The Origin of the Moon - Essay Example

?The origin of moon has always been an unresolved enigma for investigators of science. Though four different theories have been proposed, none were able to explain completely the circumstances in which moon was born and became paired in a crucial way with the earth. The earliest theory among them is that “earth had somehow spawned the moon” out (Schrunk, 1). George Darwin, son of the biological evolution theorist, Charles Darwin, had proposed this theory known as “fission hypothesis” in 1878 (Schrunk,1). This theory was based on the logic that as moon is gradually moving away from the earth (because the time taken by moon to make one revolution around the earth goes on increasing), logically it can be concluded that this is the continuation of an outward momentum initiated by the fission. Hence it is concluded that once moon might have been “closer to the earth than it is now” (Schrunk, 1). The reason for the split is also well-explained in fission theory. It is speculated that some kind of gravitational imbalance within the earth caused by its previous shape might have made it spit out a piece which became moon. The fission theory is supported by the fact that the core matter of moon is “not as massive as the earth’s” and also that same nonradioactive stable oxygen isotopes are found on the “terrestrial rocks” of both earth and moon (Gergo, 4,5). Thus this theory speculates that it was from the less dense mantle of the earth that moon was broken away as a separate entity. The weakness of fission theory is that there is a scientifically proven minimum distance near to the earth below which moon cannot exist in solid form and can exist only as “a ring of debris” (Schrunk, 1). Hence moon could never be close to earth within this minimum distance and continue to exist as a solid cosmic body. After proving this in 1873, Edouard Roche contested fission hypothesis and put forth the “co-accretion” theory (Schrunk, 1). Co-accretion theory said that earth and moon most probably might have been formed “at the same time, in the same neighborhood of the solar system” (Schrunk, 2). This is an assumption based on the similarities in structure between earth and moon like the presence of oxygen isotopes. A third theory on the origin of moon was proposed by Thomas.J.J.See (qtd in Schrunk) who theorized that “moon was … a captured satellite” (Schrunk, 2). This theory was supported by one fact which came to light in that period. It was proven that at least some of the satellites of Saturn and Jupiter might have been captured ones (Schrunk, 2). See’s theory later came to be called as “capture hypothesis” (Schrunk, 2). This theory says that owing to the gravitational dynamics within and around the area where now the solar system exists, the orbit of moon (which was then far away from the earth) came near the earth (Schrunk, 2). And earth captured moon into becoming its satellite. The fourth and latest hypothesis on the origin of moon is known as “planetesimal impact hypothesis” and this theory has evolved combining certain features of all the three theories on the origin of moon that were proposed before it (Schrunk, 2). This hypothesis takes the idea that earth was hit by a “pre-planetory body” of the size of Mars, from the capture hypothesis (Schrunk, 2). It also brings in the argument that this collision had forced out, a huge quantity of debris which is a proposition put forward by the fission hypothesis (Schrunk, 2). And finally the “planetesimal impact hypothesis” also concludes that this debris “condensed into moon,” as was suggested partially by the co-accretion hypothesis (Schrunk, 2). The collision is supposed to have happened around 100 million years after the formation of the earth (Wilkinson, 73). This great impct is also thought to have caused the tipping of the axis of the earth and “inaugurated the seasons” on earth (Wilkinson, 73). This is the theory on moon’s origin which has gained the maximum approval. This is also in the context of many drawbacks that other three theories were found to have. As per fission theory, the original proto-planet that was a combined mass of earth and moon, is supposed to have thrown off huge quantity of mass (Grego, 5). In that case, the proto-planet ought to have been revolving (at that time) with an impossibly great speed (Grego, 5). If one agrees for the sake of argument that the proto-planet was revolving with such a momentum, then again the contradiction arises why the earth-moon system has no corresponding momentum now (Grego, 5). A second anomaly in the fission theory is that it has failed to explain “why the moon’s orbit is inclined to the ecliptic by 5°” (Grego,5). As suggested by fission theory, if moon was thrown off from the equator area of earth, it would have initially orbited earth above the equator and then gradually might have been aligned with the ecliptic by the gravitational force of the sun (Grego, 5). This not being the case, fission theory falls short of explaining moon’s origin. Similarly, the co-accretion theory which says that earth and moon are sister planets, has failed to explain why there is no water or volatile substances on the moon, as on earth (Grego, 5-6). The paradox of 5° shift from the ecliptic (of the orbit of moon) that failed fission theory also challenges the validity of co-accretition theory. This is because if moon was formed in the same circumstances as earth was formed, then its orbit had to be ecliptic (Grego, 6). The “capture theory” though able to explain why moon is orbiting 5° deviant to the ecliptic, could not answer the question why the rocks in earth and moon have the same oxygen isotopes (Grego, 8). If moon is a satellite captured by earth from a far away orbit, there is no reason why earth and moon has the same oxygen isotope composition. The “planetesimal impact hypothesis” which is the fourth and most popular theory about the origin of moon is the theory that has maximum scientific logic underlying its propositions. This theory explains why the density of moon is the same as the crust of the earth and also the absence of an “iron core” in the moon (Grego, 8). According to this theory, when a planet with the size of mars collided with earth, “the core of the impactor joined the body of the Earth, while the mantles of both objects were mixed and splashed out into space” (Grego, 8). It is also supposed that a major portion of this material consolidated into moon (Grego, 8). If this is the case it is natural that earth acquired a harder core and moon which was formed of the mantle debris happened to have a less dense core. And the less quantity of iron in moon could also be accounted by this. After the collision, iron being the denser metal might have gone more speedily back to the earth while the less heavy debris condensed into moon (Wilkinson, 72). This theory is also been able to find reason for the absence of water and volatiles in the moon (Grego, 9).The high temperatures that might have been produced by the collision can be considered as the natural cause of evaporation of water and volatiles (Grego, 9). This could be the reason also for the occurrence of iron in lower quantities in moon than in earth (Grego, 9). The planetesimal impact theory is the most acceptable one also because the whole solar system has been proved to be evolving from cosmic impacts (Grego, 9). Thus this impact theory has better correlation with the theory of formation of the solar system than the other three theories. Data acquired from Apollo 11 mission also support this theory by proving that high temperatures had existed on moon in its early stages of development (Ward and Brownlee, 229). This substantiates the argument of collision and also the evaporation of water and volatiles. The scientific knowledge acquired so far about the universe in many ways agrees with the impact theory. The impact theory agrees with the spiraling out of moon from the earth as any collision is ought to have such an effect naturally (Ward and Brownlee, 231). The computer modeling of the collision and the geochemical studies conducted on the mantle of the earth have also proposed the possibility of a great impact (Shirley and Fairbridge, 513). The computer models have already suggested that such a collision is a possibility (Shirley and Fairbridge, 513). But one drawback is that the heating of the mantle caused by this “great impact” have not yet been supported on the basis of mineralogical studies (Shirley and Fairbridge, 513). But still, the great impact theory has the greatest number of points to agree with the related whole of cosmic phenomena. Works Cited Schrunk, David.G., “The Moon: resources, future development, and settlement”, Berlin: Springer, 2008.Print. Grego, Peter, “The Moon and How to Observe It”, Basel/ Schweiz: Birkhauser, 2005. Print. Shirley, James.H. and Fairbridge, Rhodes Whitmore, “Encyclopedia of Planetary Sciences”, Berlin: Springer, 1997. Print. Ward, Peter Douglas and Brownlee, Donald, “Rare Earth: Why Complex Life is Uncommon in the Universe”, Berlin: Springer, 2000. Print. Read More