The Lunar Surface

With binoculars or a small telescope, an observer can see details of the Moon's near side in addition to the pattern of maria and highlands. As the Moon passes through its phases, the terminator moves slowly across the Moon's disk, its long shadows revealing the relief of mountains and craters. At full moon the relief disappears, replaced by the contrast between lighter and darker surfaces. Though the full moon is brilliant at night, the Moon is actually a dark object, reflecting only a few percent (albedo 0.07) of the sunlight that strikes it. (Some sunlight reflects from Earth to the Moon and back again. For a few days before and after a new moon, this doubly reflected sunlight, or earthshine, is powerful enough to make the whole Moon visible. At this time an observer on the Moon would see Earth as a bright body, four times the diameter of the Moon as seen from Earth, almost completely illuminated by the Sun. The phases of Earth and the Moon are complementary, so Earth is near full when the Moon is near new, and the earthshine then is strongest.)

Beginning with the Italian scientist Galileo's sketches in the early 17th century and continuing into the 19th century, astronomers mapped and named the visible features down to a resolution of a few kilometres, the best that can be accomplished when viewing the Moon telescopically through Earth's turbulent atmosphere. The work culminated in a great hand-drawn lunar atlas made by observers in Berlin and Athens. This was followed by a lengthy hiatus as astronomers turned their attention beyond the Moon until the mid-20th century, when it became apparent that human travel to the Moon might eventually be possible. In the 1950s another great atlas was compiled, this time a photographic one published in 1960 under the sponsorship of the U.S. Air Force.

Astronomers long debated whether the Moon's topographic features had been caused by volcanism. Only in the 20th century did the dominance of impacts in the shaping of the lunar surface become clear. Every highland region is heavily cratered—evidence for repeated collisions with large bodies. (The survival of similar large impact structures on Earth is relatively rare because of Earth's geologic activity and weathering.) The maria, on the other hand, show much less cratering and thus must be significantly younger. Mountains are mostly parts of the upthrust rims of ancient impact basins. Volcanic activity has occurred within the Moon, but the results are mostly quite different from those on Earth. The lavas that upwelled in floods to form the maria were extremely fluid. Evidence of volcanic mountain building as has occurred on Earth is limited to a few fields of small, low domes.

For millennia people wondered about the appearance of the Moon's unseen side. The mystery began to be dispelled with the flight of the Soviet space probe Luna 3 in 1959, which returned the first photographs of the far side. In contrast to the near side, the surface displayed in the Luna 3 images consisted mostly of highlands, with only small areas of dark mare material. Later missions showed that the ancient far-side highlands are scarred by huge basins but that these basins are not filled with lava.

View of the Moon never seen from Earth, predominantly the heavily cratered far side, photographed by Apollo 16 astronauts in April 1972. The near-side impact basin Mare Crisium is the large dark marking on the upper left limb; the two dark areas below it are Mare Marginis (nearer Crisium) and Mare Smythii. Although the far side is well scarred with giant basins, these never filled with lava to form maria. F.J. Doyle/National Space Science Data Center

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