Although the cause of lunar rays is well understood, some oddities defy easy explanation. One such feature can be found east (right side of the image) of the conspicuous craters Kraft (left) and Cardanus (lower left). What accounts for the northward kink in the most prominent ray?

lunar notebook | By Charles A. Wood

Lunar Ray Mysteries

Even the most casual moongazer will notice the numerous milky-white crater rays so conspicuous at full Moon. Classical selenographers widely debated the origin of these rays. Walter Goodacre stated that they were "probably the greatest of all the unsolved enigmas presented by the lunar surface." Yet rays were one of the few lunar features whose origins were approached in a scientific manner by early selenographers. Careful observations demonstrated that rays were centered on and extended from bright craters, had lengths proportional to their source crater's diameters, were discontinuous, crossed over high and low areas without deviation, and often contained tiny bright crater pits. Based on these observations and terrestrial experiences, two interpretations emerged, both based on a volcanic origin for the source craters. In the first case, huge eruptions flung bright ash vast distances. Alternatively, the volcanic action that formed the craters also fractured a brittle lunar crust, allowing volcanic ash, dust, or even salts to escape from small cracks.

Where to find it

But we now know that source craters are not volcanoes, so what are the rays? The reason rays were misinterpreted is that impact cratering was inconceivable, and thus so were the immense distances that rocks could be ejected during an impact. Eugene Shoemaker's explanation of an impact origin of secondary craters led him naturally to describe the formation of rays as "feather-like splashes of crushed rock derived chiefly from the impact of individual large fragments or clusters of fragments." High-resolution photographs from Apollo cameras beautifully show that rays are the splashes of ejecta around small secondary craters.

Although we understand the mechanism responsible for crater rays, some of these features retain their air of mystery. For instance, when the Moon is nearly full, look at the long, bright rays that extend from near the northwest limb across Oceanus Procellarum. One peculiar ray seems to extend from near the crater Cardanus, passes Seleucus crater, and skirts the northern end of the Aristarchus Plateau. The ray is very unusual and is perhaps composed of unrelated ray segments. For example, the southwestern end of the ray is broken and not very straight; like other nearby rays, it probably is from the very bright crater Olbers A that is scrunched near the limb. But between Cardanus and Seleucus the ray bends in a shallow arc opening to the northwest and then abruptly straightens and heads northeasterly across Procellarum for an other 400 kilometers. Why does the ray bend? I don't know. However, the long stretch of the ray does radiate from Olbers A. Note that another ray segment passes south of Seleucus, grazes Schiaparelli, and disappears at the Aristarchus Plateau. This ray does not point back to Olbers A and has no obvious source.

To the north another strange ray appears as a straight line stretching from near the small crater Briggs B to south of Lichtenberg. While this ray also lacks a source, the mare material north of it seems darker than that to the south. In 1966 the great U.S. Geological Survey Moon mapper Don Wilhelms proposed that this strange ray was actually a "very long linear fracture" that was the source of the lava that buried ejecta from the young crater Lichtenberg. Alas for this fractured theory, this ray was measured by the German selenographer Julius Franz early this century. He projected it and a faint companion south of Struve around the far side of the Moon and predicted a source crater at 107 degrees W, 19 degrees N. Lunar Orbiter photographs reveal the fresh ray crater Ohm at 113 degrees W, 18 degrees N! Sadly, Franz never lived to be vindicated, but the strange ray is perfectly normal, luring us around the limb to the far side.

October 1999

CHARLES WOOD is an avid Moon watcher and a researcher specializing in planetary science at the University of North Dakota.