Observing the Moon: Chains of Craterlets
A mental exercise practiced by habitual observers of the moon is to try to figure out what caused certain features. If you read any literature about the moon, you'll find that professional astronomers do not always agree among themselves. One such feature is the strings, or chains, of tiny craters that are particularly abundant on certain areas of the lunar surface. There are quite a few of these chains that radiate in almost straight lines from the walls of the prominent crater Copernicus.
Most are so small that they can be seen only with a fairly large telescope that has excellent resolution. These are generally explained as the result of debris that was ejected by the massive impact from the object that created Copernicus. Pieces of the debris probably broke apart and/or skipped across the surface creating strings of tiny craters. Other chains of small craters that do not radiate from a nearby impact site can be explained as the result of the low-angle impact of a random piece of rock either from space or from an impact site much farther away.
But curiously, some of these chains are not oriented in a straight line, as one would expect. In the following image of Copernicus, notice the chain that meanders up from the lower right; this is just one of many that is visible in amateur-sized telescopes.
In a 4-inch refractor at 150x, additional craterlets were visible here, making the chain appear more as a continuous arc. If this chain were the result of ejecta, why is there a gentle curve to the line of craterlets? One explanation I've read is that some of these may not be a string of impact craterlets at all, but a series of sinkholes caused by subsidence beneath the crust. In effect, this chain would really be a type of rill. Whatever the cause, they're interesting to observe, while pondering what was going on back in the early, violent days of the moon.Published in the December 1998 issue of the NightTimes