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月球分*裂说的新理论|天文地理- 台风论坛- 中国气象局上海台风研究所

在原有的

这是近年来关于月球成因的新假设。1986年3月20日，在休士顿约翰逊空间中心召开的月亮和行星讨论会上，美国洛斯阿拉莫斯国家实验室的本玆、斯莱特里和哈佛大学史密斯天体物理中心的卡梅伦共同提出了大碰撞假设。这一假设认为，太阳系演化早期，在星际空间曾形成大量的“星子”，星子通过互相碰撞、吸积而长大。星子合并形成一个原始地球，同时也形成了一个相当于地球质量0.14倍的天体。这两个天体在各自演化过程中，分别形成了以铁为主的金属核和由硅酸盐构成的幔和壳。由于这两个天体相距不远，因此相遇的机会就很大。一次偶然的机会，那个小的天体以每秒5千米左右的速度撞向地球。剧烈的碰撞不仅改变了地球的运动状态，使地轴倾斜，而且还使那个小的天体被撞击破裂，硅酸盐壳和幔受热蒸发，膨胀的气体以极大的速度携带大量粉碎了的尘埃飞离地球。这些飞离地球的物质，主要有碰撞体的幔组成，也有少部分地球上的物质，比例大致为17:3。在撞击体破裂时与幔分离的金属核，因受膨胀飞离的气体所阻而减速，大约在4小时内被吸积到地球上。飞离地球的气体和尘埃，并没有xx脱离地球的引力控制，他们通过相互吸积而结合起来，形成全部熔融的月球，或者是先形成几个分离的小月球，在逐渐吸积形成一个部分熔融的大月球。

人们对月球起源的假设（来自维基百科）

荷兰阿姆斯特丹Western Cape大学的Rob de Meijer和VU大学的Wim van Westrenen提出了解释粉裂说的新理论：The standard theory of the origin of the Moon is called the giant impact hypothesis. It supposes that early in the Solar System's history, a massive object smashed into the Earth, cleaving it into two unequal parts. The smaller of these condensed into the Moon.

The best simulations of this process suggest that about 80 percent of Moon ought to have come from the impactor and 20 percent from the Earth.

That's hard to reconcile with the measured make up of Moon rock, which is almost identical to Earth rock in terms of isotopic content. Some planetary geologists say this could be explained if, soon after the impact, the debris mixed well before forming into solid bodies. But others counter that this might explain the similarity in the isotopic ratios of lighter elements such as oxygen but can't easily account for the identical ratio of heavier elements such as chromium, neodymium and tungsten.

But there's another theory called the fission hypothesis that could account for the similar isotopic content. This idea is that the Earth and Moon both formed from a rapidly spinning blob of molten rock. This blob was spinning so rapidly that the force of gravity only just overcame the centrifugal forces at work.

In this system, any slight kick would have ejected a small blob of molten rock into orbit. This blob eventually formed the Moon.

The fission hypothesis has been studied for 150 years but ultimately rejected because nobody has been able to work out where the energy could have come from to kick a lunar-sized blob into orbit.

Now Rob de Meijer at University of the Western Cape and Wim van Westrenen at VU University in Amsterdam say they know where that kick might have come from.

Their idea is that centrifugal forces would have concentrated heavier elements such as uranium and thorium near the Earth's suR FAce on the equatorial plane. High concentrations of these radioactive elements can lead to nuclear chain reactions which can become supercritical if the concentrations are high enough.

The question is how concentrated could these elements have become. De Miejer and van Westrenen calculate that it is quite possible for the concentration to be high enough for a runaway nuclear reaction.

Their theory is that the explosion of a natural nuclear georeactor after it became supercritical ejected the material that eventually formed the Moon.

They also say that there ought to be telltale evidence that such an explosion took place, particularly in the lunar abundance of helium-3 and xenon -136, which would both have been produced in great quantities in a natural georeactor.

Future measurements from the suR FAce could provide the evidence needed to confirm their theory but the analysis will not be easy. It is well known that the solar wind deposits vast amounts of these substances onto the lunar suR FAce so that will have to be taken into account.

Of course, georeactors are by no means hypothetical. The most famous is in Oklo in Gabon, not so far from the equator, where a natural nuclear reactor was clearly in operation until about 1.5 billion years ago, leaving telltale signs in the uranium deposits now being mined.

One interesting corollary of this discussion centres on the origin of this theory which is credited to George Darwin, son of the more famous member of this family. Not content with settling the debate over the origin of the species, could it be that the Darwin family might eventually account for the origin of the Moon, too?

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