...eresting effects. The most obvious is the tides. The Moons gravitational attraction is stronger on the side of the Earth nearest to the Moon and eaker on the opposite side. Since the Earth, and particularly the oceans, is not perfectly rigid it is stretched out along the line toard the Moon. From our perspective on the Earths surface e see to small bulges, one in the direction of the Moon and one directly opposite. The effect is much stronger in the ocean ater than in the solid crust so the ater bulges are higher. And because the Earth rotates much faster than the Moon moves in its orbit, the bulges move around the Earth about once a day giving to high tides per day. But the Earth is not completely fluid, either. The Earths rotation carries the Earths bulges get slightly ahead of the point directly beneath the Moon. This means that the force beteen the Earth and the Moon is not exactly along the line beteen their centers producing a torque on the Earth and an accelerating force on the Moon. This causes a net transfer of rotational energy from the Earth to the Moon, sloing don the Earths rotation by about 1.5 millisecondscentury and raising the Moon into a higher orbit by about 3.8 centimeters per year. The opposite effect happens to satellites ith unusual orbits such as Phobos and Triton. The asymmetric nature of this gravitational interaction is also responsible for the fact that the Moon rotates synchronously , i.e. it is locked in phase ith its orbit so that the same side is alays facing toard the Earth. Just as the Earths rotation is no being sloed by the Moons influence so in the distant past the Moons rotation as sloed by the action of the Earth, but in that case the effect as much stronger. hen the Moons rotation rate as sloed to match its orbital period such that the bulge alays faced toard the Earth there as no longer an off-center torque on the Moon and a stable situation as achieved. The same thing has happened to most of the other satellites in the solar system. Eventually, the Earths rotation ill be sloed to match the Moons period, too, as is the case ith Pluto and Charon .Actually, the Moon appears to obble a bit due to its slightly non-circular orbit so that a fe degrees of the far side can be seen from time to time, but the majority of the far side left as completely unknon until the Soviet spacecraft Luna 3 photographed it in 1959. Note there is no dark side of the Moon all parts of the Moon get sunlight half the time. Some uses of the term dark side in the past may have referred to the far side as dark in the sense of unknon eg darkest Africa but even that meaning is no longer valid today! The Moon has no atmosphere. But evidence from Clementine suggested that there may be ater ice in some deep craters near the Moons south pole hich are permanently shaded. This has no been confirmed by Lunar Prospector .There is apparently ice at the north pole as ell. The cost of future lunar exploration just got a lot cheaper! The Moons crust averages 68 km thick and varies from essentially 0 under Mare Crisium to 107 km north of the crater Korolev on the lunar farside. Belo the crust is a mantle and probably a small core roughly 340 km radius and 2 of the Moons mass. Unlike the Earths mantle, hoever, the Moons is only partially molten. Curiously, the Moons center of mass is offset from its geometric center by about 2 km in the direction toard the Earth. Also, the crust is thinner on the near side. There are to primary types of terrain on the Moon the heavily cratered and very old highlands and the relatively smooth and younger maria. The maria hich comprise about 16 of the Moons surface are huge impact craters that ere later flooded by molten lava. Most of the surface is covered ith regolith, a mixture of fine dust and rocky debris produced by meteor impacts. For some unknon reason, the maria are concentrated on the near side. Most of the craters on the near side are named for famous figures in the history of science such as Tycho Copernicus , and Ptolemaeus. Features on the far have more modern references such as Apollo, Gagarin and Korolev ith a distinctly Russian bias since the first images ere obtained by Luna 3 In addition to the familiar features on the near side, the Moon also has the huge craters South Pole-Aitken on the far side hich is 2250 km in diameter and 12 km deep making it the the largest impact basin in the solar system and Orientale on the estern limb as seen from Earth in the center of the image at left hich is a splendid example of a multi-ring crater. A total of 382 kg of rock samples ere returned to the Earth by the and programs. These provide most of our detailed knoledge of the Moon. They are particularly valuable in that they can be dated. Even today, 20 years after the last Moon landing, scientists still study these precious samples. Most rocks on the surface of the Moon seem to be beteen 4.6 and 3 billion years old. This is a fortuitous match ith the oldest terrestrial rocks hich are rarely more than 3 billion years old. Thus the Moon provides evidence about the early history of the Solar System not available on the Earth. Prior to the study of the Apollo samples, there as no consensus about the origin of the Moon. There ere three principal theories co-accretion hich asserted that the Moon and the Earth formed at the same time from the Solar Nebula fission hich asserted that the Moon split off of the Earth and capture hich held that the Moon formed elsehere and as subsequently captured by the Earth. None of these ork very ell. But the ne and detailed information from the Moon rocks led to the impact theory that the Earth collided ith a very large object as big as Mars or more and that the Moon formed from the ejected material. There are still details to be orked out, but the impact theory is no idely accepted. The Moon has no global magnetic field. But some of its surface rocks exhibit remanent magnetism indicating that there may have been a global magnetic field early in the Moons history. ith no atmosphere and no magnetic field, the Moons surface is exposed directly to the solar ind .Over its 4 billion year lifetime many hydrogen ions from the solar ind have become embedded in the Moons regolith. Thus samples of regolith returned by the Apollo missions proved valuable in studies of the solar ind. This lunar hydrogen may also be of use someday as rocket fuel. ...
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