The Moon is the nearest body to us in the solar system,and as consequence of the Apollo missions is the only extraterrestrial object that has yet been explored directly by humans.As a consequence of that exploration by both manned and unmanned spacecraft we now know a great deal about our Moon.
Distant from Earth(Orbit):384,400 Km
So we have a historical words which said by president Kennedy:
"I believe that this nation should commit itself to achieving the goal before this decade is out,of landing a man on the Moon and returning him safely to the Earth.No single space programe in this period will be more impressive to mankind or more imprtant in the long-range exploration of space:and more will be so difficult or expensive to accomplish.
(President John F. Kennedy,to a joint session of Congress may 25,1961)
The Moon was first visited by the Soviet spacecraft Luna2 in 1959.It is the only extraterrestrial to have been visited by humans.The first landing was on only 20 july,1969,and the last was in december 1972.The Moon is also the only body from which samples have been returned to Earth.In the summer 1994,the Moon was very extensively mapped by the little spacecraft clementine and a gain in 1999 by Lunar prospector.
The image in the right show the first foot print by the mission apollo 11 commander,Neil Armstrong,as he sets foot upon the surface of the Moon 10:56 pm EDT ,july 20,1969
The Moon has been known since prehistoric times.It is the second brightest object in the sky afetr the Sun.As the Moon orbits around the Earth once per month,the angle between the Earth,the Moon and the Sun changes;we see this as the cycle of the Moon's phases.The time between succesive new moons is 29.5(709 hr.,slightly different from the Moon's orbital period(measured against the stars)since the Earth moves a significant distance in its orbit around the Sun in that time.
Tides and Gravitational Locking:-
A-Tidal Coupling and Gravitional Locking
Some important consequences of tidal forces in the solar system include:
1-Tidal forces distort any body experiencing differential gravitional forces.This will normally occur for bodies of finfite extent in gravitational fields because of the strong distant dependence of the gravitational force.Thus,not only the oceans,but the body of the Earth is distorted by the lunar gravity.However,because the Earth is rigid compared with the oceans,the"tides" in the body of the Earth are much smaller than in the oceans.
2-There is a limmiting radius for the orbit of one body around another,inside of which the tidal forces are so large that no large solid objects can exist that are held together only by gravitational forces.This radius is called the Roch Limit.Thus,solid objects put into orbit inside the Roche limit will be torn apart by tidal forces,and conversly,solid object cannot grow by accerting into largeer objects if they lie inside the Roch limit.A famous example is the rings of saturn:because they lie inside the Roche limit for Saturn,they cannot be solid objects held together by gravitation and must be composed of many small particles.
Obivously solid objects can exist inside the Roche limit(for example,spacecraft)but they must be held together by forces other than gravity.This is true of a spacecraft,where chemical forces between the atoms and molecules are much than the gravitional forces.
3-The tidal are reciprocal.Not only will the Moon induce tides in the body of the Earth and the Earth's oceans,but by the same argument the gravitational field of the Earth will induce differential forces and therefore tides in the body of the Moon.Again,because the body of the Moon is quite these LUnar tides will be very small,but they occur.
4-This reciprocal induction of tides in the body of the Earth and the Moon leads to a complicated coupling of the rotational and orbital motions of two objects.These tidal forces and associated couplings have the following general effects:
The interior of the Earth and Moon are heated by the tides in thier bodies,just as a paper clip is heated by constant bending.This effect is very small for the Earth and Moon.But we shall see that it can be dramitic for other objects that experience forces.For example,we shall see that the tidal forces exerted by jupitar on its moon Io are so large that the solid surface of Io is raised and lowered by hundereds of meters twice in each rotational period.This motion so heats the interior of Io that it is probably mostly molten;as a consequence,Io is covered with active volcanos and is the geologically most active object in the Solar System.
The tidal coupling of the orbital and rotational motiontends to synchronize them.In the simplest instance,the period of rotation for the two bodies and the orbital period eventually become exactly equal because of this tidal coupling(and as a result,the size of the orbit is changed in such a way as to converse angular momentum for the entire system).This is called gravitaional locking or tidal locking,because as the two objects revolve around their common center of mass each keeps the same side turned toward the other.
B-Tidal Coupling in the Earth-Moon System
Thus,the fact that the rotational period of the Moon and the orbital period of the Earth-Moon system are of the same length is not an accident.Presumably this was not always true,but over billions of years the tidal coupling of the Earth and the Moon has led to this synchronized synchronization.In the case of the Earth-Moon system the synchronization is not yet complete.The Earth is slowly decreasing its rotational period and eventually the Earth and Moon will have exactly the same rotational period,and these will also exactly equal the orbital period.At the same time,the sepration between the Earth and Moon will slowly increase in just such a way as to conserve angular momentum for the entire system.
Thus,billions of years from now the Earth will always keep the same face toward the Moon,just as the Moon already always keeps the same face toward the Earth.We will encounter other examples of such tidal locking in other pairs of objects in the Solar System.
The surface of the Moon has two hemispheres with rather asymmetric properties; as a consequence the nature of the Lunar surface that we can see from the Earth is substantially different from the surface that is always hidden from the Earth.
The Near Side
The face of the Moon turned toward us is termed the near side (right). It is divided into light areas called the Lunar Highlands and darker areas called Maria (literally, "seas"; the singular is Mare). The Maria are lower in altitude than the Highlands, but there is no water on the Moon so they are not literally seas (Recent evidence from the Clementine spacecraft suggests that there may be some water on the Moon, contrary to previous assumptions). The dark material filling the Maria is actually dark, solidified lava from earlier periods of Lunar volcanism. Both the Maria and the Highlands exhibit large craters that are the result of meteor impacts. There are many more such impact craters in the Highlands.
The Far Side
The side of the Moon unseen from the Earth is called the far side.The far side is different alot than the Near side . In particular, there are one small maria, as illustrated in the image shown to the left of a portion of the far side surface. In this figure a number of meteor impact craters are visible.So,lunar geologists believe that the far side is very representative of how the Moon looked 4 billion years ago.
The amount of cratering is usually an indication of the age of a geological surface: the more craters, the older the surface.So, the Earth has a relatively young surface because it has few craters. This is because the Earth is geologically active, with plate tectonics and erosion having obliterated most craters from an earlier epoch. but the surface of the Moon is much older than Earth. The maria are younger than the highlands, because they have fewer craters.
The oldest surfaces in the Solar System are characterized by maximal cratering density.
The Moon's density is smaller than Earth's density though,Moon have alot of volcanic basalistic lavason the Earth,but Earth's density is more cause of the dense of iron/nickel core.The Moon is coverered with a gently rolling layer of powdery soil with scattered rocks that is called the regolith; it is made from debris blasted out of the Lunar craters by the meteor impacts that created them. Each well-preserved Lunar crater is surrounded by a sheet of ejected material called the ejecta blanket.
One striking difference between the Lunar surface material and that of Earth concerns the most common kinds of rocks.
-On the Earth, the most common rocks are sedimentary, because of atmospheric and water erosion of the surface.
-On the Moon there is no atmosphere to speak of and little or no water, and the most common kind of rock is igneous ("fire-formed rocks")
.In Gelocally point of view Lunar have the following characteristics:
1-The Maria are mostly composed of dark basalts, which form from rapid cooling of molten rock from massive lava flows.
2-The Highlands rocks are largely Anorthosite, which is a kind of igneous rock that forms when lava cools more slowly than in the case of basalts. This implies that the rocks of the Maria and Highlands cooled at different rates from the molten state and so were formed under different conditions.
3-Breccias, which are fragments of different rocks compacted and welded together by meteor impacts, are found in the Maria and the Highlands, but are more common in the latter.
4-Lunar Soils contain glassy globules not commonly found on the Earth. These are probably formed from the heat and pressure generated by meteor impacts.
The Anorthosites that are common in the Lunar Highlands are not common on the surface of the Earth (The Adirondack Mountains and the Canadian Shield are exceptions). They form the ancient cores of continents on the Earth, but these have largely been obliterated by overlying sedimentary deposits and by plate tectonic activity.
The Lunar rocks may also be examined according to the chemicals that they contain. Such analysis indicates:
1-They are rich in refractory elements, which are elements such as calcium (Ca), Aluminum (Al), and Titanium (Ti) that form compounds having high melting points.
2-They are poor in the light elements such as hydrogen (H).
3-There is high abundance of elements like Silicon (Si) and Oxygen (O).
The high concentration of rare metals like Titanium, and the availability of abundant amounts of Silicon and Oxygen has led to serious proposals about mining and manufacturing operations in the future for the Moon.
Age of Lunar Material
The abundances of radioactive elements in rock samples can be used to tell the age of the rock in a process called Radioactive Dating.
Scientists have studied the ages of rocks in cratered regions and have determined the rate of cratering at various epochs in the Moon's past. By studying the light-colored highlands, they found that there was a period of heavy bombardment very early in the Moon's history, from about 4.6 to 3.8 billion years ago. After that, the rate dropped off substantially and much less cratering has occurred since then.
Rock samples from large impact basins show that about 3.1 to 3.8 billion years ago several asteroid-size objects impacted the Moon, just as the period of heavy bombardment was ending. This was shortly followed by lava flows which filled in the basins and formed the dark maria. This explains why craters are relatively few and isolated on the dark maria, but dense and overlapping in the highlands. No lava flows occurred on the highlands to erase the original blanket of craters from the time when the Moon's surface was showered with the debris of the early solar system.
The Formation of the Moon:-
An extremely important question is that of how the Moon was formed and came to have its present orbit around the Earth.
Five Serious Theories
Five serious theories have been proposed for the formation of the Moon (not counting the one involving green cheese):
The Fission Theory :
The Moon was once part of the Earth and somehow separated from the Earth early in the history of the Solar System. The present Pacific Ocean basin is the most popular site for the part of the Earth from which the Moon came.
The Capture Theory :
The Moon was formed somewhere else, and was later captured by the gravitational field of the Earth.
The Condensation Theory :
The Moon and the Earth condensed together from the original nebula that formed the Solar System.
The Colliding Planetesimals Theory :
The interaction of earth-orbiting and Sun-orbiting planetesimals (very large chunks of rocks like asteroids) early in the history of the Solar System led to their breakup. The Moon condensed from this debris.
The Ejected Ring Theory :
A planetesimal the size of Mars struck the earth, ejecting large volumes of matter. A disk of orbiting material was formed, and this matter eventually condensed to form the Moon in orbit around the Earth.
Constraints from Recent Data
A detailed comparison of the properties of Lunar and Earth rock samples has placed very strong constraints on the possible validity of these hypotheses. For example, if the Moon came from material that once made up the Earth, then Lunar and Terrestrial rocks should be much more similar in composition than if the Moon was formed somewhere else and only later was captured by the Earth.
These analyses indicate that the abundances of elements in Lunar and Terrestrial material are sufficiently different to make it unlikely that the Moon formed directly from the Earth. Generally, work over the last 10 years has essentially ruled out the first two explanations and made the third one rather unlikely. At present the fifth hypothesis, that the Moon was formed from a ring of matter ejected by collision of a large object with the Earth, is the favored hypothesis; however, the question is not completely settled and many details remain to the accounted for searching.
Interior and Gelogical Activity:-
Before the Apollo missions we knew almost nothing about the interior of the Moon. The Apollo missions left seismometers on the lunar surface that have allowed us to deduce the general features of the Lunar interior by studying the seismic waves generated by "moonquakes" and occasional meteor impacts.
The Structure of the Interior
Our present picture of the Moon's interior is that it has a crust about 65 km thick, a mantle about 1000 km thick, and a core that is about 500 km in radius. A limited amount of seismic data suggests that the outer core may be molten. There does appear to be some amount of differentiation, but not on the scale of that of the Earth. It has no magnetic field to speak of, but magnetization of Lunar rocks suggests that it may have had a larger one earlier in its history. Although there is a small amount of geological activity on the Moon, it is largely dead geologically (the energy associated with the Earth's seismic activity is about 10^14 times larger than that of the Moon). Most Lunar seismic activity appears to be triggered by tidal forces induced in the Moon by the Earth.
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Last Modified : 5 Sep. 2001
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