National Geographic : 1969 Feb
18th-century Jesuit astronomer, Maximilian Hell of Vienna. When the moon is full, another feature becomes strikingly visible-the enormous sys tem of bright rays flaring in all directions from several of the youngest and most prominent craters: Copernicus, Kepler, and-most nota bly-54-mile-wide Tycho, whose rays can be seen with the naked eye (page 235). The rays seem to be ejecta-debris sprayed from a crater at the time of its formation, like milk splashed out of a pan when a ball is thrown forcefully into it-and scars opened up by the ejecta. Probably, say most scientists, these large craters were created by the im pacts of comet nuclei or asteroids. The object that blasted out Tycho is esti mated to have been at least two miles in diam eter. So violent was the impact that "the moon must have shaken like a bowl full of jelly," to quote Harold Masursky of the U. S. Geologi cal Survey, one of the principal interpreters of the Lunar Orbiter photographs (page 212). Some of Tycho's rays reach more than 1,000 miles. Because there is no air resistance, be cause the surface gravity is low (1/6th that of earth), and because of the sharp curvature of the moon, ejected material travels up to ten times as far on the moon as it would on earth. Less prominent and far more puzzling are the sinuous rilles, strange narrow channels or valleys that meander like rivers for as much as 200 miles (page 237). Scientists are truly perplexed by these features. They seem to originate in craters, and some specialists think they were carved by lava or ash flows. To others they suggest underground lava or wa ter channels that have collapsed or subsided. And a few say that only surface water could have cut such distinct, wandering channels. Many observers suspect that the moon holds large quantities of water in the form of per mafrost extending deep under the insulating surface layer. If the water rises to the surface, however, it is difficult to explain satisfactorily what pre vents its swift disappearance by evaporation in the moon's intense vacuum. Possibly this sudden evaporation cools the near-surface water so much that it freezes into a protective layer of ice and mud. Beneath this temporary shield, flowing water might gradually etch a rille. In all cases the rilles simply peter out; none shows a delta such as would character ize a similar stream on earth. Sinuous rilles are comparatively rare; only fifty or so are known. A more common and much different kind of channel, called linear rille, goes in a relatively straight line instead of meandering. Linear rilles represent tension cracks or faults in the crust. More than a thou sand of them have been catalogued. Occasional low domes add variety to the lunar surface, especially in the region known as the Marius Hills, near the crater Marius (4H on the map). As much as 6 miles across and 1,000 feet high, they resemble low vol canic domes on earth. They provide signifi cant evidence that at least part of the moon's formations are volcanic (page 220). Approximately 5,000 markings on the bat tered face of the moon have been given names by the International Astronomical Union, which must approve all lunar nomenclature. Only a handful of the myriad formations on the back side have yet been officially tagged, and these bear names suggested by the Rus sians, whose Luna 3 first photographed that side in 1959. s I write these words, the full moon has just risen above the buildings across the way. From my office window I can view the black-and-silver face in all its glory. I am intrigued by the thought that, although I can not see them, some very special and expen sive instruments designed by man lie at 23 different sites scattered across that glittering moonscape. For that is the number of space craft-17 U. S. and 6 Soviet-that have crashed or soft-landed on the moon as of this autumn night. With the best of earthbound telescopes (and the best are very good indeed), lunar photographs have been able to show nothing smaller than 800 feet across. That is roughly 215 He made an Instrument to know If the Moon shine at full or no... And prove that she is not made of Green Cheese. SAMUEL BUTLER, fHUDIBRAS"