Two other ray craters you’ll be able to spot easily are Kepler (to the left of Copernicus) and Proclus (near the western edge of the Sea of Crises). And there are literally thousands of smaller craters — some with rays, some without — that pop into view during the full Moon as bright spots, peppering the dark maria like stars in a stony sky. Some of these are visible only as bright rings, marking the crater rims.
The fact that we can see these crater rays so clearly has to do with their relative youth, geologically speaking. Scientists now know that the uppermost portion of the lunar surface is gradually darkened over time by the constant bombardment of the solar wind, a stream of high-energy subatomic particles emitted by the Sun. Then there’s the continual churning of the topmost surface by micrometeorite impacts, which mixes light-colored dust created by recent impacts into the older dust darkened by the solar wind.
Over many millions of years these darkening and mixing effects erase ray patterns, and they darken the bright rims and walls of young impact craters. The rayed craters we see didn’t form yesterday — Tycho is estimated to be 100 million years old, and Copernicus 800 million — but recently enough that these dark-ening and mixing effects haven’t wiped away their bright, feathery mantles.
The Makeup of Maria
If you’re starting to get the idea that the full Moon gives us a window into how our satellite has changed over time, you’re right. And when we look at the lunar maria under full illumination, these dark expanses of frozen lava let us glimpse how the Moon’s volcanic activity has evolved. Notice that the two big ones on the left side of the Moon — the Ocean of Storms and the Sea of Rains — are slightly lighter in color than the ones on the right side. You’ll notice an especially striking difference in brightness along the southeast-ern edge of the Sea of Serenity, where a band of dark lava stands out against the lighter-hued expanse that makes up most of this mare deposit.
These differences stem from variations not only in age — older mare deposits are generally darker than younger ones — but also in chemical makeup. Unlike the bright lunar highlands, which are largely com-posed of rocks rich in light-colored minerals, the maria are formed from basalt, a volcanic rock whose min-erals are generally dark. But basalts have various compositions too; for example, more abundant titanium creates a darker basalt. In the Sea of Serenity, the dark band is relatively rich in titanium, and, at 3{1/2} billion years old, it’s been on the surface about 500 million years longer than the lighter mare plains.
Higher magnification (use eyepieces with smaller numbers on their barrels) will show you a host of in-triguing features, like a dark ring surrounding Tycho that stands out against the bright ray pattern. The dust in this band is enriched with tiny, dark, glassy droplets that formed during the heat of the impact. Then there are the small, very dark patches on the floor of the crater Alphonsus, arranged at the points of an imaginary right triangle, that are deposits of volcanic ash. And be sure to check out the crater Aristarchus, which you’ll find to the upper left of Copernicus and Kepler. This brilliant crater is the brightest lunar fea-ture we can see from Earth. The impact that formed Aristarchus punched through the coating of mare lavas into the lighter-hued crustal rock beneath; the crater’s walls and floor are now covered with this bright de-bris.
And finally, move southward from Aristarchus to the western edge of the Ocean of Storms to spot the feature called Reiner Gamma, a small bright oval that is one of the Moon’s most mysterious features. It’s actually a collection of swirl-shaped markings that have no topographic relief, as if they were painted on the mare surface. Measurements made from orbiting Apollo spacecraft revealed that, mysteriously, Reiner Gamma is unusually magnetic. Scientists believe this strong local field may deflect the incoming solar wind from striking the swirls, allowing them to remain bright. And no one can say what material makes up the swirls; they’re not part of a system of crater rays or any other feature that might explain their origin.
With all this to explore, you’ll have more than enough to fill out a night of detailed observing. Keep that in mind the next time you hear another stargazer complain that there’s nothing to look at when the Moon is full. You might just win a convert.
Andrew Chaikin, author of 'A Man on the Moon,' explores the Man in the Moon from his home in southern Vermont.-- Return to Page 1© 2006 Reprinted with permission from Sky Publishing Corp.
The surface of the Sea of Serenity (Mare Serenitatis) must have been flooded with molten rock multiple times, because the flows near its margin are darker (due to their higher titanium content).
The enigmatic bright swirl known as Reiner Gamma is a puzzle to lunar geologists. About 20 miles across, it coincides with a region of enhanced magnetism. (Thierry Legault)
