noticeably brighter than the rest of the moon's daytime surface, he will always be remembered as the Sun who cast new light on the moon.