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Viewing the Moon's Geology

The entire moon is pockmarked with craters of every size, from microscopic pits to basins hundreds of miles in diameter. The largest is the South Pole-Aitken Basin, which is about 1,600 miles (2,600 kilometers) across. Objects (asteroids, meteoroids, and comets) that struck the moon — very long ago, for the most part — caused these craters. The microscopic craters, which scientists have found on rocks brought back by astronauts from the surface of the moon, are caused by micrometeorites — tiny rock particles flying through space. All the craters and basins are known collectively as impact craters to distinguish them from volcanic craters.

The moon has experienced volcanism, but it took a form different from Earth's. The moon has no volcanoes, or large volcanic mountains with craters at the top. But it does have small volcanic domes, or round-topped hills like those that occur in some volcanic regions on Earth. In addition, sinuous channels on the lunar surface (called rilles) appear to be lava tubes, also a common landform in volcanic areas on Earth (such as Lava Beds National Monument in northern California). Most notably, the moon has huge lava plains that fill the bottoms of the large impact basins. These lava plains are called maria, the Latin word for seas. (When you look up and see the Man in the Moon, the dark areas that make up some of his features are the maria.)

Some early scientists thought that the maria could be oceans. But if they were oceans, you could see bright reflections of the sun from them, just as you do when you look down at the sea from an airplane during the day. The larger, bright areas in the Man in the Moon are the lunar highlands, which are heavily cratered areas. The maria have craters, too, but fewer craters per square mile than the highlands, which means that the maria are younger. Huge impacts created the basins where the maria are located. These impacts obliterated preexisting craters. Later, the basins filled with lava from below, wiping out any new craters that had formed after the huge impacts. All the craters you can see in the maria now are from impacts that happened after the lava froze.

In the late 1990s, a NASA spacecraft called the Lunar Prospector obtained indirect evidence indicating that there may be frozen water in the bottoms of a few craters near the North and South Poles of the moon, where the sun never shines. The area includes the South Pole-Aitken Basin, a likely target for future space missions. The sun, at best, is low on the horizon near the poles of the moon; the crater rims block the sun from shining on parts of the crater bottoms. The ice may have come from comets that struck the moon long ago, because comets are largely ice and occasionally impact celestial bodies. But evidence suggests that no other water is present on the moon.

Observing the near side

The moon is one of the most rewarding objects to observe. You can see it when the sky is hazy or partly cloudy, and at times it's visible during the day. You can see craters with even the smallest telescopes. And with a high-quality small telescope, you can enjoy hundreds, and maybe thousands, of lunar features, including impact craters, maria, lunar highlands, rilles, and other features, including

  • Central peaks: Mountains of rubble thrown up in the rebound of the lunar surface from the effects of a powerful impact. Central peaks are found in some, but not all, impact craters.
  • Lunar mountains: The rims of large craters or impact basins, which may have been partly destroyed by subsequent impacts, leaving parts of their walls standing alone like a range of mountains, although not the type of mountain you see on Earth.
  • Rays: Bright lines formed by powdery debris thrown out from some impacts. They extend radially outward from young, bright impact craters, such as Tycho and Copernicus.

If you want to be able to distinguish one crater, rille, or lunar mountain range from another as you look through your telescope, you need a moon map or a set of lunar charts. These inexpensive items are available from astronomy and other scientific hobby supply houses and sometimes from map stores. Here are some good sources for these maps:

  • Edmund Scientific sells a full-color moon map poster (about $7) with feature identifications. The laminated version is better for use with your telescope; in the cool night air, unprotected paper can get wet with dew.
  • Skyandtelescope.com offers the English-language edition of a highly regarded lunar guide, Atlas of the Moon by Antonin Rukl (about $45).

Remember, these maps and charts show only one side of the moon: the lunar near side.

For almost anything you want to see on the moon, the best viewing time is when the object is near the terminator, which is the dividing line between bright and dark. Details of lunar features are most evident when features are just to the bright side of the terminator. (A telescopic view is the closest you can get to the terminator unless you head for California to meet Governor Schwarzenegger or join the NASA astronaut corps.)

During a month, which is approximately the period of time from one full Moon to the next, the terminator moves systematically across the lunar near side so that at one time or another, everything you can see on the moon is close to the terminator. Depending on the time of the month, the terminator is either the place on the moon where the sun rises or the place where the sun sets. As you know from experience on Earth, shadows extend farther during sunrise or sunset and continually shrink as the sun gets higher in the sky. The length of the shadow when the sun is at a known altitude is related to the height of the lunar feature that casts it. The longer the shadow, the taller the feature.

About the worst time to look at nearly anything on the moon is during a full Moon. During a full Moon, the sun is high in the sky on most of the lunar near side, so the shadows are few and short. The presence of shadows cast by features on the moon helps you understand the surface relief — the way landforms extend above or below their surroundings. But a full Moon isn't the time to seek relief.

Joining the dark side

You don't need a chart of the far side of the moon, because you can't see the far side; only the lunar near side is visible from Earth. Our view is limited because the moon is in synchronous rotation, meaning that it makes exactly one turn on its axis as it makes one orbit around Earth (the orbital period of the moon, which is the same as its "day," is about 27 days, 7 hours, and 43 minutes).

Astronomy supply houses and science stores sell moon globes, however, that depict the features of the entire moon, meaning the lunar near side and the far side. The Soviet space program first photographed the far side of the moon, which it did by snapping pictures with a robotic spacecraft very early during the Space Age. Since then, many different U.S. spacecraft, including the Lunar Orbiters and Clementine, have thoroughly mapped the moon.

Quite an impact: A theory about the moon's origin

According to the Giant Impact theory, the moon is composed of material blasted out of the mantle of Earth by a huge object — with up to three times the mass of Mars — that struck young Earth a glancing blow. Some of the rock from the mantle of that long-vanished impacting object also was incorporated into the moon, according to the theory.

The giant impact on young Earth knocked all this material up into space as a vapor of hot rock. It condensed and solidified like snowflakes. The snowflakes knocked into each other and stuck together, and before you knew it, the moon had formed. It came together in powerful impacts of the last big pieces of accumulated rock, with the heat from each impact melting the rock.

All the impacts that caused the craters that we now see on the moon happened later, and most of them date back to more than 3 billion years ago.

The moon is less dense than Earth as a whole, and about as dense as Earth's mantle (the layer beneath the crust and above the core), according to this theory, because it was made from mantle material. (Density is a measure of the amount of mass that's packed into a given volume. If you have two cannonballs of the same size and shape, they have the same volume. But if one ball is made of lead and one is made of wood, the lead ball is heavier and has a higher density.) This theory predicts that the moon shouldn't have much of an iron core, if any. And a small core in a small object (meaning the moon) would have cooled and frozen long ago if it ever contained liquid iron. So the moon shouldn't be able to generate a global magnetic field. And that's exactly what space measurements tell us. The Lunar Prospector, a satellite put in orbit around the moon in the late 1990s, detected magnetic fields, but only at isolated places. The Lunar Prospector scientists concluded that the fields are fossil magnetic fields, produced in an unknown way, long ago.

The Giant Impact theory is currently our best guess. Unfortunately, we have no test for it at this time. For example, the theory predicts no special kind of rock that we could look for in the hundreds of pounds of lunar rocks that the Apollo astronauts collected. However, NASA is considering a future mission to the South Pole-Aitken Basin. In that huge crater, astronauts or robot rovers may find rocks knocked out from so deep inside the moon that they were beneath the surface layer that melted after the moon formed. Studies of those rocks may tell scientists if the Giant Impact theory is accurate.

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