月球形成背景资料 2.3.3

queenie924

For a long time, the fundamental question regarding the history of the Moon was of its origin. Early hypotheses included fission from the Earth, capture, and co-accretion. Today, the giant impact hypothesis is widely accepted by the scientific community.
[edit]Fission hypothesisThe idea that the early Earth, with an accelerated rotation, expelled a piece of its mass was proposed by George Darwin (son of the famous biologist Charles Darwin). It was commonly assumed that the Pacific Ocean represented the scar of this event. However, today it is known that the oceanic crust that makes up this ocean basin is relatively young, about 200 million years old or less, whereas the Moon is much older. This hypothesis cannot account for the angular momentum of the Earth-Moon system.
[edit]Lunar captureThis hypothesis states that the Moon was captured, completely formed, by the gravitational field of the Earth. This is unlikely, since a close encounter with the Earth would have produced either a collision or an alteration of the trajectory of the body in question, so if it had indeed happened, the Moon probably would never return to meet again with the Earth. For this hypothesis to function, there would have to be a large atmosphere extended around the primitive Earth, which would be able to slow the movement of the Moon before it could escape. This hypothesis is considered to explain the irregular satellite orbits of Jupiter and Saturn. In addition, this hypothesis has difficulty explaining the similar oxygenisotope ratio of the two worlds.
[edit]Co-accretion hypothesisThis hypothesis states that the Earth and the Moon formed together as a double system from the primordial accretion disk of the Solar System. The problem with this hypothesis is that it does not explain the angular momentum of the Earth-Moon system, nor why the Moon has a relative small iron core compared to the Earth (25% of its radius compared to 50% for the Earth).
[edit]Giant impact theoryMain article: Giant impact theory
At present the best explanation for the origin of the Moon involves a collision of two protoplanetary bodies during the early accretional period of Solar System evolution. This "giant impact theory", which became popular in 1984 (although Reginald Aldworth Daly, a Canadian professor at Harvard college, originated it in the 1940s), satisfies the orbital conditions of the Earth and Moon and can account for the relatively small metallic core of the Moon. Collisions between 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.
The theory 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 of this event appear to show that the collision must occur with a somewhat glancing blow. This will cause a small portion of the colliding body to form a long arm of material that will then shear 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).
This formation theory helps explain why the Moon possesses only a small iron core (roughly 25% of its radius, in comparison to about 50% for the Earth). Most of the iron core from the impacting body is predicted to have accreted to the core of the Earth. 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 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 independent of the Earth. While this theory explains many aspects of the Earth-Moon system, there are still a few unresolved problems facing this theory, such as the Moon's volatile elements not being as depleted as expected from such an energetic impact.^[2]

fannybeibei

upup

uwants

大粗体是啥意思?

queenie924

大粗体是啥意思?

-- by 会员 uwants (2010/8/26 21:12:29)

你看阅读机经··就是里面的那4个理论··