Wednesday, June 1, 2011

Earth's Nearest Cosmic Neighbour in Astronomy


The moon is more nearly comparable in size to the earth than the other satellites are to their planets, with the exception of Pluto and Charon. Yet the moon would not quite span the width of the United States, and its mass is roughly 1 percent of that of the earth.
LUNAR EXPLORATION 
The lunar-exploration program that began in 1964 with unmanned craft and culminated in six manned Apollo landings between 1969 and 1972 has provided us with a priceless legacy of lunar materials and data. Lunar rocks have been collected from nine different locations, six by the United States and three by the Soviet Union (the most recent being August, 1976). The samples returned amount to more than 2000 individual samples, weighing about 382 kilo­grams (843 pounds). 
Five instrument packages were left on the lunar surface, the last surviving one operating until October 1977. the seismometers in these packages detected meteoric impacts and many lunar quakes during their operating life span of about 8 years.
The Apollo program also carried out an extensive effort to photograph and analyze the lunar surface. The result is maps of some parts of the moon better than those of some areas on earth. X-ray and radio­activity studies from orbit have yielded estimates of the chemical composition of about one-quarter of the lunar surface, an area about the size of the United States and Mexico together.
THE LUNAR SURFACE
Because of its small mass, the moon's history has been vastly different from the earth's. With a small mass comes a weak gravitational attraction; as a result the moon retains almost no atmosphere. It has no surface water, either free or chemically combined in the rocks (as in earth rocks), although some water may be trapped under its surface. It also has no general mag­netic field, but its rocks suggest that a strong one existed in the very distant past. However, the moon is far from a simple, featureless satellite.
Galileo's subdivision of the lunar surface into ma­ria, the low-lying, almost circular dark regions in Fig­ure 7.15, and terrae, the rough, crate red highlands, is still significant in terms of lunar history and terrain­shaping processes. The maria are covered with layers of basaltic lava similar to the lavas that erupt from terrestrial volcanoes in Iceland, Hawaii, and else­where. The highlands are a lighter-colored rock that are older than the rocks of the maria.
Even with earth-based telescopes observers over the years have recognized a variety of other surface features, such as a range of sizes of impact craters, rugged mountain ranges, and deep winding canyons, or rilles.
Over eons of time small meteoroids have pulver­ized the lunar surface, leaving a dusty layer, some 1 to 20 meters deep, covering the lunar terrain. Known as the regolith,' it is the lunar "soil" on which the astro­nauts left their footprints. Since this soil contains no water or organic matter, it is totally different from solids formed on the earth by water, wind, and life. It could only have been formed over billions of years on the surface of an airless body.
More than just bits of ground-up lunar rocks, the regolith has also been exposed to cosmic-ray parti­cles, subatomic matter flowing from the sun, and a fine dust from interplanetary space. Without an at­mosphere to shield it the layers of the regolith contain both the record of lunar events and that of events in the larger solar system.
CRATERS ON THE LUNAR SURFACE
A tremendous number of impact craters pit the moon, evidence of cataclysms that altered the crust during its past. More than 30,000 are visible by telescope. The total, down to bushel-basket size, may well exceed a million. The great walled plains, or supercraters with low profiles, like Clavius or Grimaldi have structures similar to those of the maria but on a smaller scale. Thei r diameters are between 200 and 300 kilometers.
Next in size on the moon's front side are some three dozen impact craters from 80 to 200 kilometers in diameter. A third of them have conspicuous Iight­colored streaks, called rays, radiating outward in all directions up to several hundred kilometers long, such as the well-known ray craters- Tycho, Coperni­cus, Kepler, and Aristarchus. Many of the small sec­ondary craters, as well as the ray systems, were ap­parently formed by a rain of debris ejected from the primary crater after a large body struck the surface.
Impact craters are reasonable circular, with the in­terior rim steeper than the outer rim. The larger cra­ters have terraces on their inner walls and frequently have a fairly smooth floor from which a few low peaks rise. Beyond the craters the terrain is hummocky and overlain with the ejected material from the cratering activity. Even moderate-sized cra­ters, like the larger ones, have high walls. In these craters having a central peak, the peak is believed to have been created by the elastic rebound of rock from below the surface after the initial impact. Others have bare floors, presumably because they were flooded with lava; the crater Plato is a good example.
The impact craters are not all of the same age with the crater Clavius. The rim of the major crater is eroded and worn, whereas the half dozen or so small ones in the center have sharper and higher rims. Clearly the impacting bodies that produced the small craters superimposed on the rim of the large crater must have fallen more recently than that one which formed the large crater. An in­spection of crater photographs shows that they have not only a spectrum of size but also a range in erosion and wear; that is, they are not all the same age. As an example, the craters Copernicus and Tycho are about 600 and 200 million years old, respectively.
Volcanically produced craters, formed mostly dur­ing the moon's early history, are present but in smaller numbers than those of impact origin. Volcanic craters are not always circular., Their rims slope at about the same angle, both inside and outside, and their rims and floors are shallower than those of impact craters. Thus thermal activity as a terrain-forming mechanism has not been absent from lunar history, although it is markedly less important than on the earth.
LUNAR SURFACE FEATURES
Highlands constitute about 80 percent of the surface of the moon. Although fractures are observed in the lunar crust, there is no evidence of folded mountain belts (as on the earth) nor other indication of thermal­tectonic activity. Lunar mountain ranges were appar­ently produced in conjunction with the formation of the impact basins that are now seen as maria. Most mountain chains are on or near the periphery of the roughly circular maria. The mountains bordering the maria rise more steeply on the side facing them than on the other side. Many have lofty peaks, occasionally rising over 7000 meters above the surrounding plains.
Beyond the eastern edge of Mare Imbrium a nar­row valley cuts across the lunar Alps Mountains. This feature has long been known from photographs taken from the earth. From photgraphs taken by an orbiting spacecraft we now know that the Alpine Valley is a deep trough some 3 to 10 kilometers wide and over 100 kilometers long. Narrow channels (rilles), which resemble chasms or gorges, cut many ki lometers across the lunar terrain, frequently without interrup­tion. Running lengthwise down the middle of the Al­pine Valley is a very conspicuous rille. Rilles may be lava channels, part or all of which were roofed when filled with flowing lava. Now these tubes have collapsed and are partly choked with rubble from the days of active lava flows.
FAR SIDE OF THE MOON 
Topography on the far side of the moon is strikingly different from that on the near side. Craters are everywhere, but few have steep slopes. The face averted from the earth has no extensive mountain ranges and no large lava-flooded basins comparable to Mare Imbrium on the near side although some small basins exist on the far side. The far side thus lacks the near side's extensive lava flooding.
The moon's center of mass is displaced from its geometric center about 2 kilometers earthward. One consequence is that the lunar crust facing the earth is about half as thick as that of the far side. Perhaps this variation explains why the near side has more volcanic activity, which accounts for its large deposits of dark mantling material. It may also help explain why the basins on the far side are only partially filled.

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