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An illuminating look at the full moon.

Just as the full moon makes its monthly debut, the brightness of the lunar surface rises dramatically, far exceeding the luminosity of four quarter moons. For more than a century, astronomers have attributed this surge to a phenomenon known as shadow hiding, in which particles the size of sand grains on the moon's rocky surface play the dominant role in reflecting sunlight.

As seen from Earth, sunlight strikes a less-than-full moon at an angle, not head on. In the shadow-hiding scenario, this illumination causes sand-grain-sized dust particles on the lunar surface to cast shadows on neighboring particles, making the moon look darker from Earth. In contrast, when the moon is full and sunlight strikes head on, the shadows are hidden by the particles that cast them. This would seem to account for the full moon's enhanced brightness.

Now, however, astronomers report that an entirely different phenomenon causes the jump in brightness. Simulating the sun by shining laser light on lunar soil samples, these researchers found that a mechanism called coherent backscattering accounts for the brightening. In backscattering, smaller, soot-grain-sized particles that are stuck to the sand-sized particles on the lunar surface play the featured role. When the moon is full, certain rays reflected by the smaller particles pair up to produce an intensity of light greater than the two rays could produce separately. Thus, the full moon appears brighter than expected, explains Bruce W. Hapke of the University of Pittsburgh.

Coherent backscattering may also account for the brightness surge of other planetary moons, Hapke says. If so, the surface character of each moon may differ from that suggested by reflection measurements. Hapke speculates, for example, that Jupiter's moon Europa may have a fluffier layer of surface ice than scientists thought.

He and his co-workers, Robert M. Nelson and William D. Smythe of the Jet Propulsion Laboratory in Pasadena, Calif., describe their work in the April 23 SCIENCE.
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Title Annotation:coherent backscattering rather than shadow hiding accounts for disproportionate full moon brightening
Publication:Science News
Article Type:Brief Article
Date:May 22, 1993
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