|
Origin of the Moon refers to any of the various explanations for the formation of the Moon, Earth's natural satellite. The leading theory has been the giant impact hypothesis.〔 However, research continues on this matter, and there are a number of variations and alternatives.〔 Other proposed scenarios include captured body, fission, formed together (condensation theory), planetesimal collisions (formed from asteroid-like bodies), and collision theories.〔(Theories of Formation for the Moon )〕 The standard GIH (Giant Impact Hypothesis) suggests a Mars-sized body called Theia impacted Earth, creating a large debris ring around the Earth which then formed the system.〔 However, the Moon's oxygen isotopic ratios seem to be essentially identical to Earth's. Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each solar system body. If Theia had been a separate protoplanet, it probably would have had a different oxygen isotopic signature from Earth, as would the ejected mixed material. Also, the Moon's titanium isotope ratio (50Ti/47Ti) appears so close to the Earth's (within 4 ppm), that little if any of the colliding body's mass could likely have been part of the Moon. ==Giant impact hypothesis== (詳細はplanetesimals are now recognized to lead to the growth of planetary bodies early in the evolution of the Solar System, and in this framework it is inevitable that large impacts will sometimes occur when the planets are nearly formed. It is thought to have originated in the 1940s with Reginald Aldworth Daly, a Canadian professor at Harvard. The hypothesis requires a collision between a body about 90% the present size of the Earth, and another the diameter of Mars (half of the terrestrial radius and a tenth of its mass). The colliding body has sometimes been referred to as Theia, the mother of Selene, the Moon goddess in Greek mythology. This size ratio is needed in order for the resulting system to possess sufficient angular momentum to match the current orbital configuration. Such an impact would have put enough material into orbit about the Earth to have eventually accumulated to form the Moon. Computer simulations show a need for a glancing blow, which causes a portion of the collider to form a long arm of material that then shears off. The asymmetrical shape of the Earth following the collision then causes this material to settle into an orbit around the main mass. The energy involved in this collision is impressive: trillions of tons of material would have been vaporized and melted. In parts of the Earth the temperature would have risen to 10,000 °C (18,000 °F). The Moon's relatively small iron core is explained by Theia's core accreting into Earth's. The lack of volatiles in the lunar samples is also explained in part by the energy of the collision. The energy liberated during the reaccreation of material in orbit about the Earth would have been sufficient to melt a large portion of the Moon, leading to the generation of a magma ocean. The newly formed moon orbited at about one-tenth the distance that it does today, and became tidally locked with the Earth, where one side continually faces toward the Earth. The geology of the Moon has since been more independent of the Earth. While this hypothesis explains many aspects of the Earth-Moon system, there are still a few unresolved problems facing it, such as the Moon's volatile elements not being as depleted as expected from such an energetic impact. Another issue is Lunar and Earth isotope comparisons. In 2001, the most precise measurement yet of the isotopic signatures of lunar rocks was published.〔 Surprisingly, the Apollo lunar samples carried an isotopic signature identical to Earth rocks, but different from other Solar system bodies. Since most of the material that went into orbit to form the Moon was thought to come from Theia, this observation was unexpected. In 2007, researchers from Caltech showed that the likelihood of Theia having an identical isotopic signature as the Earth was very small (<1 percent). Published in 2012, an analysis of titanium isotopes in Apollo lunar samples showed that the Moon has the same composition as the Earth〔 (Titanium Paternity Test Says Earth is the Moon's Only Parent (University of Chicago) )〕 which conflicts with the moon forming far from Earth's orbit. To help explain problems with this, a new theory published in late 2012 posits two bodies—each five-times the size of Mars—collided, then re-collided, forming a large disc of debris that eventually formed the Earth and Moon.〔 The paper was called “Forming a Moon with an Earth-like composition via a Giant Impact,” by R.M Canup.〔 A late 2012 study on the depletion of zinc isotopes on the Moon, supported "the giant impact origin for the Earth and moon".〔(Paniello, et al. - Zinc isotopic evidence for the origin of the Moon )〕 In 2013, a study was released that indicated water in lunar magma was 'indistinguishable' from carbonaceous chondrites and nearly the same as Earth's, based on the composition of isotopes.〔(Earth-Moon: A Watery “Double-Planet” )〕〔(A. Saal, et al. - Hydrogen Isotopes in Lunar Volcanic Glasses and Melt Inclusions Reveal a Carbonaceous Chondrite Heritage )〕 GIH theory was again challenged in September 2013, with a growing sense that lunar origins are more complicated.〔(Daniel Clery -Impact Theory Gets Whacked )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Origin of the Moon」の詳細全文を読む スポンサード リンク
|