The Moon has been a part of our collective consciousness since prehistoric times. Recognizing the Moon's influence on the female menstrual cycle, the tides, and other cycles in nature, our ancestors made holes in sticks, reindeer bones and mammoth tusks to represent the number of days between the phases of the Moon. The first calendars were lunar calendars used by ancient peoples throughout the world for agriculture, hunting, and religious purposes.
Even as the Sun began to be used more and more to mark the passage of time, the Moon continued to haunt the human imagination. The earliest stories about other worlds and space flight concerned the Moon. Because of its proximity, it was the first celestial body to be gazed at with the telescope, the first to be surveyed by unmanned spacecraft, and the first to be walked upon by man.
Yet, more than three decades after the Apollo missions there are still much that is not known about the Moon. Its origin is still open for debate, questions concerning its internal structure remain unanswered, strange surface features are unexplained, and lunar transient phenomena -- lights and glows in the vicinity of the Moon and on the surface which have been observed for hundreds of years -- continue to mystify astronomers to this day.
The first three
theories have been rejected for various reasons: First, it is almost impossible
that the Earth could have captured an object as large as the Moon in a nearly
circular orbit. Second, the Moon could not have split off from Earth and
be in the orbit that it is in today. Third, significant differences in the
composition of the Earth and Moon suggest that they do not share a common
origin. The fourth is the most plausible theory suggested to date. It is also
somewhat reminiscent of the ancient Sumerian creation myth, Enuma Elish
. According to Zecharia Sitchin's
interpretation of it in
Genesis Revisited , the
Moon originated not as a satellite of Earth but of another planet, Tiamat,
which was located beyond the orbit of Mars. One of Tiamat's moons, Kingu,
collides with Tiamat and part of the remnants of the collision forms the
Earth and Moon.
The dark areas are flat, low-lying regions with few craters called maria or seas. Maria are comprised of basalts are made of iron, manganese, and titanium and are similar to rock created by lava flows from Earth's volcanoes. Rock samples from the maria collected by the Apollo astronauts have been dated to be between 3.1 and 3.8 billion years old . Tracking data for the Lunar Orbiter spacecraft in the 1960s revealed the presence of strong areas of high gravitational acceleration located over circular 'ringed' maria. These mass concentrations (mascons) may be caused by layers of denser, basaltic lavas that fill the mare basins.
The lighter regions on the Moon are highlands, much more cratered, and made up of light-colored rock called anorthosite, some of which has been shown to be over 4 billion years old . Anorthosite contains lighter elements such as calcium and aluminum -- as opposed to the heavier elements that make up the maria. Anorthosite is found only in the oldest mountain ranges on the Earth.
The surface of the Moon is covered by a dark gray, sandy soil, called regolith. Lunar regolith is a layer of debris from 2-20 meters thick that is created by meteoric impacts.
Not even trace amounts of water were found in the rocks gathered by the Apollo astronauts. However instruments aboard the more recent Clementine and Lunar Prospector missions have returned evidence of water at the Moon's poles . It is thought that water in the form of ice mixed with regolith may exist in there in areas that are permanently in shadow.
Below the lunar surface lies the crust and mantle. These two layers may be as thick as 800 km -- much thicker than the Earth's crust and mantle. Lunar geologists speculate that beneath the mantle is a partially molten layer and perhaps a small iron core. It is possible that the core might be frozen, which would account for the lack of any significant magnetic field on the Moon. Unlike the Earth, the Moon has no plate tectonics, active volcanoes, or magnetic field and thus no magnetosphere (a protective layer that provides a shield from cosmic rays). It also has virtually no atmosphere.
Little is known for certain about the Moon's deep interior. After it had been used, Apollo 12's lunar module was crashed on the Moon. Seismometers on the lunar surface measured the reverberations of the impact for over an hour. After crashing Apollo 13's third stage rocket booster, the Moon "reverberated like a bell" for almost four hours. In attempts to explain this, some scientists have suggested that there might be large cavities within the Moon, or that it might even be hollow .
After many years of debate, it was decided in the 1960s that the Moon's craters are formed primarily by meteoric impacts rather than volcanism. However, the nature and origin of crater rays remain unexplained. Although most scientists believe the rays are material ejected by impacts, other explanations have also been proposed. Equally enigmatic are formations which geologists call 'special features'. Examples include narrowly-spaced furrows or grooves, crater chains and other structures that repeat themselves, unusually bright regions, trenches and tubes leading into/out of craters, straight lines and other highly geometrical patterns on the surface. It has been suggested that these so-called special features may be artificial in origin.
Early in the 19th century, the astronomer Franz von Paula Gruithuisen announced that through his telescope he had seen a walled city on the moon , located near the crater Schroter. Many other such claims followed in the 1960s and 1970s based on the wealth of photos taken by the Apollo astronauts and Lunar Orbiter spacecraft. In 1966 Lunar Orbiter 2 imaged a group of objects that cast extremely long shadows on the edge of rectangular depression at the western edge of the Sea of Tranquility. The length of the shadows at first suggested the objects were towers or spires of some kind. This provocative image soon became a cold war joke -- Soviet ICBMs on the Moon!
But to William Blair, an anthropologist at Boeing (the company that made the Lunar Orbiter), the objects appeared to be laid out in a geometrical pattern. 30 years later, Lan Fleming conducted a thorough investigation of these objects, which had become known as the 'Blair Cuspids'. As a follow-up to this work, in his paper "3-D Analysis of the 'Blair Cuspids' and Surrounding Terrain," Mark Carlotto reveals that the depression first noted by Blair appears to be the deepest part of a larger network of rectilinear collapses of the surface. He also shows correlations between the arrangement of the objects, the geometry of the rectilinear collapses, and orientation of subtle lineaments in the surface.
In the spring of 1994, the Clementine spacecraft  collected over one million images of the Moon. Following its brief mission, once gain interest in unusual lunar surface features intensified. In "Towards Lunar Archaeology" Alexey Arkhipov describes a series of tests developed to analyze over 80,000 of Clementines's HIRES images and detect artificial features on the Moon. He identifies a number of unusual features on the lunar surface that resemble terrestrial archaeological ruins and structures.
For centuries astronomers have observed lunar transient phenomena (LTP) -- lights that appear and in some cases move across the lunar surface. In 1968 NASA published a report describing hundreds of LTP sightings . Most lunar scientists believe that LTPs are natural phenomena, however, more than 40% of LTP reports do not have an obvious explanation . A sighting (ULO-092196) made by the Lunascan project in September 1996 is described in "Anomalous Object Tracked Near Moon" by Francis Ridge and Lan Fleming. Their analysis suggests that object ULO-092196 was in lunar orbit at a time when there were no known man-made objects in the vicinity of the moon.
The Moon is the only natural satellite in the solar system having an almost perfectly circular orbit, and is the only satellite whose rotational period is the same as its orbital period (so the same side faces Earth). The gravitational pull of the Moon, which is responsible for our tides, also stabilizes the tilt of Earth's rotational axis. In doing so, it helps the Earth to maintain relatively stable climactic conditions over long periods of time, which may have been a key factor in the development of life on Earth.
It is hard to imagine what life would be like without the Moon. Some have even suggested that the Sun-Earth-Moon system is no accident. According to Issac Asimov, "There is no astronomical reason why the moon and the sun should fit so well, it is the sheerest of coincidences, and only the Earth among all the planets is blessed in this fashion."
We have explored on the Moon, analyzed its rocks, and probed its interior. Yet, even now at the beginning of the 21st century, our nearest celestial neighbor, in many way, is as much of a mystery to us as it was to our ancestors.
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